Toxicity of acephate to larvae of gypsy moth as a function of host plant and bioassay method

We examined toxicity of acephate to third-instar gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), under different conditions of administration method, availability of food to larvae during bioassay, host plant, and activity of detoxifying enzymes. Larvae that had been fed field-collected foliage of white alder (Alnus rhombifolia Nutt.) were less susceptible 48 h after treatment with topically applied acephate if they were allowed to continue feeding on foliage during the bioassay period (LD₅₀= 60.6 μg/g larva ) than if they were not (LD₅₀= 13.5 μg/g larva ). All surviving larvae were replaced on their original food plant after the 48-h bioassay; of these, 14.4% of the larvae not fed during treatment died before pupation, compared with 1.3% of the larvae fed alder during treatment. The LD₅₀ obtained for topically treated larvae reared and treated on Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, (51.1 μg/g larva) was comparable to that obtained for larvae fed alder (60.0 μg/g larva) throughout treatment. Larvae treated orally with acephate, however, were slightly more susceptible when reared on Douglas-fir (LC₅₀, 20.3 ppm ) than when reared on alder (LC₅₀, 27.0 ppm ). Post-treatment mortality in orally treated larvae was 10.3% in those fed alder and 9.5% in those fed Douglas-fir. Higher cytochrome P-450 activities in larvae reared on Douglas-fir apparently did not enhance tolerance to acephate. Both sexes of orally treated larvae took significantly longer to pupate than did controls on both foliage types, as did topically treated males fed Douglas-fir. Pupal weight generally was slightly, but not always significantly, higher in treated than untreated larvae under all dietary and treatment regimes.