Larval feeding behaviour and toxicity of <i>Bacillus thuringiensis</i> against <i>Ctenopseustis obliquana</i> on avocados

Abstract

Ctenopseustis obliquana is a serious pest of avocado in New Zealand Laboratory experiments to gain a greater understanding of the interaction between larval feeding behaviour and environmental degradation of Bacillus thuringiensis (Bt) residues were carried out Larval C obliquana were found to feed equally during light and dark periods and the application of Bt sprays in the evening did not consistently result in higher mortality than sprays applied in the morning First instar larvae were more susceptible to Bt sprays than third instar larvae However when first instar larvae were provided with leaf discs that had been sprayed with Bt seven days previously mortality was equivalent to that of larvae feeding on unsprayed leaves Mortality of first and third instar larvae provided with Bttreated leaf discs collected from within the shade of the avocado tree canopy (zero and one day after spraying) was significantly greater than that of larvae provided with Bttreated leaf discs collected from the outer perimeter of the canopy Results indicated that even very short exposure to sunlight (ie 3060 minutes) was sufficient to cause a significant reduction in the toxicity of Btsprayed leaves to C obliquana larvae