Download The Theoretical Role of Predators in Sterile

The Theoretical Role of Predators
in Sterile-Insect Release Programs
By E.
Unh'crsity
SHIPP AND
A. \V.
OSBORN!
of New South Wales, Kensington,
Knipling (1955, 1959) has suggested eradicating animal
populations by releasing large numbers of sterile males
to mate with wild females thereby reducing the birthrate.
If the process were repeated at intervals it should
theoretically lead to the eradication of a pest population.
~[ore recently Monro (1963) has proposed the release
of sterile animals to overload essential resources, thus
"flushing out" a proportion of the fertile animals. With
successive introductions
of sterile animals the fertile
population could be reduced further, irrespective of the
influence of sterility on birthrate.
The release of a pest species to tide a parasite over unfavorable periods was suggested by Smith and DeBach
(1953).
Subsequently Huffaker
and Kennett
(1956)
proposed releasing both the cyclamen mite and its predator so that an early equilibrium could be reached with a
consequent reduction of the damage done by the former.
Birch (1963) has recently reviewed the use of ecological
methods in pest control.
The present paper considers the theoretical interrelations between predators and prey in programs aimed at
eradication of the prey species by the release of sterile
insects. In this context parasites may be considered to act
in the same manner as predators.
If predator numbers are correlated with those of their
prey, including sterile individuals, let us consider the
situation when sterile animals are released into a population that has otherwise only minor fluctuations in density.
The short term result should be an increased survival of
fertile prey, as the sterile animals will provide some of
the food requirements of the predators. This will occur
during the time before the predator population has increased to a new level. However, the progeny of the pest
species will be reduced by matings involving sterile animals, and the total population will subsequently decline.
The mortality of the predators will lag behind that of the
prey resulting in an increased predation pressure, tending
to hasten the decline. Predation will thus reinforce the influence of the sterile matings, which become more effective
when the number of fertile animals is reduced.
In populations that show seasonal fluctuations releases
of sterile animals will generally begin when the numbers
are declining. Predation will already be at a high level,
due to the time lag between predator and prey mortality,
even without the influence of the released animals.
The proportion of the prey destroyed by the predator
will diminish as the total numbers decline due to decreased likelihood of encounter. When the fertile population is at a low level the addition of sterile animals will
cause an increase in the proportion of the total population
killed by the predator, and if predation is at random the
proportion of fertile animals killed will likewise increase.
Furthermore
if the release of sterile animals ceases
1 We
thank A. K. O'Gower,
E. Robinson and R. McLau{:hlin
of the University
of New South Wales for reading and critiCizing
drafts of the manllscript.
\Ve gratefully
acknowledge
the support
for sugar·cane
leafhopper
studies
given by The Colonial
Sugar
Refining Company Ltd .• Sydney.
N,S,W'
J
Australia
abruptly, the relatively high predator density should
again produce a temporary increase in pressure, thus assisting the complete eradication of the pest population.
In sterile-male eradication programs to date natural
parasites and predators have not been sufficiently effective to draw attention to their influence on the pest populations, although predators have hampered some release
operations (Steiner et al., 1965). However in a current
investigation (Shipp et al., 1966) of the possible use of
radiation-sterilized Perkinsiella vitiensis (Kirk) to eradicate this sugar-cane leafhopper from Fiji, it has become
necessary to consider the influence of an egg predator,
Cyrtorhinus mllndulus (Bred).
This mirid feeds on the
eggs of several species of leafhopper, however in the
sugar growing areas of Viti Levu alternate prey species
do not occur in significant numbers. C. mundulus has been
artificially introduced into Hawaii (Muir, 1920) to control P. saccaracida, another leafhopper on sugar cane,
where it has proved to be well adapted to reduce significantly populations of its prey (Swezey, 1936).
The influence of an egg predator, in an eradication program involving release of sterile insects, differs from that
of any other predator in the following respects :1. Egg predators do not destroy released insects, nor
do they reduce the longevity of wild individuals. It appears likely that this may have a bearing on the frequency
of releases required to achieve optimum reduction of the
pest population.
2. The numbers of an egg predator may be regulated
within limits by the release of sterile females. For this
to occur both the sterilized females and the normal females mated with sterile males must lay eggs, and the
predator must be able to obtain nutrient from these eggs.
The proportion of fertile eggs eaten should be greater
when predator numbers are higher, due to increased likelihood of encounter.
Predators specific to other immature stages of the prey
will not destroy insects released as adults, but their
numbers cannot be regulated by the release of sterile
females.
In certain circumstances the release of sterile females
with the males may be considered to be sufficiently
deleterious to require sexing prior to release. In these
situations the advantages gained through predator pressure may offset these disadvantages and obviate the
necessity for the development of a technique for separating
the sexes thereby reducing the amount of handling required. Undue handling may adversely affect the mating
capacity of the sterile males.
The benefits derived from predators will be related to
their effectiveness at different prey densities, the size and
duration of natural population fluctuations, as well as the
relative length of the life cycles of predator and prey
species. Advantages gained through the efficient use
of parasite or predator populations in eradication programs based on releases of sterile insects may be summarized as follows: (i) the cost of some programs may be
115
reduced by the combination of these two factors and by
elimination of sexing and handling procedures; (ii) there
may be an increase in the range of pests for which
sterile-male release programs are feasible. The decline
of a naturally fluctuating population may be hastened
thus increasing the duration of the low-density period;
(iii) there is an additional safeguard in the availability
of predator pressure when sterile releases cease.
The use of predators in sterile-male eradication programs is being investigated by us in a study of sugarcane leafhoppers of the genus Perkillsiella. Further studies may reveal the possible advantages to be gained from
predators in other sterile-male eradication programs. It
is conceivable that situations may occur where the effects
of sterile insects proposed by Monro (1963) may also
operate in conj unction with those described in the present paper. Sterile females released in a sterile-male eradication program may thus simultaneously
maintain a
predator population and overload a resource of the prey.
The effectiveness of Knipling's sterile-male eradication
technil'J.ue might then be supplemented by both "flushing"
and increased predator effectiveness.
Birch, L. C.
of pests.
141-6.
REFERENCES CITED
1963. Population ecology and the control
Bull. World Health Organ. 29, Suppl.:
LEITERS TO THE EDITOR
Dear Sir:
Both T. A. Brindley and J. G. Watts have written
persuasive statements regarding the proposed "Professional Division" in the March BULLETIN, but neither has
clearly defined the distinctive purposes of such a division.
I suspect that part of the difficulty in understanding what
I hope would be the purpose of the division is the fact
that the word "profession" has two distinct meanings.
In the general sense, all members who have formal
training in entomology and are employed as entomologists
are "professional
entomologists."
To understand
the
other meaning "the profession as distinguished from the
science of entomology" we must recognize that entomology has become vastly more complicated than it was a
generation ago when some of us first entered the field.
The best entomologists would not claim to be qualified
in specialties other than their own. Top physiologists do
not claim to understand systematics well enough to work
in that area and top biologists would not claim to be
"competent" in toxicology. More specifically, the complications involved in control operations are such that
only a specialist can expect to adequately evaluate all of
the factors involved in such recommendations. The selection of a control measure from the wide variety of
chemical and non-chemical methods available, the timing,
the avoidance of health hazards or hazards to wildlife,
the economics, the legal status, these and many other
factors place the entomologist who recommends the control measure to be used against a specific pest in one of
the most complicated specialties in entomology.
Moreover this specialty, more than any other in entomology, deals with the general public which is completely
dependent upon the competence of the entomologist. This
public, untrained in entomology, currently has no way of
knowing whether he is dealing with a "professional entomologist" or a laborer who has picked up a few technical
phrases.
Many States require licenses for pest control operators
but few if any provide similar safeguards on agricultural
or home recommendations.
The layman in entomology
116
Huffaker, C. B., and C. E. Kennett.
1956. Predation
and cyclamen-mite populations on strawberries
in
California. Hilgardia 26(4): 191-222.
Knipling, E. F. 1955. Possibilities of insect control or
eradication through the use of sexually sterile males.
J. Econ. Entomol. 48(4) : 45!H52.
Knipling, E. F. 1959. Sterile-male method of population
control. Science 130: 902-4.
Monro, J. 1963. Population control in animals by overloading resources with sterile animals. Science 140:
496--7.
Muir, F. 1920. Report of entomological work in Australia, 1919-1920. The Hawaiian Planters' Record
23 (3): 125-30.
Shipp, E., A. W. Osborn, and P. Hutchinson.
1966.
Radiation sterilization of sugar cane leafhoppers of
the family Delphacidae. Nature (In press.)
Smith, H. S., and P. DeBach.
1953. Artificial infestation
of plants with pest insects as an aid in biological
control. Seventh Pac. Sci. Congr. Proc. 4: 255-59.
Steiner, L. F., E. J. Harris, W. C. Mitchell, M. S. Fuji.
moto, and L. D. Christenson.
1965. Melon fly
eradication by overfiooding with sterile flies. J. Econ.
Entomol. 58(3): 519-22.
Swezey, O. H. 1936. Biological control of the sugar
cane leafhopper in Hawaii. The Hawaiian Planters'
Record 40(1) : 57-101.
is now conditioned to "professional" physicians, dentists,
lawyers, etc. But only a licensed pest control operator
among entomologists is labeled "professional" to the public.
There are basically two approaches to protecting the
public from charlatans and incompetents. One is for the
government to require licensing and the other is for a
professional society to establish standards that can be
recognized by the public. One or the other or both is
inevitable in a field of growing importance such as insect
control. Therefore it is reasonable for the Entomological
Society of America to begin setting professional standards on which the public can rely. Such standards would
be applicable only to those entomologists who specialize
in insect control such as extension entomologists, commercial field men, pest control operators, and a few others.
These then are the members who should be interested
in a "professional division" if its objectives are this specific. This should be no more divisive than our present
sections, and if such a division does a good job of estab·
lishing suitable standards it will greatly improve tbe
public image of all entomologists.
Sincerely,
WILLIAM M. UPHOLT
PROPOSALS
FOR HONORARY
MEMBERSHIP
Members of the Society are invited to make nominations
of eligible individuals for honorary membership in the
Society. The criteria for honorary membership are stated
and discussed in the report of the Standing Committee
on Honorary
Membership for 1965, published iv the
BULLETIN12(1) : 74-5, March 1966.
Proposals for honorary membership should be accompanied by a brief summary of the professional career
of the individual nominated, and of his or her Society activities that are considered to be pertinent in connection
with consideration for honorary membership. Proposals
may be sent at any time to the Executive Secretary at the
Society headquarters.
To be considered this year, nominations must be received in the Society offices prior to
October 3, 1966.