Download Oecophylla longinoda. in cocoa agro­ecosystems in order  to suppress cocoa pests such as capsids?  

The behaviour of Oecophylla longinoda.
How to manipulate or use Oecophylla ants in cocoa agro­ecosystems in order to suppress cocoa pests such as capsids?
Research subject of Machiel van Wijngaarden and Monique van Kessel in the framework of their MSc study at the
Laboratory of Entomology, Wageningen University
Research proposal
Keywords: Oecophylla longinoda, cocoa, pest management…
Students: Monique van Kessel*, Pieter Machiel van Wijngaarden*
Supervisors: Godwin Ayenor^, dr. A van Huis*
*Laboratory of Entomology, Wageningen University, PO Box 8031, 6700 EH Wageningen, The
Netherlands
^University of Ghana
Index
Preface
Introduction
Research area
Material and Methods
References
‘’In 1476, the farmers of Berne in Switzerland decided, according to this story, there was only one way
to rid their fields of the cutworms attacking their crops. They took the pests to court. The worms were
tried, found guilty and excommunicated by the archbishop.’’ (Reference)
PREFACE
Ants as a biological agent for controlling pests is already known and practiced for a long time
(Van Mele, 2000). The earliest report of Oecophylla ants at work among the orange trees is described
in a book on tropical and subtropical botany written by Hsi Han in AD 304. "The people of Chiao-Chih
sell in their markets ants in bags of rush matting. The nests are like silk. The bags are all attached to
twigs and leaves which, with the ants inside the nests, are for sale. The ants are reddish-yellow in
colour, bigger than ordinary ants. In the south if the kan trees do not have this kind of ant, the fruits
will all be damaged by many harmful insects, and not a single fruit will be perfect." (REFERENCE)
Around the 12th century the Chinese farmers already connected their fruit trees with bamboo sticks to
provide the ants a passage to move from one tree to another (Friederichs, 1920). This measure
resulted in a spread of ants through whole of their orchards.
Nowadays there are several places where ants do a very useful job as a biological pest agent.
Vietnam, Cote d'Ivoire, and countries in the North of Australia are some of the places where the
benefits of this little creature have already been recognized and used. Ghana, as one of the major
cocoa-producing countries, could also use the advantages of ants in their struggle against pests
which threaten the yield of the cocoa plantations.
The research questions and methods which will be described in this proposal just represent a global idea of the
research on O. longinoda, and the use of Oecophylla ants in the biological control of cocoa capsids, which still
can be done. Mainly due to the limited time available for our research (approximately five and a half months),
we will not be able to carry out all the described experiments in real terms. Another important reason for the
somewhat global way in which we describe methods in this proposal, is a lack of practical knowledge
concerning cocoa and pest management. We hope we will be able to get to a more specific research plan in
meeting with Godwin Ayenor, a phd student participating in the ‘Convergence of Science’ project, who will
supervise us during our stay in Ghana. INTRODUCTION
COCOA PLANTATIONS AND ITS PROBLEMS
Major problems
Pests and diseases in cocoa plantations are a large threat for cocoa trees. Around the year
1999, 29.4% of the crop was lost due to capsid insects, cocoa swollen shoot virus and fungal disease
pod root (Gray, 2000). The same values were predicted for the following years. These numbers
indicate how serious pests and diseases threaten the income of the farmers. Each year almost a third
of potential income is lost.
There are several major problems cocoa crops have to deal with. Fungi are a problem all over the
world and for Ghana this is no exception. An example of a deadly fungus attack is shown in figure 1.
Fungi are difficult to control, because the ground acts as a natural barrier by which they are protected
and they are often widespread through the infected area. For example the fungi Phytophthora
megakarya occurs in Ghana, which destroys part of the crops (Opoku et al., 2002).
Another major problem is the cocoa swollen root virus. This virus is transmitted by the mealybug.
Mealybugs belong to the Homopteras, family Pseudococcidae. An infection leads to interveinal
chlorosis, leaf mosaic, stem and root swellings, and pod distortion (Ollennu, 2004). This disease can
be easily governed, because the vectors of this disease can not travel over great distances. So, for
example a 10 meter long corridor would already prevent the disease from spreading. The only
problem is to make this a common policy and bring the knowledge to the people (Ollennu et al.,
1989).
However, our research will mainly focus on insect pests and particularly capsids. These insects are
another cause of great yield loss in the cocoa plantations (REFERENCE).
Introduction: insects
Two capsid species are the mean cause of damage done to cocoa plantations by insects,
namely Distantiella theobroma (Dist.) and Sahlbergella singularis Hagl (Pictures) (REFERENCE).
When capsids have injected their histolytic saliva, they leave a lesion behind. The shape of the
lesions often appears to be somewhat characteristic for each capsid species. For example, lesions
resulting from the feeding of Distantiella and Sahlbergella tend to be elliptical with the long axis
parallel with that of the stem, while for some other species the lesions are more or less round.
The lesion which is left behind is very frequently infected by pathogenic fungi. Calonecrtia rigidiuscula
is the most common one (REFERENCE). This is the reason why many trees can die from just a
relatively low number of capsids being present. Therefore these pest insects have to be controlled
efficiently in order to prevent much damage as a result of fungi.
From August till December the capsid population is at its highest level of impact (REFERENCE),
attacking young branches and jorquettes (young sprouted branches) which they prefer. By attacking
planted seedlings the capsids can delay the plantation to bear fruit several years (REFERENCE).
A more complete list of insects which threaten the cocoa in Ghana is given on website 3. For the
paper version of this article, the part about which species threaten cocoa, is pasted below:
“In addition to capsids, termites have in recent times been of economic importance in some parts of
Ghana (Ackonor, 1994). Sporadic attacks by other insect pests of minor importance also occur,
causing varying degrees of damage. These insects include the pod feeders Bathycoelia thalassina
(H.-S.) (Heteroptera: Pentatomidae) and Pseudotheraptus devastans (Dist.) (Heteroptera: Coreidae)
(Owusu-Manu, 1971; Padi, et al., in press), the stem borer Eulophonotus mermeleon Fldr.
(Lepidoptera: Cossidae) (Anon, 1995) and the pod borer Characoma stictograpta Hmps. (Lepidoptera:
Noctuidae) (Akotoye, 1975). Anon (1995) observed that damage caused by termites and stem borers
is becoming increasingly important in the whole country and recommended that CRIG must intensify
research for a quick solution to these problems. Other insect pests which are particularly important on
young cocoa include the Lepidopteran foliage pests Earias biplaga Wlk., Anomis leona Schaus.,
Orgyia basalis Wlk. and Prodenia litura F., the psyllid Tyora tessmanni (Aulm.), the aphid Toxoptera
aurantii (Fonsc.), the thrip Selenothrips rubrocinctus (Giard.) the stem tip feeder Tragocephala spp.
(Coleoptera: Cerambycidae) and Empoasca devastans Dist. (Homoptera: Fulgoroidae) which causes
the tip burn disease (Collingwood & Marchart, 1971; Entwistle, 1972)”.
CONTROLLING METHODS
Control by insecticides
Control of pests through insecticides is an option, but there are many disadvantages. First of
all, natural populations which are neutral or even beneficial are also being destroyed or distorted.
Besides that, important natural enemies of the pests are wiped out or suppressed by the insecticide
as well.
Second, the effectiveness of the insecticide reduces as the pest insects become more and more
resistant to the chemicals. In the long term the insecticide will not be of any use anymore and might
even have a negative effect on the environment.
Third, the use of insecticides can suppress a primary pest, but meanwhile promote a secondary pest.
This is demonstrated in the study by Dunn (1963). The common insecticide in these days, lindane,
suppressed the pest S. singularis, which was sensitive to that insecticide. Yet, by destroying this
species, D. theobroma flourished and replaced S. singularis. The outbreak of this secondary pest was
possible because D. theobroma was much more resistant to lindane.
The fourth disadvantage is the effect on health of producer and consumer. The effect on health of the
people who eat the products is not too dangerous -otherwise they would have been forbidden- but
they are not healthy either. However, this is not the major problem. The farmers, who have to apply
the insecticide, are in contact with a much higher dosage when spraying the insecticide on their crops.
Therefore the insecticide must be harmless for mammals –which automatically mean the insecticide
will be harmless for humans too- (REFERENCE).
The last disadvantage is the insecticide affecting the taste of the cocoa. By applying insecticide the
quality of the cocoa is being reduced (Leiter & Harding, 2004).
Biological control
The reduction of pests without, or at least with less pesticide, can be done in many different
ways. Research has been done on the use of parasites and pheromones to control pests in cocoa
(Beevor et al., 1993; REFERENCE). Fungi can be used for example to promote the plants’ defence
system. Other crops can be sown which have a negative effect on the pest.
Another possibility is the promotion of natural enemies (natural enemy enhancement/ conservation
biological control). Conditions are favoured for the natural enemy of the pest, for example by adding
additional food sources or growing host plants at the border of the field. More individuals of the natural
enemies are able to survive and attack the pest, resulting in a reduction of the pest population size.
Ants are often not specialists, and therefore not specific enemies of one species of insects. Their
generalistic predation and aggressiveness makes them very effective for biological control, so they
can be used for introduction in areas where pest insects are too numerous.
Aim of the research
At this moment there is an inefficient controlling of pest insects by insecticide, which is shown
by the amount of crops lost due to pest insects. The purpose of this research is to give an alternative
to this chemical control, by investigating the possibility of the application of Oecophylla longinoda as a
biological agent for controlling the major pests in cocoa plantations in Ghana.
During the study of literature the question about which species to introduce can be answered quite
easily. The Oecophylla ants are quite common in Ghana (REFERENCE) and considered to be a good
agent for biological control (Way & Khoo, 1992). Since native species are most easily to introduce and
less dangerous on a long-term (because it is their own environment) this research will focus on O.
longinoda. Although many articles are found on the use of Dolichoderus thoracicus as a biological
control agent, this ant species occurs to be absent in Ghana. Therefore this species, although well
competent for biological control, will not be discussed in this paper as a possible biological control
agent (Way and Khoo, 1991).
The following question will guide our research:
• How can we introduce and maintain Oecophylla longinoda colonies in order to suppress
capsid pests in cocoa plantations in Ghana and why is O. longinoda not being effectively
used already?
This question can be divided into many subjects. All these subjects have to be investigated to carry
out a proper research in the field. The basic information is required for a good outline of the field
research which will be preformed in Ghana from June till November. Godwin Ayenor, a phd student
participation in the project ‘Convergence of Science’, will supervise us during the research.
OECOPHYLLA LONGINODA
Introduction
As mentioned before, weaver ants (Hymenoptera, Formicidae, Formicinae, Oecophylla) belong to the
group of insects which can be used in biological control of capsid pests in cocoa plantations. For these arboreal
Oecophylla ants two species have so far been described (website 1). The genus Oecophylla occurs in tropical Asia, Australasia and Africa and the most well known and widely used
ant for biological control within this group is Oecophylla smaragdina (Way, 1954 and Way & Khoo, 1992).
Van Mele & Cuc (2003) mention all countries in which weaver ants have been reported (table 2). The African
representative of the genus is Oecophylla longinoda and has, among others, been described by Latreille
(Vanderplank, 1960). Oecophylla longinoda has five varieties (as reviewed in Way, 1954), but Vanderplank
(1960) questions whether or not there are sufficient grounds to consider these five varieties of O. longinoda and
O. smaragdina as separate species. Also Way & Khoo (1992), mention that the biology of both O. longinoda
and O. smaragdina can be treated as one, though their geographical distribution is very distinct.
About O. longinoda relatively little literature has been found, and if so, it was often quite ‘old’. About O.
smaragdina a bit more has been written and since we assume that both species show significant similarities in
ecology, in some cases we use information on O. smaragdina for both Oecophylla species. Castes and nest building
Oecophylla colonies may cover up to 1600 m², comprising approximately a million workers and brood
and the life of a colony might exceed five years if not being destroyed by other ants (Vanderplank, 1960 and
Way & Khoo, 1992). Five different castes of O. longinoda can be distinguished; small and large workers, and
the reproducing males, virgin females and queens, according to Way (1954) and Vanderplank (1960). However,
virgin females are not being mentioned in other studies, so in this research only four castes will be considered.
The (female) workers are thus dimorphic with major (large) workers outnumbering the minors (small workers),
which is a common feature among ants (Greenslade, 1972). Small workers can often be found inside the nest where they attend the developing brood and the sexual forms,
and sometimes also honey­dew­producing Homoptera within the nest. The functions of large workers are
defending the colony, foraging, nest­construction and attending Homoptera outside the nest (Way, 1954 and
Van Mele & Cuc, 2003). Queens are the largest of the colony with a big abdomen for egg production. Queens
start life winged, but lose their wings soon after the mating flight. Males are much smaller than the queen,
winged, and their only task is mating with the queen which results in a longevity of only a few days (Van Mele
& Cuc, 2003). It has been shown by studies of, among others, Way (1954) and Vanderplank (1960) that within a
colony there is always only one queen present. The queen moves between the nests (escorted by several
hundreds of large workers). This would then result in many nests within a colony without a queen. However,
Van Mele & Cuc (2003) speak of one or several queens within a colony, in one (dry season) or in more (wet
season) nests. They also mention that more queens are being produced during the wet season than during the
dry season (Van Mele & Cuc, 2003). Peng et al. (1998), which developed a method to determine which nests within an O. smaragdina colony
contains a queen, also often found more than 1 queen (up to 6) per colony always occurring in just 1 nest.
Within queen­nests there always were large numbers of large workers, eggs, small larvae and just a few small
workers were found. However no pupae or medium­ or large larvae were present. According to Peng et al.
(1998) it seems that the queens stay in one nest to lay eggs, and that larvae later on are carried to other nests.
This is contrary with findings of Vanderplank (1960). The study by Way (1954) showed that queen­less
colonies eventually die out, unless brood is being added from time to time (also reviewed in Van Mele & Cuc,
2000). Nests are always being built in tree/shrub crowns. The nest­building process involves a highly organised
process. First, the ants seem to determine whether or not there are leaves enough to form a nest. Secondly, the
large workers start drawing leaves together, sometimes forming chains up to twelve workers to bridge the gap
(Vanderplank, 1960). When the leaves are in the right position they are held together by workers, whereas
others start carrying medium­sized larvae in their mandibles. The larvae then are continuously stimulated to
secrete silk from one leave to the other which will hold the leaves together (Way, 1954 and Van Mele & Cuc,
2003). Small shelters of only a few leaves are sometimes being build in the same way over clusters of Homoptera
which are being attended (Way, 1954 and Van Mele & Cuc, 2003). Way (1954) also observes that the part of
the tree selected for nest­building varies according to the season, and seems to depend on both sunlight and
wind direction. Distribution
As O. smaragdina mainly occurs in the tropical parts of Asia and Australia, O. longinoda can thus be
found in West­ and east Africa (as reviewed in Löhr, 1992). In the article by (Way, 1954) the general
environmental conditions determining the ants’ abundance are briefly being described. O. longinoda ants are
especially abundant in areas with relatively high rainfall and temperature, suitable soils, and evergreen tree and
bush vegetation (Way, 1954). The article by Van Mele & Cuc (2003) describes O. smaragdina prefers
temperatures between 26 and 34 ºC and relative humidities between 62 and 92 %. For O. longinoda we assume
preferences will be quite similar. Since Oecophylla ants do build nests of living leaves, vegetation should not be deciduousness, leaves must
exceed a certain size and leaves must be strong but flexible, though often Oecophylla can adapt the foliage of
most trees for nest­building (Greenslade, 1972 and Van Mele & Cuc, 2003). Sometimes Oecophylla ants are
also satisfied with trees for nest­ building having smaller leaves, but then leaves should be abundant (Van Mele
& Cuc, 2003). Beside this, choice of host plants for nest building might also depend on the ability of the plant to
support suitable Homoptera clusters. O. longinoda is less present or totally absent in areas with higher altitudes
(Way, 1954).
In Zanzibar O. longinoda ants has been found colonising some 89 species of trees and shrubs belonging to 35
families in Zanzibar (Way, 1954), but other numbers and species may hold for other areas. Feeding habits & foraging activity
O. longinoda is intermediate in its feeding habits, being both an active predator but also feeding on
honeydew produced by several Homoptera species. According to Vanderplank (1960) both food sources even
appear to be essential for the survival and reproduction of a colony. Because ants benefit from several
Homoptera species they often take care of them in several ways. First by protecting them from various enemies
(though often accidentally), secondly by removing honeydew and fungi contaminations, and third by offering
shelter (Way, 1954a). The ants thus live in a close mutualism with certain Homoptera species (Greenslade,
1972). However, Oecophylla does keep Homoptera populations at levels at which they provide the required
amount of food for the ants, and thus sometimes attacks and kills part of the Homoptera populations. The
Homoptera species Stictococcus sjostedti Cockerell and Planococcoides njalensis (Laing) are the most common
in cocoa and important in honeydew production for ants. Also Parastictococcus multispinosus has been
observed being attended by O. longinoda (Bigger, 1981).
O. longinoda is a generalist and therefore predates on a great variety of insects, among which are many pests
(Vanderplank, 1960). Oecophylla ants are being observed foraging on both the ground and over trees and shrubs
attacking most insects with which they come into contact (Way, 1954). Way (1954) briefly lists the insects
which were observed being killed by O. longinoda in a study in Zanzibar. Among these insects were several
Hemiptera Heteroptera (e.g. Coreid species), Hymenoptera species (e.g. honey bees, wasps and some other
ants), Coleoptera species (e.g. Carabidae, Curculionidae, and Cetoniidae), but also limbs and wings of
grasshoppers, cockroaches and other Arthropod species were being found in their nests. A study by Wojtusiak
et al. (1995) reported cases of capture of very large prey by workers of O. longinoda, and even remains of
vertebrate prey were being found in their nests. They compare the hunting behaviour of O. longinoda with that
of army ants (Wotjusiak et al., 1995). In many studies it has been observed that Oecophylla worker colour depends partly on diet. If colonies
exclusively feed on sugar sources (or some specific insect species), only small, yellow workers are being
produced which show decreased aggression. In case of feeding on insect prey only, the newly produced workers
are relatively larger, deep red coloured and are much more aggressive (Greenslade, 1972 and Vanderplank,
1960). It can therefore be suggested that feeding Oecophylla ants exclusively with insects would result in highly
aggressive ants, which could be advantageous in biological control. Besides that, they would require less
Homoptera to anticipate their needs. However, according to Way (1954a), ant populations which solely
dependent on insect prey can only maintain at relatively low populations levels. Besides that, a higher
aggressiveness could also lead to a greater loss of beneficial insects. If additional prey would be added higher
population numbers might be achieved, but this should be thoroughly investigated. Probably there will be an
equilibrium state which gives the optimum predation with negative effects of aggressiveness as low as possible.
According to Greenslade (1972) O. smaragdina ants are diurnal and are thus mainly active during the period of
daylight. Dejean (1990) looked specifically at the circadian rhythms of activity of O. longinoda in a study
conducted in Zaire. In his introduction he describes several contradicting conclusions from studies which
investigated the activity pattern of Oecophylla species. Dejeans’ (1990) study clearly shows that O. longinoda
ants are diurnal in foraging behaviour, but that there is also a low nocturnal level which is associated with
guarding of workers in the ‘central’ territories (located within the immediate areas around the nest, often in the
tree canopy). Foraging occurs mainly in the ‘ground’ territories (located in areas where the number of potential
prey is consistent and high, often on ground levels) (Dejean, 1990). There seems to be a clear distinction
between prey captured in ‘central’ territories (winged insects) and those captured in the secondary territory
(‘crawling’ insects). Dejean (1990) suggests this might be the result of either ‘worker specificity’, relative
abundance of flying versus walking prey in each territory, or both. It appears that light is the main factor in
determining foraging activity (Dejean, 1990).
Territoriality & orientation In several studies by among others Dejean and Hölldobler & Wilson, aspects of territoriality and trail
pheromones have been looked at. Since this does not seem to be an important aspect of our study we will not go
into too much detail about this subject, but few notes will be given below. Weaver ants are strongly territorial at both inter­ and intraspecific level. Therefore O. longinoda secretes a
colony­specific territorial pheromone produced in the rectal sac. If deposits of another colony of ants are being
detected by workers, these will respond with an increased amount of aversive and aggressive behaviour
(Hölldobler & Wilson, 1977). According to Dejean (1991) this anal drop deposition has two important
functions, namely territorial and orientational marking. Beugnon & Dejean (1992) show that trail pheromones
can even be used for orientation after a period of 11 months! The chemical trail also appears to be resistance to
water, which Beugnon & Dejean (1992) consider as an adaptation to the high amount of rainfall in the West
African rain forest. Orientation by weaver ants is achieved by the use of several methods. Jander & Jander (1998) show that O.
smaragdina ants use a directional light compass. Like has been shown for other ant genera, they also conclude
that compass orientation based on the polarised light of blue sky strongly outweighed compass orientation on
light coming directly from the sun. In times of absence of sufficient 'light', O. smaragdina ants use a landmark
compass, and they even seem to respond to changes in the earth's magnetic field and thus also possibly use a
magnetic compass (Jander & Jander, 1998).
Natural enemies
According to Greenslade (1972) it has been recorded that vertebrates (such as birds and frogs)
occasionally predate on Oecophylla. Also the pseudoscorpion Paratemnus salomonis sometimes feeds on them.
Except for these predators it are especially several other dominant ant species which possibly threaten
Oecophylla ants (as reviewed in Greenslade, 1972). Several ant species which are dominant over Oecophylla are mentioned in the literature. These include
Anoplepis longipes, Anoplepis custodiens, Pheidole punctulata (Way, 1953, Vanderplank, 1960), Iridomyrmex
myrmecodiae, Pheidole megacephala (as reviewed in Way, 1953 and as reviewed in Way & Khoo, 1992,
Vanderplank, 1960), Pheidole devastans (as reviewed in Way & Khoo, 1992), Dolichoderus thoracicus (Van
Mele & Cuc, 2003), Crematogaster castanea, Crematogaster rectinota (Vanderplank, 1960), Crematogaster
clariventris (Bigger, 1981) and Vanderplank (1960) mentions several ant species which do not destroy whole
colonies but kill solitary queens which are attempting to establish a new colony. Although these ant species all
might outcompete or kill Oecophylla ants, some are also potential biological control agents for several pests in
many different plantations and areas (Way, 1953, Way & Khoo, 1992). Vanderplank (1960) describes that
Pheidole species often outcompete Oecophylla ants during the dry seasons when conditions are more favourable
to them, whereas during the wet season Oecophylla wins territory from Pheidole ants. According to website 1,
several Crematogaster species and Pheidole megacephala are quite common in cocoa plantations in Ghana. Adult workers of individual Oecophylla colonies are mutually antagonistic (as reviewed in Way & Khoo, 1992),
but striking is the absence of antagonism towards immature forms of other colonies (Way, 1954). OECOPHYLLA IN PEST MANAGEMENT
Examples
In the introduction several examples are given of ants which are being used as a biological
control agent. The example of citrus farmers in Mekong Delta, Vietnam, which have a long tradition of
managing the weaver ant O. smaragdina is one of them (Van Mele & Cuc, 2000).
However in the last century, since the introduction of insecticides, most of the original and historical
(which were often biological) ways of reducing pests, have become replaced by ‘chemical
management’ (Way & Khoo, 1992). The insecticide was considered as the new way of dealing with
pests. Now the old knowledge has to be revalidated and presented to the farmers as the best
alternative.
Yet biological control in Northern Australia is carried out with O. smaragdina in commercial cashew
plantations. This was based on the old age knowledge and extensive scientific research (Peng et al.,
1995, 1997, 1998). Another example of experiments in which pest insects were controlled by O.
smaragdina and D. thoracicus is given by Van Mele et al. (2002). It is reported that O. longinoda can
effectively control several pests on coconuts including the devastating coreid P. wayi Brown on
coconuts in East Africa and P. devastans in Cote d'Ivoire. Also the mirid D. theobroma on cocoa in
West Africa and other examples of among other O. smaragdina as a control agent are being
described (all referred to in the article by Way & Khoo, 1991).
The examples above show that the use of ants in pest management is very common, and especially
being practised in Asia. They prove that ants, and particularly O. Longinoda, could be used as a
biological control agents.
Ant husbandry
In many of the examples it was necessary the ants were looked after. This is called ant
husbandry and involves several ways of for example field manipulations (Van Mele & Cuc, 2000).
Aspects are listed below.
Requirements
To introduce ants in cocoa farms, nests have to be moved into the plantations. This change of
habitat will not be a big problem if the plantations meet with the requirements of ants needed for
survival. There is not much known about adaptations to habitat change. So we do not know how ants
react on transferring their nests to other habitats. However, an indication of the sources which have to
be included in the environment is available from many studies. These will briefly be discussed below.
First the abundance of Homoptera, which provide an essential food source by production of
honeydew, should meet the ants’ requirements (Way, 1954). Of an predatory species Formica rufa,
which also has been considered as a biological control agent (Way & Khoo, 1992), a diet consist of
62% honeydew, 5% resin, fungi, carrion and seeds and 33% insect prey. This indicates that
Homoptera are the greatest source of food. Observations of transport of Homoptera by O. longinoda
to new nesting sites confirm this statement. Ants give some protection against enemies of
Homoptera, but to what extend is not yet clear and often exaggerated (Nixon, 1951). O. longinoda is
attending Homoptera species according to many studies (Way, 1954a, Vanderplank, 1960).
Homoptera are controlled in both number and position (Way 1954a). If there are more individuals than
needed to provide the ant colony with sufficient honeydew, the Homoptera are predated upon.
Second, weaver ants need trees with big leaves to construct their nest. The abundance of the right
leaves is a prior to introduce ants. Even when whole nests are introduced, there should be still some
vegetation suitable for the ants to build their nest in case they move to other nesting sites. After an
average of 85 days they tend to move to another site to build a new nest, this because the leaves
inside the old nest are rotten or climatic circumstances are not optimal anymore (Way, 1954).
Although ants have often been observed in coconut palms, the ants do have preference for other tree
species for nest-building -for example clove trees- and rather not for coconut or cocoa (Way & Khoo,
1991). Leaves should be big enough to fold but not too tough, so not too much effort has to be put in
the folding. However, while ants are so often observed in coconut palms, we doubt whether or not
coconut leaves meet preferred leave characteristics for O. longinoda ants.
Climatic circumstances affect the ants in their way of living and wind is one of the factors ants do
react on (Way, 1954). Yet farmers do not have to participate in these processes, because the ants will
move themselves if they have to and farmers can not influence climatic changes. Only the fact that
ants move when wind changes, should be known, so a farmer would not be disturbed by observations
of moving ants. Besides that, seasonal dispersion of nests and Homoptera clusters should be familiar
to farmers (Way, 1954a).
One of the factors which could actively be influenced and might have some advantages, is the
creation of shadow. Room (1971) states that shadow favours O. longinoda in comparison to
competing species. Bigger (1981) gives another reason why creating shadow is not a waste of time; it
prevents much damage by mirids, thribs and leaf eating caterpillars.
Cocoa trees form tight and dense vegetation between overhead canopy and ground vegetation,
hereby creating a close resemblance to natural situations. Also insect fauna composition often
becomes very ‘natural’ (Bigger, 1981). However, growing other trees than Theobroma cocoa L. would
even better resemble the natural situation and thus support natural balance.
Little is known about the effects different composition of the underground can have. It is known that
O. longinoda forages on the ground and never in the soil itself, so a more open underground will
probably be favoured (Bigger, 1981).
Disadvantages
The use of Oecophylla in biological control is not without any disadvantages. And even after
Oecophylla ants were considered to be a potential biological agent, different perspectives of authors
did not give much intelligibility about the relation between the disadvantages and the advantages and
how they were correlated (Way, 1954, Way & Khoo, 1992). However, the latter study does not
emphasize the disadvantages as much as the first one. The different opinions about the
disadvantages result in different suggestions about the way ants should be treated.
One of the disadvantages is the aggressiveness of the ants. When harvesting the beans or coconuts
the ants will attack. This interference is not beneficial to the image of ants, especially to the local
farmers which have to stand not only one, but many bites during the harvest. The effect of a bite will
be different for each person, but it is generally experienced as painful, though pain will not last for
long. A description of a bite is found in the article by Vanderplank (1960). “Way (1954a) gives his
observations on the predation by Oecophylla in Zanzibar. My observations do not differ in any way
except he mentions that ‘the “poison” does not seem to increase pain caused by the bite, nor does
the pain persist after the removal of the ant.’ I cannot agree with this statement. I have allowed soldier
ants to bite the back of my hand, and before they are able to squirt any fluid over the site of the bite,
cut their abdomens off with a sharp pair of scissors. The bite of the ant without any fluid is not painful,
but the fluid makes the bite quite painful and in my case I find the pain persists for 10-15 min after the
ants have been removed. The local people also find the bite painful; hence Oecophylla’s Kiswahili
name maji ya moto or ‘hot water ant’
A solution to this problem is given, but it is not sure whether this is applicable or not. Following an
often suggested strategy, the ants should be kept out of the harvest area by applying insecticides
during the period of harvesting. As far as we know, this strategy has not yet been applied in the field,
only suggested. However, we certainly doubt whether this strategy would work, for which we think
there are many reasons.
First, the ants are reduced in number and driven out of the area they normally forage in (cocoa fields).
Because this management has never been carried out, the adaptation of ants to this kind of control is
uncertain. The ants might not survive or will abandon the nests to find a more undisturbed place, and
therefore possibly totally disappear from the plantations.
Second, the amount of ants is thus being reduced, -especially that of large and small workers- and
the colony might easily being overrun by competing ants.
Both problems occur after the harvest, but control of pests with chemicals is absent for a at least a
certain period. The farmer could introduce the ants again next growing season, like the Chinese
already did in AD 304. For a broad application of O. longinoda there has to be a large stock of nests
which can be introduced in the cocoa plantations. This is only possible with sufficient amount of nests.
Because of the reduction of natural habitats of the ants, these might not be available (REFERENCE).
Therefore a breeding programme would be necessary to obtain the required ants. As far as we know
there is no experience or research done on breeding ants and in particular that of Oecophylla.
An alternative which does not require a huge amount of nests is the transfer of nests to outside the
plantation (REFERENCE). This must be done just before harvesting; otherwise the pest could still
destroy a lot of yield. The nests must be transferred to a place where the ants are able to survive until
the next growing season, so the requirements described above must be met in the area the ants are
kept. Also the level of disturbance must be reduced as much as possible or otherwise the ants could
be chased away. It has not yet been tested how disturbance affects the presence of ants, as far as we
know. Also, transfer of nests is still somewhat a problem.
Another disadvantage is the requirement of Homoptera which the ants need for their honeydew. Ants
have a certain preference for species of Homoptera they attend. O. longinoda is reported to attend
Stictococcids and S. zanzibarensis, which are both considered as quite harmless, when occurring in
small numbers (Vanderplank, 1960). Oecophylla largely depend on these insects for survival (Way,
1954). These Homoptera are attended by the ants in exchange for their honeydew, however
Homoptera might become a serious threat to the cocoa. If they become too numerous, they affect the
yield negatively by causing severe mechanical damage or by being vectors of plant virus diseases (as
reviewed in Way, 1954a). Hence the essential presence of Homoptera to provide the ants an
important food source is considered to be a negative aspect of the natural pest control by O.
longinoda. However Way states in his article (1954a): ”To summarize: O. longinoda workers transport
the Saissetia zanzibarensis (a Homoptera species), establish them at suitable feeding sites, and
maintain the number of individuals in a cluster at a level which does not cause obvious injury to the
twigs of the host plant.” “Under experimental conditions the young twigs of clove trees were damaged
by S. zanzibarensis, but this was not observed in the field”. This is contrary to what Way announces in
another article (1954). In this article the size of the population is determined for the damaging the
Homoptera causes. This is not considered in the article of Way. It is not clear whether it depends on
which tree the O. longinoda and especially the Homoptera live on, the species which is attended by
O. longinoda, the type of area or the setup of the experiment. Our opinion is that the size of the
population and observations in the field should be taken into account; where upon a good overview
can be given of the balance between the protection of Homoptera given by O. longinoda and the
damage caused by these aphids.
O. longinoda ants are quite a large Formicidea species. This enables them to attack large insects and
even small vertebrates (Wojtusiak et al., 1995). This aggressiveness against a large variety of insects
makes it more likely that this species might not only attack pests , but also beneficial insects
simultaneously. Another problem is the neglection of small species, which also can develop into
pests. Insects of 1 mm or less in length are not noticed or left alone (Way, 1954). And among these
little fellows, there are some which are quite hazardous to the cocoa trees. So an important potential
source of infections and carriers of diseases are overlooked.
Advantages
However, all the authors cited above do see Oecophylla as a potential biological agent. The
advantages outweigh the disadvantages according to the results of researches which have been done
in the past. Therefore the effectiveness of ants against pests must be high. References for this
statement can be found in Way & Khoo (1992), Van Mele & Cuc (2003) and Way (1954). So if O.
longinoda is considered to be a beneficial biological agent, what are the advantages?
As described before, O. longinoda is a very aggressive species. Besides the disadvantages for the
harvesters, this is one of the major advantages the ant introduces in the battle against pests. If any
potential food source comes across the path of O. longinoda workers, it will be attacked and, if
possible killed. So the numerous pests, which will be encountered many times, will be reduced in
number. Also, a possible secondary pest outbreak is less likely, because these will be attacked by the
ants as well. This is one of the advantages for a generalist as O. longinoda. O. longinoda is not likely
to become a pest itself, because the food sources will not be sufficient to provide for too large
colonies, although it is not excluded.
While “small-size” species are actually neglected, the presence of O. longinoda is not without any
affect on the populations of these little insects. Way (1954) concludes that the abundance of O.
longinoda causes the population sizes of “mini-insects” to decrease. So this suppression, not by
predation but by ‘presence’, could be added to the beneficial aspects of O. longinoda. Yet to what
extend and how effective this control by ‘presence’ reaches, is not known. Therefore it is not sure if it
is effective enough to use O. longinoda as a biological agent for these species. Nevertheless
observed trees, occupied by O. longinoda, did not accommodate any red spider, which cause serious
damage in other cocoa trees.
Since the abundance of O. longinoda is linked to the abundance of other species, it also influences
the populations of the Homoptera attended by the other species. This is important for certain species
of Homoptera which are considered to be a threat for the cocoa yield.
Some Pheidole species tend and benefit species of Pseudococcidae. Some of these Pseudococcidae
cause severe damage to plants or transmit virus diseases (as reviewed in Way, 1953). So, if O.
longinoda is favoured by human interference, at the expense of Pheidole species, it is also beneficial
in destroying these non beneficial Pseudococcidae because their attenders are gone. This could be
another reason why interference in favour of O. longinoda could be beneficial in fighting diseases or
pests.
Human interference might be needed before O. longinoda can be effective as a biological agent.
What we will try to achieve is to develop an easy and labour-extensive way of introducing and
maintaining O. longinoda populations in cocoa plantations. We will also try to give advice on how the
ants can be taken care of in such a way they can carry out their function as good as possible. So one
of the aspects which have to be considered next to the ecological aspects is the amount of labour our
method will acquire. It has to be applicable to serve the farmers and let them adopt the results of our
fieldwork. And when the application of control by the use of ants is too labour-intensive or too difficult,
it is likely the farmers will not adopt it. The costs for this application should also stay below the costs
spend for ‘general’ pest management. But before the fieldwork can begin, several other factors have
to be considered.
Cultural aspects
While the production was very high back in the seventies, the yield decreased in the nineties
till it was only a third of the yield of that in 1965. Several reasons are the cause of this reduction in
yield. Government interference, social structures and declined forest rent prevented great yields as
gained in the other parts of the world (Brazil and Malaise). Ghana -and most of West Africa's cocoa
producing countries- had only small stock farmers, which were not able to buy enough insecticides.
This prevented the cocoa from damage by use of chemicals as was the case in other cocoa
producing countries. While these countries produced more in quantity, they lost in quality. This
favoured the Ghanese cocoa which gave Ghanese farmers the chance to get a higher price for the
cocoa they produced, encouraging the production of cocoa in such a manner Ghana is now the
second producer of cocoa in the world (Leiter & Harding, 2004).
Cocoa is considered to be a crop with the potential to bring wealth to the Ghanese people. And
besides gold, it is the most important export product, making Ghana a relatively prosperous land.
The farmers in Ghana have to acknowledge the importance of IPM. We do not know how IPM and
classical methods of controlling pests are percepted. The project “Convergence of Science” is dealing
with this issue. Our supervisor, Godwin Ayenor, is closely related to these activities so we might direct
part of our fieldwork in this direction.
This leads us to the following question: Will we be able to introduce the ant successfully as a
biological agent and will this be adopted by farmers. To answer this question part of our research will
be focussed on the perception of ants, and especially O. longinoda, by farmers. Also they possibly will
be questioned about their experience with these insects. If necessary and practical a protocol is tried
to be established about how to familiarize farmers with these biological agents. What should be kept
in mind is the applicability of the ants as a biological agent. When the application is as simple as
possible, the farmers will adopt it faster. The amount of work has to be considered -the farmers can
not be busy whole day long with ant husbandry-.
Although not necessary a higher yield means more income -one of the objects which is tried to be
achieved- the introduction of ants will be, with present knowledge only have a positive effect on the
production of cocoa. Less insecticides, higher quality of cocoa and higher yield will be the expected
results of our field study. Results which make it worth the effort!
RESEARCH AREA
The work will be carried out in a research field at the Cocoa Research Institute of Ghana (CRIG) in
Tafo, Ghana. Describe conditions of research fields. Ghana knows a tropical climate with temperatures ranging from 21 to 32 ºC. In the South, were Tafo is located,
there are two rainy seasons, one from March to July and the other from September to October. In August there
occurs a short dry season, and there is a relatively long one from mid­October to March. The South of Ghana is
very humid with an annual rainfall averaging 2,030 mm, while the North is relatively dry (website 2). ­Vegetation? (http://www.africanconservation.com/ghanaprofile.html)
Both crematogaster clariventris and O. longinoda appear to be the most numerous species of ants in cocoa
fields in Tafo, which consist of Amelonado cocoa trees (Bigger, 1981). Bigger (1981) also describes which
other ants species are most abundant in cocoa fields and refers to different studies which estimate ant species
occurence. MATERIALS AND METHODS
­FUNDAMENTAL ASPECTS­
Structure and composition of the nest
Estimates of population size and distribution of O. longinoda and Oecophylla ants have already been
given by Way (1954) and Vanderplank (1960). Both studies only give colony size estimates of colonies
occurring in coconut and glove trees. Way (1954) states there is a difference in nest composition between
different tree species. However he mentions that outcomes of his study should be treated with reserve, since
they are based on relatively few quantitative data. For colonies in cocoa trees we have not found any estimates
yet, and therefore we will look at this in our study. • What is the distribution and composition of the O. longinoda population within and outside the
nest, of nests in cocoa plantations?
• What is the ratio of different castes of ants (small workers, large workers, males, queens, pupae and
larvae) within and outside the nest?
­Method; nest composition
To find out how the ant’s population within the nest looks like we will study ant nests in the laboratory. Nests of
different colonies of ants will be abscised from their trees, packed in plastic bags for transport and after that (as
soon as possible) be frozen at ­20 ºC for x hour. We will open the nests after defrosting and all individual ants
will be separated from the nest debris and grouped into castes (small workers, large workers, males, queens,
pupae and larvae). This will be done with x different nests from x different colonies, in x different periods
(Way, 1954, Lommen et al., 2004). Alternatively, in absence of freezing possibilities we can knockdown the
ants with an effective insecticide (Haines & Haines, 1978, Vanderplank, 1960). To describe nest composition
we will count the number of individuals per caste per nest population, determine weight of each stage, and
calculate percentages of stages (Haines & Haines, 1978, Lommen et al., 2004). Besides that, we will collect all other insects being present in the nests in order to detect possible natural
enemies of O. longinoda (see paragraph on ‘natural enemies’).
Nests will be collected in the field, where O. longinoda occurs naturally and is relative abundant.
­Measuring average body length, maximum head width, head length and wing length (etc.). ­How much time will it cost per nest? How many nests should we count?
­We can look at number of leaves of nests. Is there a correlation between population size per nest, and number
of leaves used per nest?
­Method; ant distribution outside the nest
To determine the distribution of ants outside the nest we have to observe how many and what kind of ants
(large/small workers, and perhaps males and queens) go in and out the nest. We can combine this with our
experiment in which we would like to determine (foraging) activity of the ants. Natural enemies
Natural enemies of Oecophylla ants are mainly comprised by other ant species. The main Oecophylla
predators and competitors have already been discussed in the paragraph on O. longinoda (introduction). In this study we would like to investigate which ant species specifically predate and parasitize on/ or compete
with O. longinoda in cocoa plantations in Ghana. Besides that we would like to find out whether or not there are
any other natural enemies of O. longinoda which might be possible since numerous small hymenopterans and
dipterans have been found in Oecophylla nests, and some of these have been suspected to be parasitic on
Oecophylla larvae (Vanderplank, 1960). • Which natural enemies of O. longinoda occur in cocoa plantations? • Which predatory ant species should definitely not be present in the cocoa plantation in order to
•
prevent competition with O. longinoda? And thus which plants related with these ‘enemies’ should
be avoided?
Which ant species should be reduced or be controlled in order to promote O. longinoda abundance?
­Method: Parasites/predators?
When investigating nest composition (see paragraph ‘Structure and composition of the nest’), we will not just
look at the ant population present, but also collect possible other insect species. After collecting these species,
we will try to identify them and search in the literature whether or not these insects have the potential to be
parasitic or predatory on O. longinoda. Whenever we find these insects not to be part of the diet of O.
longinoda (see paragraph ‘food consumption’) a possible conclusion is that either these insects are natural
enemies of O. longinoda or they live in, for example, a mutualism with the ants. If we suspect certain insects to
parasitise on the ants we can also, instead of kill the population of ants within the nest, try to anaesthetise them,
and select individuals (e.g. a certain number of workers and larvae) which we will observe in quarantine for the
period they live. We then might observe certain parasites eventually hatching out of the workers/larvae. ­Method: Ant enemies?
We should first determine which ant species occur naturally in the region and cocoa plantations specifically,
and which of these are known to be dominant species (literature and observations). We can also collect all ant
species occurring in the plantations and try to identify them. An ant collecting method has been described by
Bigger (1981). Secondly we will try to observe possible fights between O. longinoda and different ant species.
We might already be able to conclude out of these observations which ant species are dominating O. longinoda. Alternatively, we can put two nests with equal ant populations (estimation) of two different species (of which
one is O. longinoda) close together (lab?). They probably start to fight with each other and maybe we can then
say something about dominance of one of the species over the other. Another possibility could hold observing
which ant species occur in the same trees or else live closely together with O. longinoda, and which certainly do
not. The first ones will possibly tolerate O. longinoda, or the other way around. The ant species which do not
live closely together, while physically possible, could be either dominant or inferior to O. longinoda.
Ecosystem distribution In the study by Way (1954) 5 up to 151 nests per colony of O. longinoda have been found, Peng et al.
(1998) found 25 up to 153 nests per colony of O. smaragdina and Vanderplank (1960) even found Oecophylla
colonies with a number of nests reaching up to 192. Distribution within systems often shows a clear 'ant
mosaic', in which several distinct ant territories can be found mainly based on levels of dominance of the
species (Bigger, 1981). For this study we would like to know how many nests per colony of O. longinoda occur on average in cocoa
plantations. We would like to look at possible factors determining distribution of colonies through the cocoa
plantations. For example, does O. longinoda significantly occurs in higher numbers on specific vegetation close
or within the cocoa plantations? Which kind of ‘disturbance’ (e.g. human activities close to the plantations)
result in lower abundances? Etc. • How many nests per colony?
• Distribution of ants within the ecosystem? • How far do O. longinoda ants they spread outside the nest to forage?
Method: number of nests per colony
To determine how many nests can be found within each colony, we will transfer an x number of ants per nest to
other nests. We will observe the ants after transfer for approximately 10 minutes and if they will start to fight
with ants in the other nests we can conclude the nests do not belong to the same colony. When ants appear not
to fight, reciprocal transplants will be carried out to ensure that nests do belong to the same colony. If there are
many nests within one (cocoa) tree we can also assume the nests in this tree belong to the same colony, count
the number of nests within one tree, and only transfer ants between different trees to find out the actual number
of nests per colony. Colonies should be selected randomly, maybe in different plantations (Palmer, 2004). If we
know which nests belong to the same colonies we can try to map the colonies and might be able to draw
conclusions on for example distribution, overlap of colonies, etc. and try to relate this with other aspects such as
use of chemicals etc. (Way & Khoo, 1991). Another method for determining colony size has been described by
Peng et al. (1998). They determined the boundary of each ant colony by following the ant trail till were it ends,
and then count the nests within these boundaries (Peng et al., 1998). ­Method: distribution of ants within the ecosystem
To estimate the 'ant mosaic' pattern (Bigger, 1981) within the cocoa fields we should map the distribution of
ants. We might be able to achieve this by observing ant species occurrence through the field. We will randomly
choose trees on which we will observe which ant species occur and map this. Due to differences in levels of
dominance we will be able to see which ant distribution patterns occur (Bigger, 1981). ­Method: spreading of ants
We can possibly tag ants of a specific nest by for example the use of radioactive compounds (Vanderplank,
1960). After a time period of x days (the ants have been offered the opportunity to spread through the
environment) we can collect samples at different distances from the nest (e.g. put ant traps or collect death
ants), and therefore observe how far ants spread outside the nest (possible?).
Food consumption
Way (1954) described, among others, that O. longinoda ants feed on a wide diversity of insect species,
representing many orders of insects. Since we are interested in whether or not O. longinoda does influence
important cocoa pests, it is important to know on which insects they exactly prey on. In case O. longinoda
mainly feeds on insects which just represent minor or no pests, it might come true these ants would not be such
a good candidate for use of biological control of cocoa pests. However, this is very unlikely, regarded the
results of other studies. Dejean (1990) presented clear differences between prey species being captured in the central territory and the
ones captured in the ground territory. On the tree, prey comprised mainly winged insects, whereas workers
foraging on the ground caught insects running around or which were hiding under the leaf litter (Dejean, 1990).
Regarding these results, there might be a difference in prey capture by ants living in nests which can only be
reached by the stem of the tree in contrast to ants living in nests which can be reached both by the stem of the
tree as well as tree branches from other trees.
Probably the latter ant populations’ diet will represent a higher percentage of winged insects. • Which prey (especially cocoa pests) are consumed by O. longinoda ants, and are there any other
food sources of importance?
• Do the ants prefer to feed on capsids (the major cocoa pests) above other insects?
• Is there a difference in prey­diet between ants living in nests which can only be reached by the stem
of the tree, and ants living in nests which can be reached both by the stem of the tree and connecting
branches of surrounding vegetation? (Is there a wider diet range if ants can also disperse between
tree crowns?)
­Method; food consumption
We will collect all prey which are being transported into the nest during x hours of several (x number) O.
longinoda populations which can only be reached through the stem of the tree in which the nest in located. The
prey insects can be grouped into pests and non­pests, and the total numbers of insects per species will be
counted. This collecting will also be done for ant populations which live in nests which can also be reached
through the tree canopy. Therefore prey will be collected, which are being transferred into the nest by trails
connecting different trees.
Out of these numbers the division of insect species will be calculated, as well as the proportion of pest insects in
the diet of ants. These results are correlated with the relative abundances of the prey species and conclusions
can be drawn concerning food preferences. Other feeding behaviours will be observed, such as attending
Homoptera species for their honeydew. Outcomes will be taken into account in the experiments described in
'Introduction of nests/ Transfer of nests/ Maintenance of nests'. ­Method; prey preference
When known what kind of prey O. longinoda feeds on, preference tests can be carried out with the most
important prey insects (e.g. most important pest/capsid species). Orientation and communication of the ants
• Do they use chemical cues (communication) or for example sunlight as well?
­­> Has already been described by several studies, for O. smaragdina. ­APPLIED ASPECTS­
Activity of the ants
Dejean (1990) showed that O. longinoda ants are the most active during the period of daylight, and thus
activity seems to be coupled with light presence/intensity. As mentioned in the introduction, the highest activity
occurs between 6.00 am and 6.00 pm. However, this are data about nests located in citrus trees in Zaire, and
since we would like to know whether or not there is a difference, activity will be measured again, but now for
nests located in cocoa trees in Ghana. • At what time of the day the ants are the least active? ­Method; foraging activity
We will count how many ants (and of which castes) go in and out the nest (which can only be reached by the
stem of the tree in which the nest is located), to determine overall activity. However, high activity or numerous
ant trails might make carrying out the method described above quite difficult. If this method will not give
qualifying results or observations are thus hard to make, the method described by Greenslade (1972) can be
used. This involves drawing a line close to the nest (e.g. on a foraging trail) and counting the number of ants
crossing this line. Countings will be made every 20 minutes for x minutes, during 24 hours, for 2 days. Foraging
activity and caste ratio will be calculated. ­Foraging activity is correlated with caste ratio, different colonies, daytime, and way of disturbance (naturally
or experimentally)?
Introduction of nests/ Transfer of nests/ Maintenance of nests In order to introduce Oecophylla nests in cocoa plantations many aspects should be taken into
consideration before actually being practiced. Two main topics must be considered: ant competition and
environmental conditions (which might become favourable through manipulations). This has been more
extensively described in the introduction, but a short summary is given below. As mentioned before, other ant species are often the main natural enemies of Oecophylla. Therefore it is of
great importance to reduce ant competition as much as possible, since otherwise introduced O. longinoda
populations might possibly totally disappear from a system (Way & Khoo, 1992 and Van Mele & Cuc, 2003).
This aspect will be looked at in the paragraph ‘interaction with environment’. If we know which ant species
might harm O. longinoda populations, we can for example remove nesting sites of these ants. Competition
between different colonies of O. longinoda should be minimised as well, for example by removing possible
connections between trees which have been colonised by different colonies (Van Mele & Cuc, 2003). The second aspect to consider is providing the most favourable conditions for Oecophylla populations for
establishment and maintenance, which can be obtained by several different manipulations (Way & Khoo, 1992).
Quality and quantity of food sources, possibilities to spread through a plantation, reduction of the use of
chemicals, habitat quality, number of good nesting sites and so on (Way & Khoo, 1992, Way & Khoo, 1991,
Van Mele & Cuc, 2000) are factors which should be looked at. Often additional food sources have been offered to Oecophylla populations in order to increase survival,
reproduction and longevity. In some plantations Oecophylla populations are being encouraged by supplying
them with honey or sugar (Vanderplank, 1960), others are being supplied with food such as fish or chicken
intestines, as done in citrus orchards in the Mekong Delta (Van Mele & Cuc, 2000). Supplied honey or sugar
does not only directly encourage ants, but also indirectly by attracting insects, such as bees and wasps, which
can serve as prey (Vanderplank, 1960). According to Van Mele & Cuc (2003) only 2 to 3 times a year
additional food should be provided to prevent ants living from your food sources only. Place the food sources at
different parts of the plantations to keep the populations more evenly distributed. Tree connections can be made
by placing bamboo sticks or nylon ropes between different trees. Of course the reduction of human disturbance is very important in maintaining stable O. longinoda populations.
The reduction of use of chemicals, such as insecticides and pesticides, is very important, since these do not only
negatively influence the target organisms, but also O. longinoda ants and other beneficial organisms (Van Mele
& Cuc, 2003). At least one of the nests within the colony being transferred should contain a queen in order to be able to
successfully transfer ant colonies. As discussed before, colonies often contain only one, or just a few queens,
and transplanted queen­less populations are much less stable than those which do have a queen. Those without a
queen even often eventually die out (Vanderplank, 1960 and Peng et al., 1998). Besides that, according to
Vanderplank (1960), it is impossible to introduce a ‘queen’ Oecophylla to a ‘queen­less’ colony, since all his
different methods of trying to failed. The article by Peng et al. (1998) describes a method for locating O.
smaragdina queens without destructive sampling. They found that within a colony the tree with the queen nest
had most ant trails connecting to other trees in the colony and that trees with queen ant nests also had more ant
nests than the others. Trees with queen ant nests can not be characterised on tree size, species and position
within the colony (Peng et al., 1998). The most suitable moment for transfer is when O. longinoda shows the least activity, since then most ants will
be within their nests. Dejean (1990) showed a clear diurnal pattern in activity of O. longinoda ants, with
foraging activity being highest in between approximately 6.00 am and 6.00 pm. We therefore assume that the
best moment for transfer of ant nests is between 6.00 pm and 6.00 am. However, these moments have been
estimated for Zaire, and depend on light intensity (Dejean, 1990). We thus think the best moment for transfer of
nests is in the period of darkness, for then the ants are considered to be the least active. At last, vegetational conditions should be matching the optimal criteria; deciduousness, relatively large leaves,
and high leave quality are best for nest building of Oecophylla ants (Greenslade, 1972). Also environmental
conditions should be considered which is most optimally a tropical climate (relatively high rainfall,
temperature, and diurnal and seasonal variation less pronounced) (Way, 1954). And of course certain
Homoptera species should be present for providing honeydew (Bigger, 1981). • Can the nests be transferred from one farm to another, or one plantation to another, and how?
• Which Homoptera species should definitely be present?
• Should additional food sources be supplied (does this significantly lead to stronger/larger colonies)?
• At which location should the nest be placed? (Do we for example need a ‘fake’ nest structure?)
­Method: Homoptera presence?
Compare colony survival, growth and longevity of ant populations which attend different Homoptera species.
Observe which associations between ants and Homopteras can be seen, and score which are most abundant. We
can use the scoring system used by Löhr (1992); for ants from 0­4 (0: no ants, 1: ≥1≤20 ants, 2:>20≤100 ants,
3:>100≤250 ants, 4: ≥250 ants seen foraging on the for example the tree stem) and for aphids 0­5 (0: no aphids,
1:single aphids on lower leaves only, 2: small groups of aphids on lower leaves only, 3: sizable colonies of
aphids on lower leaves and few aphids on youngest open spadices, 4: high numbers of aphids on older and
young spadices, someone unopened spadices and spear leaf, 5: high aphid numbers all over the upper part of the
crown and all leaves and fruits). ­­> for coconut palms ­Method: effects of different factors?
An experiment to compare colony survival, growth and longevity of different O. longinoda populations under
different conditions has to be designed. For example the effect of offering additional food sources, effect of
constructing tree connections, optimal time of transfer of nests, effect of occurrence of different environmental
conditions and many more could be considered. If we, for example, want to know whether or not there is a significant positive effect on ant populations when
providing additional food sources, comparing yield of cocoa fields having ‘natural’ O. longinoda populations
with each other could give some clues. We have to make sure these cocoa fields comprise approximately the
same size of O. longinoda populations (e.g. approximately same number of nests), that other ‘disturbance’
factors are kept at equal levels and that multiple replicas are being compared. These experiments should be
started at the beginning of our research period, since we have to look at long­term effects. We will offer half of
the number of fields additional food sources (distributed at several different places in the field) about x times
during the period of x months. Cocoa yield (see the paragraph on 'number of ants required for capsid control')
and population sizes (e.g. number of nests) will be estimated at the end of the research period, and compared
with each other. When offering additional food sources does positively influence ant populations, and therefore
possibly cocoa yield, a significant difference between both tests is to be expected (control fields versus fields in
which food sources have been offered). Similar experiments can be conducted for different factors.
Number of ants required for capsid control
It has often been described that in order to develop sustainable biological control measures, one of the
most important things to achieve is an equal or even higher cocoa yield as achieved with 'general' management.
Therefore a certain level of pest suppression, which should be at least equal to the suppression occurring in
absence of biological control, should be achieved. In order to find out which population level of Oecophylla
ants should be maintained, we first have to know whether damage by capsids can be reduced by ants at an equal
(or even higher) level compared to the reduction as a result of chemical use. Therefore, we would like to
estimate the mean population (/nest/colony) size needed for sufficient capsid suppression. We will conduct two
studies. First we will compare average cocoa yield of two plantations; one with Oecophylla ants (in densities
occurring under ‘natural’ conditions) and one without Oecophylla ants but managed through the ‘general’ use of
insecticides. The other study will involve the comparison of cocoa yield in fields with different Oecophylla
densities (ranging from level 0, natural conditions, up to x, introducing ants or adding several encouraging
manipulations). Among others Vanderplank (1960), has already shown that in coconut palm plantations there
was indeed a correlation between yield increase and increase in percentage of Oecophylla present. We have to take into account that there might be severe Homoptera damage in case of high Oecophylla
densities. However, this does not per se have to happen since there probably is a maximum density determined
by one of the following factors; available food sources, inter­ and intraspecific competition, available nesting
sites etcetera. The ant density at which Homoptera damage will be a problem might thus never be reached. This
confirms several studies (e.g. as reviewed in Van Mele & Cuc, 2003) which say that Homoptera outbreaks have
never been associated with Oecophylla populations. • How many nests of ants are adequate on a certain number of cocoa trees/certain surface of cocoa
plantations to keep capsids at more tolerable levels?
• What is the minimal ant density needed to keep this level? • Is the protection against pests by Oecophylla as sufficient as it is by the use of chemicals?
­Method; effect of ant predation on crop yield
The performance of the cocoa trees, with and without O. longinoda populations, will be compared by the
number of cocoa beans and the damage score (0; no damage, 1; slight damage but no reduction in bean size, 2;
damage and slight reduction of bean size, 3; damage and much reduction in bean size. (Proper definitions of
damage and size reduction, and other possible effects can only be given after observations in the field)
­Is this sufficient to eventually describe loss in yield?
­Can other measuring methods be used? (e.g. such as damage to foliage expressed by loss in number of leaves
or surface damage?) (Löhr, 1992)
Interactions between different colonies It is known that Oecophylla ants do show intraspecific competition. Ants of different colonies do not
accept each other in their territories, and when colonies are forced to mix, it results in one colony killing the
other (Vanderplank, 1960). Vanderplank (1960) has tried several methods, which often are successful in
honeybee populations, for mixing different ant colonies, but all failed. We therefore suggest to keep contact
between different Oecophylla colonies as low as possible. Different ant colony territories should definitely not
show overlap, and it even might be better to minimise the number of colonies used in one plantation. However,
there might be a danger to be dependent on just one or a few colonies. • How can we avoid inter­boundary fight amongst nests that tend to reduce their numbers? In other
words, how can we increase their numbers without upsetting the ecosystem? ­Method; reduction interaction between different colonies
Besides minimising contact between different O. longinoda colonies, it might be possible to reduce
aggressiveness in other ways. When forced to introduce more than one colony, physical barriers could be
created to prevent contact between species. Maybe we can do something with pheromones of different ant colonies?
à Or leave this part out; we do not have a very specific idea on how to solve this problem.
Use of chemicals (Insecticides/pesticides/fungicides)
Chemicals do not often only influence pest insects but also the beneficial organisms and humans.
Several studies have already shown the harmful effects of chemicals on Oecophylla ants. Vanderplank (1960)
for example shows considerable increased movement of Oecophylla colonies or even total disappearance of
colonies living in fields which were threatened severely with chemicals. In the course of biological control it
might be very good to show that chemicals do have a very negative effect on O. longinoda ants, and a nice
outcome would be as well if biodegradable or bio­insecticides appear to have less negative influence on O.
longinoda ants. • Are they susceptible in the presence of other cocoa insecticides­ both synthetic and biodegradable
or bio­insecticides such as Aqueous Neem Seed Extracts? • How long does it take for an ant population to fully recover after spraying events?
Method: Effect of the use of chemicals
First we will select chemicals which are widely being used in cocoa plantations and try to estimate the general
doses in which they are being applied. Like Vanderplank (1960) did, we will define an x number of plots onto
which we will apply several different chemicals in different doses. We try to start with approximately equal
number of nests (/colonies) of O. longinoda in each plot, and apply chemicals at certain time intervals
(depending on general application recommendations). Re­counting the number of nests (/colonies) after a
period of x months will tell us what kind of influence the chemicals have and which of those are the worst for
maintenance of O. longinoda colonies is. After calculating the population increase/decrease we could also see
how long it would take for a population to recover till ‘normal’ size, if spraying has stopped. Cultural aspects
As described in the introduction there are also few very important cultural aspects which should be
involved in the research, especially when it comes to true application of our (and other) findings concerning the
use of Oecophylla ants in the control of cocoa capsids. First of all farmers should be made aware of the
alternative possibility of farming, that is in a biological way. They should know not all insects are pests, and
many even may contribute to a higher yield and a better natural environment. The IPM program of course is a
very good way of participial learning in which knowledge concerning biological control measures can reach
farmers effectively. Farmers should be made familiar with the advantages of the use of Oecophylla like a
healthier environment, the saving of money and a possible higher yield. Therefore we would like to spend the very last part of our thesis on transforming our scientific findings into a
language understandable for farmers. The possible findings which could be of interest for biological control of
capsids in cocoa plantations should thus eventually become part of the 'Convergence of Science project' in
which the farmers actively contribute to a better knowledge on biological control. ­Method; cultural aspects We will not thoroughly investigate the cultural aspects involved in the possible use of O. longinoda in control
of cocoa capsids, but we will report the things we will intercept concerning this subject as much as possible and
try to give some possible research implications on this. If time lets us to, we might even carry out a small
interview research in which we will ask questions about experiences with ants, perception of the ants, about
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LITERATURE WHICH HAS NOT BEEN FOUND, OR WHICH HAS NOT BEEN USED YET
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