CYROMAZINE TOXICITY IN DIFFERENT LABORATORY STRAINS OF THE TOBACCO HORNWORM (LEPIDOPTERA, SPHINGIDAE)

Three different strains of tobacco hornworm, Manduca sexta (L.), were treated with cyromazine ingestion of cyromazine-treated artificial die t or by intrahemocoelic injection. The effect of cyromazine on larval growth and the onset and severity of poisoning symptoms were similar i n the wild type green-pigmented strain and the white-pigmented mutant. Feeding times of 4 h or greater and injected doses of 22.6 mu g/g lar va or more resulted in lower weight gains than were observed with cont rols. Elongation caused by exposure was evident within 12-24 h. The in cidence of cuticular rupture ws 55 and 67% in the dietary exposure tes ts and 24 and 22% in the injection tests for the green and white strai ns, respectively. The response of the black strain to cyromazine diffe red by the route of administration. Like the other strains, dietary ex posure times of 4 h or greater led to smaller weight gains than in the controls. Injected doses of 2.8 eta g/g larva or more caused a decrea se in the weight gain of the treated versus controls. A small proporti on (21%) of black larvae consuming treated diet developed cuticular ru ptures, whereas injected treatments had a higher incidence (87%). The differences in the pigmentation of the white and black strains had bee n linked to high and low juvenile hormone titers, respectively. The gr eater susceptibility of the juvenile hormone-deficient black strain ra ises the possibility that the mode of action of cyromazine involves a hormonal component. In a separate series of experiments, the poisoning symptoms of cyromazine involves a hormonal component. In a separate s eries of experiments, the poisoning symptoms of cyromazine were attenu ated or eliminated by periods of starvation of 1-3 d following exposur e to the chemical. Starvation for 3 d preceding treatment attenuated b ut did not eliminate signs of poisoning.