Monitoring nontarget Lepidoptera on Ribes cereum to investigate side effects of an operational application of Bacillus thuringiensis subsp kurstaki

The abundance of nontarget Lepidoptera on the shrub Ribes cereum Douglas was monitored from 1997 to 2000 in an Interior Douglas-fir forest in British Columbia to assess potential side effects of an operational program to control the western spruce budworm, Choristoneura occidentalis Freeman. The treatment was a single application of Bacillus thuringiensis subsp. kurstaki (Btk) (Foray 48B) at the rate of 30 BIU/ha in 2.4 liter/ha. The guild of leaf-feeding Lepidoptera on R. cereum was dominated by Gelechia ribesella Chambers, and Euhyponomeutoides gracilariella Busek, which made up 24 and 62% of the guild, respectively. The remaining 14% of the guild comprised numerous sparsely distributed species (at least 45 different species based on morphotypes). Total larval abundance was significantly lower on plants that were sprayed with Btk than on plants that were covered to exclude Btk. Covering the plants was a novel approach that enabled us to replicate the treatment within a single spray area. G. ribesella and E. gracilariella were significantly reduced by Btk, but a modest reduction of 'sparsely distributed species' was not significant. Both G. ribesella and E. gracilariella appeared to make a full recovery within 2 yr of the Btk spray, but as a group the abundance of the sparsely distributed species was lowest in the year 2000 in both the Btk sprayed area and an untreated comparison area. This suggests a general decline independent of the treatment. Microscopic examination of cadavers of the two major nontarget species showed the presence of Btk in some of the larvae reared from the treated plot, but Btk was absent in larvae from the reference plot.