Release of sterilized males

Table 10. Theoretical Effects of Release of Sterile Male Insects on a Natural Insect Population ...

Mass releases of sterile male insects have produced dramatic reductions in the populations of the Mediteranean fmit dy Ceratitis capitata in California beginning in 1981 when 40 million sterile dies were released weekly and in the codling moth Cjdiapomonella in isolated apple orchards in the Pacific Northwest. 

Genetic Control. Manipulation of the mechanisms of inheritance of the insect pest populations has occurred most successfully through the mass release of sterilized males, but a variety of other techniques have been studied, including the environmental use of chemosterilants and the mass introduction of deleterious mutations, eg, conditional lethals and chromosomal translocations (58 ndash 60) (see GENETIC ENGINEERING). 

Additional control measures include use of Bacillus thurlngiensis formulations and the release of parasites and predators. There is a continuing program to find new species that can be imported and released. A virus has also been registered for use against the insect. Work on release of sterile males is in an early stage. 

The success of this operation has stimulated investigations in many other parts of the world, particularly in the eradication of various ftnit flies. The first investigations appear to have been made in Austraha, but it was found that the infested areas of Northern Queensland and New South Wales were too extensive to offer economic control by this method. On the other hand, in South Australia, Victoria, and in the metropolitan areas of Melbourne and Adelaide, relatively infrequent outbreaks could be controlled by the release of sterilized males 40). 

There is a great deal of interest in several alternatives to traditional chemical insecticides, and much research and development work is being carried out on these by major agrochemical companies. Examples which we will briefly consider are (i) release of sterilized males, (ii) biological predators, (iii) microbial insecticides, (iv) pheromones and (v) growth-regulating hormones. 

Release of sterilized males. In this method very large numbers of, say, the male of the pest insect species are bred and are then sterilized by exposure to X- or 7-radiation. They are then introduced into the problem area in such numbers that there are far more of them than there are natural, fertile males, so that mating is much more likely to involve a sterile insect. As a result no offspring will be produced. Thus in time the population will decline and insects will no longer be a pest. 

The pesticide DDT is toxic to humans and animals repeatedly exposed to it. DDT persists in the environment for a long time. It concentrates in fatty tissues. The DDT once used to control the screwworm fly was replaced by a radiological technique. Irradiating the male flies with gamma rays alters their reproductive cells, sterilizing them. When great numbers of sterilized males are released in an infested area, they mate with females, who, of course, produce no offspring. This results in the reduction and eventual disappearance of the population. 

A major limitation to SIR programs is the requirement for the availability of large numbers of reproductively competent sterile males (2). Because there is no efficient mechanism for the specific selection of males, the current SIR programs rear, sterilize, and release both sexes. The presence of the females results in added costs to the programs for rearing and in reduced efficiency of sterile male matings after release. Implementation of improved procedures for the selection of males or removal of females early in development would offer measures necessary to reduce the costs and increase the efficiency of SIR programs. 

There have been two main lines of development in the control of insects by irradiation firstly, the disinfestation of stored products at doses sufficient to prevent reproduction of the insect, and secondly, eradication of insects in the field by the technique of sterile male release. Both lines of approach have achieved success, a plant for the disinfestation of grain being under construction in Turkey at this time, whereas in the United States, success in the elimination of the screw-worm fly has been achieved, and recently it has been announced that control of fruit flies can be effected in this way. 

Sterile Males— Males of some pest insect species may be reared and sterilized in laboratories and released in large numbers into infested areas to mate with native females. These matings produce infertile eggs or sterile offspring and help reduce the pest population. This technique has been used successfully in only a few species and is still being developed. The screw worm, which attacks cattle, is one insect on which this technique has been effective. 

Radiation doses of ss 80 J/kg have been used to sterilize the males of insect species, which are released after sterilization and mate with females. In this way, further reproduction of the species is reduced or prevented. The technique has been applied in the USA, Mexico, Egypt, Libya and other countries to eradicate screw-worm flies and other insect species that are threats to agriculture. 

Some forms of pest resistance involve. hanging the pest rather than the host plant. The best example of autocidal resistance is the sterile male technique used to control. nsects. Scientists rear thousands of males of a particular insect species under controlled conditions, then sterilize them with x-rays. These sterilized males are released in the wild to mate with females of the same species. Since the males are sterile, mated females do not produce eggs, causing populations to drop drastically over time. (The technique is restricted to insect species with females that only mate once.) This method is most successful when the sterile males compete aggressively with the natural population of fertile males. A working example of autocidal resistance occurs at the border of Mexico and California, where sterile males of the Mexican fruit fly are released to help control populations and to prevent the pests from entering California. 

Leuprolide acetate, a synthetic nonapeptide, is prescribed for the treatment of metastatic prostate cancer and endometriosis. Leuprolide acetate is presented in a number of injectable dosage forms including Lupron sterile solution for subcutaneous administration (1 and 5 mg mL 1) and Lupron depot controlled release formulation for intramuscular injection (3.75 and 7.5 mg). Zheng and Fulu [106] have evaluated the in vivo effect of leuprolide loaded micro emulsions on the genital organs of the male and female rats. In the preliminary pharmacokinetic studies, oral microemulsions of leuprolide administration resulted in 10-fold higher plasma levels of leuprolide as compared to that of saline solution (Fig. 9.4). 

Several methods have been proposed for the practical application of insect sterilants, such as contact, oral ingestion and sterile male release method. This latter method involves capture of the males, their chemosterilisation and subsequent release which means considerable excess of work compared to the usual application of pesticides. Nevertheless it seems particularly advantageous because under appropriate conditions it permits almost complete control of undesirable insect populations in several generations. 

The first work in this field was carried out on the screw-worm fly, and it was shown that the insects could be most easily sterilized by irradiating pupae. Males could be sterilized by doses as low as 2500 rad, and females, 5000 rad. In an experiment carried out in a small island off the coast of Florida, the release of 100 sterilized males per square mile per week was sufficient to outnumber the fairly abundant native males. After several weeks, the field population was greatly reduced, but eradication failed because of the proximity of the island to the mainland and hence the chance of reinfestation. On the island of Curasao, however, the experiment was completely successful, the island being saturated with sterile flies. After a period of two generations, aU egg masses collected from livestock failed to hatch, and eradication was complete in a time of approximately 20 weeks. 

Although the drawbacks are obvious—the vast numbers needed and the slowness of the processes—the method has clear advantages in that it is highly specific and will not create any adverse environmental effects. It was extremely successful in the eradication of the screw-worm from Curacao. This, however, was a rather special situation involving an island. A similar campaign in the south-western United States involving release of 4 billion sterilized male screw-worms was only partly successful. 

Ionizing radiation can be used in two ways to control insect populations (1) by killing the insect outright as in its use with stored crops and (2) by sterilization of the males followed by their release into the natural population in such competitive numbers that most matings result in sterile eggs. Although description of either of these procedures plays a minor role in meeting the objectives of this book, a discussion of the first procedure would more appropriately be found in a book on radiation effects, while the second merits at least brief mention here. 

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