Field efficacy and persistence of Bacillus thuringiensis var kurstaki against Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae) in relation to larval behaviour on grapevine leaves

Commercial formulations of Bacillus thuringiensis (Bt) var. kurstaki are used in southern Australian vineyards to control lightbrown apple moth (LBAM), Epiphyas postvittana (Lepidoptera: Tortricidae). In field experiments, Bt applied to grapevines gave variable control. To test whether the field activity may have been affected by sunlight, deposits of Bt were exposed to direct sunlight and bioassayed against neonate larvae in the laboratory. Bt exposed on vine leaves in direct sunlight lost more than 50% activity against LBAM in the first day. In the field, larvae were found to feed in areas mostly shaded from sunlight. Measurements of UV energy falling on these surfaces, indicated that Bt in shaded areas retained 60% activity after 2 d of sunlight, which would be sufficient time for actively-feeding larvae to consume a lethal dose of Bt. However, larvae on treated leaves either did not eat any tissue, or ate very little compared with those on untreated leaves. It is concluded that reduced feeding rates by LBAM larvae on Bt-treated leaves, rather than rapid inactivation by sunlight is more important in explaining the variability of Bt in the field against LBAM.