16.4.2. Insect growth regulators
Insect growth regulators (IGRs) are compounds that affect insect growth via interference with metabolism or development. They offer a high level of efficiency against specific stages of many insect pests, with a low level of mammalian toxicity. The two most commonly used groups of IGRs are distinguished by their mode of action. Chemicals that interfere with the normal maturation of insects by disturbing the hormonal control of metamorphosis are the juvenile hormone mimics, such as juvenoids (e.g. fenoxycarb, hydroprene, methoprene, pyriproxyfen). These halt development so that the insect either fails to reach the adult stage or the resulting adult is sterile and malformed. As juvenoids deleteriously affect adults rather than immature in- sects, their use is most appropriate to species in which the adult rather than the larva is the pest, such as fleas, mosquitoes, and ants. The chitin synthesis inhibitors (e.g. diflubenzuron, triflumuron) prevent the formation of chitin, which is a vital component of insect cuticle. Many conventional insecticides cause a weak inhibition of chitin synthesis, but the benzoylureas (also known as benzoylphenylureas or acylureas, of which diflubenzuron and triflumuron are examples) strongly inhibit formation of cuticle. Insects exposed to chitin synthesis inhibitors usually die at or immediately after ecdysis. Typically, the affected insects shed the old cuticle partially or not all and, if they do succeed in escaping from their exuviae, their body is limp and easily damaged as a result of the weakness of the new cuticle.
IGRs, which are fairly persistent indoors, usefully control insect pests in storage silos and domestic premises. Typically, juvenoids are used in urban pest control and inhibitors of chitin synthesis have greatest application in controlling beetle pests of stored grain. However, IGRs (e.g. pyriproxyfen) have been used also in field crops, for example in citrus in southern Africa. This use has led to severe secondary pest outbreaks because of their adverse effects on natural enemies, especially coccinellids but also wasp parasitoids. Spray drift from IGRs applied in African orchards also has affected the development of non-target beneficial insects, such as silkworms. In the USA, in the citrus-growing areas of California, many growers are interested in using IGRs, such as pyriproxyfen and buprofezin, to control California red scale (Diaspididae: Aonidiella aurantii); however, trials have shown that such chemicals have high toxicity to the predatory coccinellids that control several scale pests. The experimental application of methoprene (often used as a mosquito larvicide) to wetlands in the USA resulted in benthic communities that were impoverished in non- target insects, as a result of both direct toxic and indirect food-web effects, although there was a 1–2 year lag-time in the response of the insect taxa to application of this IGR.
Neem derivatives are another group of growth- regulatory compounds with significance in insect control (Box 16.3). Their ingestion, injection, or topical application disrupts molting and metamorphosis, with the effect depending on the insect and the concentration of chemical applied. Treated larvae or nymphs fail to molt, or the molt results in abnormal individuals in the subsequent instar; treated late-instar larvae or nymphs generally produce deformed and non-viable pupae or adults. These physiological effects of neem derivatives are not fully understood but are believed to result from interference with endocrine function; in particular, the main active principle of neem, azadirachtin (AZ), may act as an anti-ecdysteroid by blocking binding sites for ecdysteroid on the protein receptors. AZ may inhibit molting in insects by preventing the usual molt-initiating rise in ecdysteroid titer. Cuticle structures known to be particularly sensitive to ecdysteroids develop abnormally at low doses of AZ.
The newest group of IGRs developed for commercial use comprises the molting hormone mimics (e.g. tebufenozide), which are ecdysone agonists that appear to disrupt molting by binding to the ecdysone receptor protein. They have been used successfully against immature insect pests, especially lepidopterans. There are a few other types of IGRs, such as the anti-juvenile hormone analogs (e.g. precocenes), but these currently have little potential in pest control. Anti-juvenile hormones disrupt development by accelerating termination of the immature stages.