Download B. A. Keddie Biological Control March, 2007 HISTORICAL

B. A. Keddie
Biological Control
March, 2007
HISTORICAL PERSPECTIVE
- term biological control has been used in a variety of ways.
- many people have tried to expand the definition of the term possibly in an
attempt to add legitimacy (environmental sensitivity) to their own particular
research endeavours.
- some people include in biological control:
1). the development and selection of plants which are more resistant to insects
secondary plant substances - plant resistance.
e.g.
2). cultural techniques - crop rotation or habitat modification.
3). genetic techniques - sterile male release; sterile insect release.
4). "third and fourth generation pesticides" analogues of insect hormones, or
compounds such as pheromones.
5). microbials which are applied in a manner so that the mode of action is different
than that
of a natural system, e.g. Bt the bacteria is applied like a chemical; the
dose/formulation used
eliminates any need for the pathogen to replicate in the
host or become established in the
environment.
6). products of biotechnology are sometimes classified as biological control;
new term(s) has been coined for these products e.g. biorational pesticides.
- may all be useful control techniques, alternatives to the use of chemical
insecticides and all that implies while demonstrating a greater awareness of the
biology of the system, but they don't fit the original definition of biological control.
- exclude from biological control, except for comparative purposes.
- described as part of biological insect pest suppression programs.
- include some as part of natural control.
- coined by Harry S. Smith in 1919 to apply to:
the control or regulation of pest populations by natural enemies,
i.e predators, parasites and pathogens, often referred to as
"classical biological control".
Handout 1 (upper) = components of natural control of population numbers.
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see HANDOUT 1
definitions:
density-independent factor: any factor causes mortality independent of the population density.
e. g. heavy frost killing plants.
density-dependent factor: a population-regulating factor that changes in intensity with changes in
population density. e. g. Intraspecific competition - how does this operate? Would this type of
competition have any use in pest management?
nonreciprocal: resource doesn't change but interactions of population using the
resource change as their populations fluctuate.
e. g. space - aphids sensing crowding may produce migratory forms before aphid populations
impact the plant - "controls" the numbers of users without changing the space available.
reciprocal: vary in numbers (magnitude) in response to changes in host numbers.
e. g. as the host population increases, predators, parasitoids and pathogens may
increase and decrease as host poulation decreases.
- original definition restricts the term biological control to predators, parasites and pathogens,
among all natural control factors as they relate to the control of insect pests.
- some argue that biological control must also be simply the reestablishment of interactions that
once existed and once reestablished humans need to do very little.
- define more on a case-by-case basis.
- expanded to include biological control of plants using microbes or insects.
HISTORICAL
- the earliest use of one insect species to control another insect species in agroecosystems
probably predates the written record.
- as one might expect, the earliest records (records of 1700 years) come from the Chinese.
- e.g. book published in China around 900 A.D. describes the use of predatory ants to control
caterpillars and large boring beetles by citrus growers.
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ants, Oecophylla smaragdina F weaver ant
- build large paper nests in trees.
- contain large numbers of individuals.
- can be transported from wild to domestic trees.
- can be transported from tree to tree.
- alternatively poles set up between trees to facilitate movement among trees.
- become a source of revenue-farmers bought and sold colonies.
- a current practice as an alternative to chemical control (1987).
- around 1775, a European traveler noted that the Yemenese transported predatory ants from the
mountains to oases to feed on insects feeding on date palms.
- the major pest of the date palms were also ants.
- indicated their ability to differentiate between two similar species at least on the basis of their food
sources.
- considered to be the first documented case of movement over a substantial distance of natural
enemies for the purposes of biological control.
- two early examples of biological control demonstrate the use of one species of predatory insect to
control another insect species, however the definition does not exclude other types of predators.
e. g. ,the red locust, Nomadacris septemfasciata Serville was a serious pest of sugarcane in
Mauritius in the 18th century.
- mynah bird was imported from India in 1762 and by 1770; it was credited with the successful
control of this locust.
- first documented case of international movement of a biological control agent.
- birds have been used to help suppress forest insect pests in the Soviet Union; nesting boxes are
provided to increase bird populations.
importance of birds often overlooked until birds missing.
- in North America, Amish farmers in Iowa often provide nesting boxes for birds and encourage
swallows to nest on their farm buildings. One farmer estimated that on his farm he could count
some hundreds of nesting pairs and a minimum of 2000 fledglings within 300 yards of the centrally
located farm building. He stated that insects were not a problem on any of his crops and thought
that the birds were at least partly responsible for this situation.
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- in Europe the early observers of natural history began to generate the information which later
would be formalized as " biological control".
- why were predators used first?
- readily visible for the reasonably careful observer of animal behaviour.
see HANDOUT--Réaumur (1736)
- understanding of the interactions in the natural history of insects was sufficiently sophisticated
that by 1734 Réaumer suggested that the eggs of aphidiverous flies (lacewings) be introduced to
greenhouses to keep them free of aphids.
- another student of natural history, De Geer in the 1760's is reported to have said, "we shall
never be able to guard ourselves against insects but by means of other insects".
- observation of the phenomena of parasitism and the realization of its potential use in pest
control and the use of pathogens for pest insect control are more recent developments.
PARASITISM
- 1602, first person to describe insect parasitism was Ulysses Aldrovandi published a description
of the larvae of the braconid, Apanteles glomeratus, emerging from the cabbage butterfly, Pieris
rapae (L.) where they spin their external cocoons. Unfortunately he misinterpreted these
structures as the butterfly eggs.
- 1668, Francesco Redi described aphids parasitized by an ichneumonid, again not understanding
the actual process however.
(Note: Redi disproved spontaneous generation).
- 1701-1710 several people, including van Leeuwenhoek (microscopy), described the parasitic
nature of the interactions between insect species including Aphidius sp. emerging from aphid.
- once nature of the parasite-host relationship was understood, descriptions of the biology of
numerous parasites were published -1750's.
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- One of the first to comment on the use of a parasitic insect to control a pest (1800) was Dr
Erasmus Darwin-(grandfather-Charles).
- noted the destruction of cabbage butterfly larvae infestations after the deposition of eggs by an
ichneumonid on the backs of caterpillars.
- also recommended the use of coccinellid beetles to control aphids in greenhouses.
- gathering and storing of parasitized caterpillars for later release of adult parasites was
proposed by Hartig in Germany in 1826.
- in 1855 Asa Fitch suggested importation of European parasitoids to use against wheat midge
in the U.S.
- first importation into N.A. occurred in 1875, 1881, release, by 1883/84 established (Cotesia
glomerata) – used against Pieris rapae (cabbage butterfly).
PATHOGENS
Diseases in insects have been noted for a long time, certainly much before the nature of
infectious disease was understood.
- early descriptions largely focused on insects that were of economic importance; e.g. diseases of
honeybees and other insects were described by:
- Aristotle, in ancient Greece then the Roman writer, Virgil, described honeybee diseases
some 300 years later.
- descriptions of diseases occurring in the silkworm were published in Japan perhaps a 1000
years ago, while the silkworm and related species were reared in China for at least 3000 years-possibly literature exists from these earlier periods.
- silkworms themselves can be considered to play a central role on insect pathology as a
discipline, and one may even consider them as central to the development of the whole concept of
infectious disease. With time the rearing of silkworms for the production of silk spread from Asia to
Europe and North America. At various times devastating outbreaks of diseases destroyed large
numbers of insects and uninfected stocks from one part of the world were sent to another part for
considerable profit. In addition these diseases created a demand for causes and cures. One of
the first individuals to take up the challenge was a man named Agostino Maria Bassi who
examined silkworms with a disease known as calcino (in Italy) or muscardine (in France). He was
able to demonstrate that the disease was caused by a "vegetable parasite" or fungus; and that the
growth of the fungus leads to death of the silkworm and the production of an infectious agent which
can be transmitted by inoculation, contact or contaminated food. He was even able to
demonstrate that the infectious agent could be destroyed by certain chemicals. In fact, Bassi
demonstrated the GERM THEORY OF DISEASE.
- the date of these discoveries was 1833, although he didn't present his findings until 1834.
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Louis Pasteur also was recruited to study the diseases of the silkworm, although not until
around 1865. Pasteur was able to isolate several different microorganisms from silkworms each of
which caused diseases. These included bacteria and protozoa and he also described a disease
that we now know to be caused by a small RNA virus, which of course he could not identify.
Certainly one would suspect that the research Pasteur conducted on insect diseases aided in his
efforts to understand vertebrate diseases.
He also suggested using the protozoan against the grape phylloxera and recommended
searching for a fungus that could infect these insects and which, once obtained, should be sprayed
in infested vineyards.
The first scientific field test of microbial control was conducted by a Russian, Krassilstschik,
who used the fungus Metarhizium anisopliae (Metchnikoff) against the sugar beet curculio,
Cleonus punctiventris Germar.
- 50-80% mortality in test plots.
- by 1891 a commercially produced product was available in France, another fungus, Beauveria
sp.
- in the 1930's a bacterium, Bacillus thuringiensis, was tested and it became commercially
available shortly thereafter.
Each category will be described in more detail:
(1). def'n and description.
(2). some examples of these organisms.
(3). what do you need to know to use successfully.
(4). some examples of introductions, case histories.
Predators:
Def'n: an animal which feeds upon other animals(prey) that are usually smaller and weaker than
itself, frequently devouring them completely and rapidly.
Characteristics of predatory insects
1).
2).
3).
4).
require several to many prey individuals to complete development.
adults generally deposit eggs near prey populations.
immatures active, mobile search for and devour prey.
adults frequently predatory as well.
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5). as a rule polyphagous - advantages/disadvantages.
6). prey is killed very quickly [contrast to pathogens which usually take days to
kill their hosts while parasitoids (parasites) often take days to weeks].
7). many predators are long-lived - consider the polyphagous feeding habits then
there may be a need to select for synchronized development of predator and
prey.
Predatory insects come from a wide variety of taxonomic groups.
- e.g. thrips, true bugs, beetles, Neuroptera, flies, Hymenoptera, other arthropods such as spiders
qand mites (note many of these groups also have phytophagous pest species.
- important role of predators in the regulation/natural control of pest populations is often
overlooked; perhaps their role is best demonstrated by excluding them from the system, even if
unintentional.
Recall: secondary pest outbreaks that frequently occur with chemical
insecticide applications. Worldwide spider mites have frequently been observed as
secondary pests--studies have shown that the elimination of predators (often
predatory mites) by broad spectrum insecticides has generated these outbreaks.
Several research projects examining the means to prevent these secondary outbreaks which
fall into the "biological pest suppression" category have been implemented in this system:
1). selecting for insecticide tolerant predators.
2). identifying and cloning of genes from prey which are involved in insecticide
resistance and insertion into a predator's genome in order to make it at
least equally resistant to the insecticide applied against the prey.
Use of predators in biocontrol
Useful observations
- predators often function well at low population densities of the prey.
- probably unable to respond to large increases in host density quickly enough to suppress pest
population explosions.
- the number of predatory species, even in simple agroecosystems can be astounding; for
example: (includes birds, rodents, spiders, mites, insects).
300-600 spp.
cotton
Arkansas
observations of predators feeding on cotton bollworm
- predatory insects > 28 species on plant.
- predatory spiders >19 species on plant.
- many of these and other species were observed to be feeding on `
caterpillars on the ground.
Note – these observations were on a single pest species and recorded over a two
year period.
>1000 spp.
>1000 spp.
alfalfa
soybeans
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California
Florida
- may present a problem -- how does one determine which predator or group of
predatory species are important for controlling a potential pest.
- likely no single answer because the "most important" predator of a pest can
vary from:
i. season to season
ii. within a season
iii. field to field
Example of Successful Introduction
Cottony cushion scale: a pest of citrus crops, controlled by the Vedalia beetle in California.
- cottony cushion scale(Icerya purchasi) were discovered in 1868 in northern California on
Acacia.
- by 1886, damage to the citrus industry in Ca. was extensive -- many growers had pulled out or
burned their trees.
- yet by 1892, the pest had been reduced to insignificant numbers.
How was this accomplished?
- in the mid 1880's, inquiries to the USDA Division of Entomology stimulated
C.V. Riley to send an assistant to look over the situation; Riley also recruited a
competent amateur entomologist to work on the problem.
- Riley also sent letters to different parts of the world in search of information about this insect and
potential natural control agents.
- an Australian scientist sent him information about a potential parasite and as a result, Riley
recommended that this parasite be investigated and an expedition mounted to collect some of
these insects.
- although the appropriate government agency refused to spend money for this endeavour, by
adroit maneuvering Riley collected enough funds to send an entomologist to Australia -- this insect
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was present in both New Zealand where it was a significant pest and Australia, where it was a not
a pest -- Riley reasoned that Australia was the native range and a likely source of natural controls.
- finding the scale insects and therefore the natural controls was difficult, however a few parasites
were observed as well as a predator, the coccinellid, Rodalia cardinalis. (Vedalia beetle).
- these agents were collected as well as some others and shipped to the U.S.
- thousands of potential control agents representing several species were released but only around
480 beetles.
- within two years these beetles had been distributed to many parts of the state and the citrus
industry rescued.at a total cost of around $5000.00.
Why did it work?
- these beetles are predators yet they responded quite rapidly in terms of their reproduction -- this
is a somewhat unusual case.
- firstly, unlike many predators, these insects are monophagous - feed exclusively on these scale
insects.
- secondly, they clearly have sufficient fecundity to produce rapid population growth.
- thirdly, they had few competitors -- i.e. no other species which used the cottony cushion scale as
a substantial part of their diet.
- results with the Vedalia beetle created a lot of optimism -- find the right biocontrol agent and
pests can be controlled-a very difficult proposition.
- discuss interactions of predators and pesticides.
-timing, bioaccumulation.
Question – around 1947 a resurgence in this pest was observed in California
– why did this occur?
- vedalia beetle is a Coccinellid; ladybird beetles of various species have been promoted as
biological control agents--many commercial suppliers - are they effective?
i. if you bought 10,000 adult beetles and put them in your backyard garden to feed on aphids,
you would find that over a period of days some number of aphids would be destroyed, but success
would be limited.
ii. this is likely the result of the following characteristics:
-these beetles tend to disperse - obligatory dispersal phase.
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-polyphagous.
however placed in a greenhouse their effectiveness may be substantial.
PARASITE (PARASITOIDS)
Definition: a species which lives on or in a another species, the host, feeding upon it, but
usually not killing it. A parasite often needs only one or part of one host to reach maturity.
- several insect orders have parasitic members.
- insects are parasitic on vertebrates and invertebrates.
see Handout
Vertebrates:
- most are ectoparasites - live on the "surface" of the host.
- host is usually not killed.
- exopterygotes: adults and nymphs - parasites.
- endopterygotes: larvae - parasites.
- the interest for biological control purposes rests largely with insects which parasitize other
insects.
Invertebrates:
- these are mainly protelean parasites - immature forms are parasitic, adults
are free-living.
- the endopterygotes are well suited for this dual lifestyle.
- many of these parasites are endoparasitic, living protected inside their host,
usually killing the host at the end of larval development.
- the set of features which I have described for these parasites has led some
people to give them a special name - parasitoid.
(some even think of their behaviour as a specialized case of predation).
Parasitoids - described as having the following characteristics:
(1).
(2).
(3).
(4).
(5).
develop as larvae in or on a single host.
eggs are laid in, on, or nearby a host.
they consume all or most of the host body.
frequently they pupate in or on the host.
free-living stage emerges from the pupa and searches for new hosts.
- parasitoids attack and develop in all insect stages, e.g egg parasitoid
- parasitoids may attack an earlier stage than the one from which emergence may
occur, e.g. attacks larva, emerges from pupa = larval-pupal parastoid.
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- host range variable; broad vs narrow.
broad host range - e. g. Compsilura concinnata (Miegen) Tachinidae which
has 200 lepidopteran larval hosts.
- introduced from Europe to NE U.S. to control gypsy moth.
- Western records include California and B.C.
Question:
Was it a good choice for a biocontrol agent?
Selecting and Searching For Potential Biocontrol Agents
- difficult process.
- search another area where insect is a major pest or in areas where your pest is found, but not at
significant numbers (which area?).
- finding the host insects may be difficult - then you have to find the parasitised insects.
- alternative strategy - rear pest species and place on vegetation in the search area
and
monitor for parasitoid activity.
- if successful - you collect some parasitised larvae and rear out the parasitoid.
- compare with selecting predators.
Question:
Do you think you could use these (found parasitised pests and reared out
parasitoids) in a biocontrol program simply based on the fact that some insects
were observed to emerge from the body of the host.
- complicated by the fact that there are different kinds of parasitoids-host interactions.
See Handout
(1). primary parasitoid- an insect parasite of any arthropod which itself is not
a parasite.
(2). there are parasitoids which parasitize other parasitoids; called secondary
parasitoids (or hyperparasitoids). also tertiary ...
- if simply collected parasitised aphids without determining which parasitoid species and its host,
could lead to a wasted effort.
- releasing a hyperparasitoid rather than helping to suppress a pest species could result in an
increase in the pest over time by reducing the effectiveness of the primary parasitoids already
present.
- other complicating factors related to other aspects of parasitoid behaviour.
(3). superparasitism - many individuals of the same species utilise a single host, so many that the
host cannot sustain them nutritionally and the parasitoids die; or if they survive the adults are
smaller than normal and less fecund - sometimes only one will survive destroying its competitors
as it feeds.
- inefficient to release more than required parasitoids - requires knowledge of host population sampling.
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(4). a. multiple parasitism - more than one parasitoid species simultaneously
parasitizes a single host; usually only one species survives.
b. a specialized form of this multiple parasitism occurs when one species preferentially
attacks hosts that already have been parasitised by another species. These specialists are known
as cleptoparasites - in the ensuing competition usually the cleptoparasite wins.
- this complex of life history strategies should convince you of the efforts required to acquire
biological control agents.
Table of Hymenoptera
see Handout
- most of the successful biocontrol progams have been accomplished with parasitoids, largely
hymenopterans.
- probably reflects large number of parasitoids in this large order - more current estimate - 500,000
parasitic species.
Order Hymenoptera
Suborder Symphyta - nearly all phytophagous.
Suborder Apocrita - large number of predatory and parasitic species.
Superfamily Ichneumonoidea (2 families).
- mostly beneficial, few hyperparasitoids.
Family Ichneumonidae (32 subfamilies).
- 20% of all parasitic insects.
- utilize larvae of Lepidoptera, Coleoptera and other Hymenoptera;
(phytophagous Hymenoptera).
- few successes in biological control.
Family Braconidae (30 subfamilies).
- host preferences; Lepidoptera and Coleoptera larvae.
- both internal and external parasites; if external frequently paralyze
the host to prevent eggs from being dislodged.
- considerable number of successful biological control cases.
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Superfamily Chalcidoidea (20 families)
- utilize nearly all orders of insects.
- preference for Coleoptera, Diptera, Homoptera and Lepidoptera.
- develop predominantly in eggs or larvae of hosts.
- the greatest number of successful cases of biological control;
possibly a reflection of their specialisation on scale insects, aphids
mealybugs, and whiteflies = major pest groups = substantial work.
Family Mymaridae --tiny insects - egg parasites.
Family Trichogrammatidae - small - egg parasites.
Of these groups the chalcids have been used with greatest success; currently there is an
effort to increase the use of Trichogramma egg parasites against spruce budworm - large rearing
facility in Guelph.
Role of Systematics:
1. critical need fo exact identification of both pest and potential biological control
agents.
2. some families (even genera) of Hymenoptera specialize on particular hosts
e. g. lepidoptera larvae or more restricted group of hosts - fruit fly larvae.
3. may avoid groups which contain a significant percentage of hyperparasitoids.
Success/Failure of Introduced Natural Enemies
1. Environmental Factors
See Handout
- climatic factors - need to be examined in total.
- voltinism - if biological control agent has a second generation and host not
etc.
2. Matching Natural Enemy and Host
- large number of natural enemies collected worldwide introduced against
Saissetia oleae (scale insect).
- 38 spp. introduced, 15 established, 1 effective.
- established that the genus was indigenous to South Africa - the only
effective control agent also came from South Africa.
- importance of additional information.
Metaphycus helvovus (Encyrtidae)
available,
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3. Importance of Biotypes
See Handout
- accidental introductions usually represent a limited number of insects, therefore from a genetic
perspective limited variability.
- can be a benefit or a problem.
e. g. Hypera postica (alfalfa weevil)
- widely distributed in native areas.
- distributed from Sweden to Egypt.
- considerable genetic heterogeneity - adapted to local conditions.
- parasitoids may be widely distributed but represent local adaptations.
Bathyplectes curculionis (Ichneumonidae)
Problem
- pest introduced, identified, and native range located, where do
you collect the parasitoid?
- what else do you need to know/do ??
i.
ii.
iii.
iv.
original source = biotype.
collect parasitoids from the source region(s).
monitor results.
not very successful - possibly "biotypes" of parasitoid.
PATHOGEN:
Definition:
- a specific cause of a disease.
- a microorganism capable of producing disease under normal conditions of host resistance and
rarely living in close association with the host without producing
disease.
- any microorganism, virus, substance or factor causing disease.
- in terms of biological control, we are interested in those agents which we can manipulate and
cause diseases in pest organisms.
- these agents include:
a. viruses b. bacteria c. fungi d. protozoa e. nematodes* (not microorganisms).
Characteristics
1) may or may not require insect host for survival.
2) may require ingestion (contaminated substrate), if not may be airborne and
encounter host by chance.
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3) requires several days to a week or more to kill host.
4) younger insects usually more susceptible.
VIRUSES
- obligate intracellular parasites.
- virtually every kind of virus that has been isolated in all other organisms (with
the exceptions of the putative slow viruses) have been isolated from insects.
HANDOUT A --Table 1
- as you might guess insect viruses which are similar to viruses which cause
disease in
humans or things precious to humans are not really considered for mass production and spraying.
See Handout
- however, one group of viruses which appear to be restricted to Arthropods:
Baculoviridae.
these are the
- Baculoviridae include two groups:
i. nuclear polyhedrosis viruses (nucleopolyhedrovirus) (NPVs)
ii. granulosis viruses (granulovirus) (GVs)
- the vast majority of the NPVs and GVs have been isolated from caterpillars;
moths); in fact GVs are restricted to the Lepidoptera.
(butterflies and
- factors which separate these viruses from all other viruses are:
i. the presence of an occluding matrix which surrounds the virions which contain
stranded circular DNA.
ii. the replicative strategy in which two different virion phenotypes are generated.
Replicative Strategy
double-
see Handout
- NPVs infect after oral ingestion; can eventually infect many tissues.
- diseased insects often climb to highest part of plant before dying -- aids in
dispersal
of pathogen.
- viruses are inactivated by UV portion of sunlight; remain viable in the soil for many
years.
Methods of Transmission (natural conditions)
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1) dying/dead insects - dispersed by wind and rain; other mechanical
contamination.
2) diseased insects - eaten by birds; pass through bird.
3) parasitoids are capable of transmission from diseased to healthy insect.
4) vertical transmission.
Comments: Considerable length of time for virus to kill its host.
- Why?
- Is this acceptable to the grower?
- narrow host range is another aspect of these viruses which has inspired the
engineers.
- latest success is to insert mite paralysis toxins.
- a baculovirus, isolated a field population of insects has been patented
by the U.S. Patent Office.
genetic
BACTERIA
- bacteria common component of the insect environment.
- most diseased-looking insects are full of bacteria but often these are opportunistic infections-i.e.
the bacteria are not the primary cause of the disease.
- isolating these bacteria will not lead to an effective control agent.
- insects are protected from bacteria by:
i. cuticle/integument.
ii. peritrophic membrane.
iii. immune system.
- loss of integrity in (i). and/or (ii) leaves only the immune system to protect
insect -- effective
against a whole range of bacteria -- stress reduces the
effectiveness of the immune system.
- lab rearing/production facilities – stress.
- some bacteria have the capacity to overcome insect defenses.
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- both Bacillus popilliae and B. thuringiensis have been studied and used
against insect pests (Lepidoptera, Coleoptera and Diptera).
extensively
- Bt is being researched by dozens of companies; currently over40,000 "strains" have been
isolated (or created).
- Bt is a spore-forming bacterium that also carries a plasmid encoded toxin; the toxin forms crystals
within sporangium.
- the toxin is largely responsible for the effectiveness of this organism as a control agent but may
be responsible for its downfall as well.
- Bt induced pathology is dependent in part on the species infected.
Lepidoptera
- immediate gut paralysis, general paralysis ∂-endotoxin alone,
death in hours (even without bacteria).
TYPE II - immediate gut paralysis, starvation, no general paralysis
death in days with bacteremia. (most leps).
TYPE III - requires crystals and spores.
death in days.
TYPE IV not susceptible to toxin.
TYPE I
-for species affected by toxin alone, the application of Bt is similar to a
convential chemical insecticide.
-don't need to spray viable spores, just toxin.
QUESTION: Is this biological control?
Bt toxins have been used in a variety of ways.
1) toxin has been placed in different bacteria - bacteria which are found in a
different habitat. e.g. rhizosphere.
2) place toxin directly into plants --defoliators exposed upon ingestion of plant
material--cotton, tomato and tobacco.
3) toxin gene has been put into baculovirus genome.
QUESTION: Do you think there are any risks or potential problems with this type of Bt use?
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FUNGI
- many kinds of fungi cause disease in insects.
- some have wide host ranges (Metarhizium anisopliae 200 hosts known; another exampleBeauvaria spp.) while others (Entomophora) can be quite specific, e. g. Entomophora
maimaiga - gypsy moth -a single species infected.
- most transmission occurs by spores and entry occurs by entry through the
integument.
see Handout
- disease cycle.
- proteinases and chitinases digest cuticle, however surface conditions critical for
spore
germination.
- once fungus enters haemocoele it can overcome immune response and proliferate.
- fungus secretes toxins that can kill the insect - fungus then uses dead insect as a source of
nutrients and fill the body with hyphae.
- if conditions are suitable the fungus can grow out of the dead "mummified " insects, produce
fruiting structures that release spores into the surrounding environment to infect the next host.
- if conditions unfavourable the fungus becomes dormant and remains inside the dead mummified
host; when conditions improve (temperature and moisture suitable) the fungus grows out of the
host and releases the spores to infect other hosts.
COMMENTS ON PATHOGENS
- increasing interest in the use of pathogens for managing insect pests in forestry
and
agriculture as well as in urban settings.
- there is a great need to understand how pathogens work with respect to:
(1) host range.
(2) transmission in nature.
(3) maintain viability for longer periods.
(4) improved production.
- probably will work best when integrated with other management toolsIntegrated Pest
Management
- unfortunately a great rush to commercialize - before the systems are completely
understood.
- repeat same mistakes:
(1) resistance.
(2) competition - commercialization - reduces free exchange of information.
(3) patents - issued for virus, why not for parasitoid?
- microbial control refers to the use of natural enemies to reduce the populations
of an insect pests--however microbes have or will be used in a variety of systems, including:
(1) plant pathogens have been used to control weeds.
(2) benign microorganisms used as competitors of plant pathogens.
(3) pathogens used to reduce animal (insect) populations.
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