Sterility in insects

Because of their ability to induce sterility in insects, hydrazine-derived compounds have long been of interest to us in our search for new reproduction inhibitors (1.-.4) In this context, we recently evalu = ated compound, ll-(2,4-dif luoropheny 1)-2-( 2-f luoro phenyl)-hydrazinecarboxamide (Figure 1), and found that it inhibited oviposition in house flies (Musca domestlea L.) when administered orally to adults. However, since infertility was accompanied by excessive adult mortality, failure to lay eggs was probably symptomatic of nonspecific toxic effects rather than effects directed at the reproduction pathway. 

Great enthusiasm existed a few years ago for the development of chemicals that caused sterility in insects. Unfortunately, most of the active chemicals, though quite effective, were powerful alkylating agents, and as such exhibited carcinogenic potential. Some very simple phosphorous amides containing the CH2—CH ... 

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. 

Most social insects are found in the order Hymenoptera. Sociality in insects is defined by the presence of one or more of the following traits (1) individuals of the same species cooperate in caring for the young (2) there is a reproductive division of labor, with usually sterile individuals working on behalf of fecund individuals and (3) there is an overlap of at least two generations in life... 

Fluoro-orotic acid was found to be an effective insect chemosterilant. When fed at a 1 per cent concentration (w/w in milk powder) for 24 h, it induced complete and permanent sterility in female adult houseflies (Musca domesticd) [205,206]. Other biological activities of 5-fluoro-orotic acid have been discussed in Part I of this review. 

Many people think the ultimate pesticide should be developed from research now being done on certain insect attractants and juvenile hormones. Isolation of naturally occurring sex attractants (pheromones) and juvenile hormones has been accomplished. The attractants could be used to congregate large numbers of insects in one place for extermination by the already existing insecticides. Alternatively, juvenile hormones have been found that prevent maturation or cause sterility in many pests. 

For Instance, many terpene derivatives mimic insect hormone actioa Juvabione (15) is the classical example of a juvenile hormone (JH) mimic that prevents egg maturation in Pyrrhocoris bugs. Aromatic terpene ethers (16), methylene dloxyphenyl terpene ethers (17), and other farnesyl derivatives also have JH activity and the latter ones (18) also cause sterility in Pyrrhocoris. For the most part JH active terpenes are among the sesquiterpenes but several monoterpenes also have insect sterilizing effects (19. 20). The acyclic monoterpene cltral reduces the fertility of rats by causing follicular degeneration (21). 

Miscellaneous Applications. PEIs and their derivatives are used as cementation auxiliaries in cmde oil exploration (459), and for breaking cmde oil emulsions (460) in cmde oil extraction. Seed coatings of water-soluble copolymers containing polyethyleneimine have been developed (461). Polyethyleneimine derivatives have positive photoresist properties (462) amidated polyethyleneimines improve the flow properties of cement (463) and with few exceptions, A/-acylaziridines act as chemical sterilizers for insects (464). 

On the other hand, the direct sterilization of insects in their natural environment (2) has found only marginal utility because the chemosterilants suitable for this application are effective only in females. From the standpoint of a population, a sterile female is equivalent to a dead female even if she remains capable of mating. In contrast, a sterile male is much more important because his mating may incapacitate several females (the so called bonus effect). Therefore, a female chemosterilant is at its best no more effective than an insecticide, whereas a male chemosterilant can be considerably more effective than an insecticide of comparable activity. 

Perhaps it. would be well, at this point, to define what we mean by the term chemosterilant, since this is a new word. A chemosterilant is a chemical capable of causing sexual sterility—that is, failure to reproduce—in insects or other organisms. 

Insect chemosterilants may act in several ways. They may cause the insects to fail to produce ova or sperm antimetabolites, when they are also chemosterilants, act in this way. Compounds that cause the death of sperm and ova after they have been produced would also be considered chemosterilants, but I do not know of any compounds of this type which are being considered for use in insect control. A third type of action, and the one in which we are most interested at the present time, is that shown by the radiomimetic compounds. These compounds apparently injure the chromatin, or genetic material, in the sperm and ova so severely that, although they remain alive and the sperm retain full motility, the zygotes, if formed, do not complete development into mature progeny. This type of action is desired because the males sterilized in this manner compete readily with normal males for the available females and transfer motile sperm to the spermathecae of the females, with the result that the mating requirements of the females are satisfied to the same extent as in a mating with a normal male. 

Chemosterilants might be used in two basic ways—as a substitute for radiation to sterilize insects that had been reared for release in large numbers or as a means of inducing sterility in a large proportion of the natural population, thus avoiding the necessity for rearing and... 

More information must be available on the toxicity of the promising compounds before the full range of possible methods of application for the control of various species can be determined. Chemosterilants can certainly be used in the control of some species to sterilize reared insects for release among natural populations. No doubt they will also prove acceptable for use with various baits and attractants, which would bring them into contact only with the species... 

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. 

Knipling, E. F. (1982) "Present status and future trends of the SIT approach to the control of arthropod pests." In Sterile Insect Technique and Radiation in Insect Control, pp. 3-23. IAEA/FAO, Vienna. 

Construction of sex-linked genetic markers in major economic pests will facilitate expansion of genetic control methods such as the sterile insect technique and backcross sterility in Heliothis virescens. 

The most important ways of direct pheromone application are their combination with traps, and their joint application with insecticides, male sterilants or insect hormones on restricted parts of the area to be protected. This method of application permits a considerable reduction in the quantity of insecticide to be used which, besides its economical advantage, is important also from the aspect of environmental protection. 

Phytochemicals, such as the precocenes, isolated from Ageratum houstonianum [126], interfere with JH biosynthesis. In sensitive insect species, application of these dichromenes to larval instars results in precocious metamorphosis to sterile adults or sterility in adult females following treatment. Pesticides based on these compounds would be useful in the control of insects which are primarily destructive in their immature stages. Other phytochemicals which potentially function as "antijuvenile hormones" include dimethyl sciadinonate, isolated from the leaves of avocado, Persea americana Mill [127, 128], Ingestion of this... 

Triorganotins, Rj Sn X, have in-vitro fungicidal and antibacterial properties which may be used for crop protection. Trialkyltin compounds are phytotoxic, but triphenyltins are well tolerated by plants and maintain a high biocidal activity. Triphenyltin acetate or hydroxide and tricyclohexyltin hydroxide are now registered in the United States and in many European and Asian countries as pesticides, but are also used as chemo-sterilants and antifeedants in insect control. 

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