Effects of sublethal doses of insecticides and of diet and crowding on Dysdercus fasciatus Sign. (Hem., Pyrrhocoridae)

Abstract

The effects of sublethal doses of insecticides and unrelated environmental stresses on Dysdercus fasciatus Sign, were studied in the laboratory. Dieldrin applied topically to fifth-instar nymphs at 02–0·6 μg/individual increased egg production, at 0·8–60 μg/individual decreased egg production, and at 0·6–60 μg/individual decreased fertility. Applied to adults, dieldrin at 2–4 μg/individual reduced egg production, and at 0·02–0 08 μg/individual (males) and 0·2–0·8 μg/individual (females) shortened the life span. In a separate experiment, dieldrin at 0·2 μg/individual reduced egg production when applied to fourth-instar nymphs, but had no effect on fifth-instar nymphs or adults. p.p'DDT at 6 μg/individual applied topically to unmated females increased egg production. Exposure of all life stages to 0·06 mg DDT/ litre for two generations was followed by increased egg production and fertility: shorter exposure at 1 mg DDT/litre had no effect, and at 6–20 mg/litre egg production and fertility were reduced. Rearing on boiled cotton seed resulted in small adults with low gg production and fertility, the eggs being longer than normal. Crowding nymphs (24 individuals/200-ml cage) had the same effect on the size of the resultant adults. Starvation caused low egg production and fertility. Fluttering of males during a free fall (a measure of activity) was increased by crowding (maximum at 2 individuals/cage), by topically applied dieldrin at 0·04–0·08 μg/individual and DDT at 3–5 μg/individual, by two-generation exposure to 0·06 mg DDT/litre, but not by feeding on boiled cotton seed.