Effects of a microbial insecticide, Bacillus thuringiensis kurstaki, on nontarget Lepidoptera in a spruce budworm-infested forest

Species in a guild of nontarget leaf-feeding Lepidoptera ontobacco brush, Ceanothus velutinus Dougl. were monitored in the fieldto assess ecological effects of one application of the microbial pestcontrol agent, Bacillus thuringiensis Berliner var. kurstaki [BTK]. TheLepidoptera were sampled to compare species richness, species even-ness, species diversity, larval abundance, and a dominance indexbetween an untreated and BTK treated site over a period of two years.The guild of leaf-feeding Lepidoptera on C. velutinus consisted of 32species. No statistically significant differences were observed in overallspecies richness, although the number of species in the untreated sitewas 30% higher two weeks after treatment. However, species richnessamong uncommon species was significantly reduced in the treated site.Also, no statistically significant differences were observed in speciesevenness or species diversity but the indices were lower in the untreatedsite in three of the four post-treatment samples. A dominace index wasconsistently higher in the untreated site. The total number of caterpil-lars per 100 sec sampling was significantly higher (5.4-fold) in theuntreated site in the early summer sample, two weeks after treatment.Also, larval abundance in the early summer sample was significantlyhigher (3.5-fold) one year later. No differences were noted in larvalabundance in the late summer sample in either year.