Toxicity of Avermectin to Larva and Adult Soybean Looper (Lepidoptera: Noctuidae) and Influence on Larva Feeding and Adult Fertility and Fecundity

Abstract

Effects of Avermectin (MK 936) on larval and adult soybean looper (SBL), Pseudoplusia includens (Walker), were investigated in the laboratory and in field-cage tests. Larvae were reared on one of three food types before treatment: artificial pinto bean diet, ‘Ransom’ soybean foliage, or P. includens-resistant ‘GATIR 81-327’ soybean foliage. Larvae reared on artificial diet were less susceptible to MK 936 than larvae reared on either type of soybean foliage. Little difference in larval susceptibility was noted between larvae reared on the two foliage types. Topical LD50 values at 48 h ranged from 0.024 μg per larva (larvae reared on ‘Ransom’ foilage) to 0.203 μg per larva (larvae reared on artificial diet). Moths allowed to feed on various concentrations of MK 936 in 8% sucrose solutions exhibited increased mortality and reduced fertility and fecundity compared with moths given 8% sucrose solutions only. The LC50 of MK 936 at 72 h was 6.1 mg/liter. Unfed and newly emerged moths released into small field cages (five male–female pairs per cage) containing cotton plants sprayed 24 h previously with MK 936 (0.16 kg [AI]/ha) oviposited an average of only 5.9 eggs per plant as compared to a mean of 31.9 eggs per plant in control cages. When moths were released into field cages 72 h postspray, no difference in eggs per plant was noted in sprayed and unsprayed cages.