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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.