Cyromazine causes decreased cuticle extensibility in larvae of the tobacco hornworm, Manduca sexta

Abstract

The substituted melamine insecticide cyromazine caused delayed mortality in tobacco hornworm caterpillars. The LC 50 for cyromazine administered in artificial diet was about 5 ppm. The insecticide induced a poisoning syndrome that included reduction of feeding and growth, impaired movement, the assumption of a characteristic long and thin body shape, and increased internal pressure. Poisoning resulted in delayed moulting with significant mortality in subsequent stages. With higher doses (20 ppm and above), lesions developed in the cuticle of the intersegmental membranes, leading to rupture and lethal loss of body fluids. Similar symptoms occurred in larvae injected with cyromazine. Cyromazine poisoning was associated with a significant decrease in the extensibility of the body wall cuticle, measured as a decrease in the rate of stretching of cuticle loops under load. This change in the cuticle's mechanical properties occurred rapidly (3–6 hr) and preceded other externally evident changes. This effect of cyromazine on the cuticle may be responsible for other symptoms of poisoning. Internal pressure may be increased because the cyromazine-affected cuticle is less readily stretched. Feeding and growth may be curtailed because the body wall cannot expand to accommodate increased body mass. Movement may be impaired because excess pressure in the haemolymph prevents the caterpillar's hydrostatic skeleton from functioning normally. Lesions in body wall cuticle may be a consequence of excessive internal pressure.