ACTIVITY OF ABAMECTIN AGAINST LARVAL STAGES OF SPODOPTERA-LITTORALIS BOISDUVAL AND HELIOTHIS-ARMIGERA HUBNER (LEPIDOPTERA, NOCTUIDAE) AND POSSIBLE MECHANISMS DETERMINING DIFFERENTIAL TOXICITY

Marked changes in the relative toxicity of topically‐applied abamectin were found between larval instars of Spodoptera littoralis, toxicity decreasing up to the fifth instar but increasing over 500‐fold (at LD50 level) in the sixth instar. By contrast, the toxicity of abamectin remained constant from fifth to sixth instar Heliothis armigera and there was an increase in the toxicity of two chemically unrelated insecticides, malathion (4‐fold) and lambda‐cyhalothrin (2.5‐fold), from fifth to sixth instar S. littoralis. Prior topical application or injection of the microsomal oxidase inhibitor, piperonyl butoxide (PB) increased the toxicity of abamectin (6–8 and 16‐fold respectively) against fifth instar S. littoralis, while topically‐applied PB increased the proportion of radioactivity present as abamectin in the ventral nerve cord of this instar following topical application with [3H]abamectin. Topically‐applied PB also enhanced the toxicity of abamectin against third (4‐fold) and fourth instar (5‐fold) S. littoralis but had no significant effect on sixth instar S. littoralis, fifth instar H. armigera, or on the toxicity of malathion and lambda‐cyhalothrin against fifth instar S. littoralis. Topical application of the esterase inhibitor, S,S,S‐tributyl phosphorothioate (DEF) significantly increased the toxicity of abamectin at the LD50 level (3‐fold) against fifth instar S. littoralis. The toxicity of injected abamectin against fifth instar S. littoralis was greater (20‐fold) than with topical application but injected abamectin was less toxic (2‐fold) against sixth instar S. littoralis and had no significant effect on fifth instar H. armigera. It is suggested that differential toxicity of abamectin is due in part to greater metabolism and reduced penetration in fifth instar S. littoralis than in sixth instar S. littoralis or fifth instar H. armigera. Copyright © 1990 John Wiley & Sons, Ltd