GROWTH-INHIBITORY AND ANTIFEEDANT EFFECTS OF AZADIRACHTIN ON 6 NOCTUIDS OF REGIONAL ECONOMIC IMPORTANCE

The comparative efficacy of azadirachtin, the major active ingredient in the botanical insecticide neem, as a larval growth inhibitor and feeding deterrent was determined for six species of noctuids of economic importance: the black army cutworm, Actebia fennica (Tausch.), the bertha armyworm, Mamestra configurata Walker, the variegated cutworm, Peridroma saucia Hubner, the zebra caterpillar, Melanchra picta (Harr.), the Asian armyworm, Spodoptera litura (Fab.), and the cabbage looper, Trichoplusia ni Hubner. When added to an artificial diet, azadirachtin inhibited neonate larval growth of all species in a dose-dependent fashion. EC50 values (dietary concentration reducing larval growth by 50% relative to controls) following ten days of feeding ranged from 0.12 to 0.24 mg kg-1 but without significant differences between species. However, when second-instar larvae were offered a choice, only larvae of P. saucia and S. litura discriminated between a control diet and diet containing azadirachtin at concentrations up to 0.4 mg kg-1. A more sensitive behavioural bioassay using fourth-instar larvae indicated that S. litura was the most sensitive to the antifeedant effects of azadirachtin (EC50 = 1.25 ng cm-2), whereas A. fennica was the least (EC50 = 40.7 ng cm-2). Topical treatment of fourth-instar larvae with 50 or 100 ng of azadirachtin resulted in significant inhibition of subsequent growth, diet consumption, and dietary utilization. However, A. fennica and S. litura are less inhibited that the other four species. The present study indicates that behaviour-based bioassays alone do not characterize the total effect of azadirachtin on noctuid larvae.