Food Consumption and Survival of Trichoplusia ni (Lepidoptera: Noctuidae) Larvae Following Intoxication by Bacillus thuringiensis var. kurstaki and Thuringiensin

Trichoplusia ni (Hübner) third instars were exposed for 3 d to Bacillus thuringiensis (Berliner) var. kurstaki or thuringiensin incorporated into artificial diet. Surviving larvae were transferred to untreated cabbage, and survival time and consumption were measured. B. thuringiensis var. kurstaki intoxication was rapid and proportional to concentration, with LC50 values of 23.1 and 13.4 g/ml diet after 2 and 3 d, respectively. A latent toxicity was observed following transfer to cabbage, which decreased LC50 and LC90 values by a further 1.8 fold. Thuringiensin toxicity was low until 6 d after initial exposure, when mortality occurred in conjunction with larval molting. Mortality increased rapidly to >90% after 12 d. Consumption of untreated cabbage by surviving larvae was almost completely inhibited by thuringiensin (97-99% reduction); feeding inhibition was the primary factor involved, although larval survival was also reduced significantly. Food consumption and duration of the larval stage of larvae surviving intoxication by B. thuringiensis var. kurstaki were also reduced, but reductions were small compared with those caused by thuringiensin.