In a variety of laboratory and field experiments, avermectins have been tested against some 84 species of insects in ten orders, most of which are pests of livestock or horticultural crops or are of general nuisance value. This work is reviewed, comparing doses used, methods of application, and responses of the insects. Avermectins (abamectin and ivermectin) are toxic to almost all insects examined, although tolerance varies and death can be uncommonly slow, taking 24 h to 30 days. There is a marked absence of information on physiological processes that are affected by the pesticides, although at the cellular level they are thought to disrupt receptors for y-aminobutyric acid and glutamic acid in the central nervous system and muscular system. At high doses, treated insects are progressively immobilized, and although initially many can move when stimulated, this ability becomes lost. Some show a disturbed water balance and become distended with fluid, while others show disruption of moulting and metamorphosis. Feeding inhibition is commonly observed at sub-lethal doses. Avermectins affect many aspects of reproduction including mating behaviour, egg development, oviposition and egg hatching. The possibility is raised that these diverse disturbances are not all due to disruption of neuromuscular or central nervous system synapses, and the need for work in this area is stressed. Field studies have shown ivermectin to be most valuable in eradicating insect pests of livestock, but the use of abamectin against horticultural pests has produced less impressive results. The limited work on non-target species is discussed, and attention is drawn to some possible environmental consequences of excreted ivermectin on dung-breeding insects.

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