Responses of nontarget Lepidoptera to Foray 48B Bacillus thuringiensis var. kurstaki on Vancouver Island, British Columbia, Canada.

Impacts of a gypsy moth (Lymantria dispar) eradication program on native, nontarget Lepidoptera were assessed in 1999, on southeastern Vancouver Island (BC, Canada). The microbial insecticide Bacillus thuringiensis var. kurstaki (Btk) was applied aerially over two areas totalling 12,805 ha on May 8, May 19, and June 8, 1999, at a dosage of 50 billion international units in 4.0 L/ha. Lepidoptera were collected from two host plant species: Garry oak (Quercus garryana) and common snowberry (Symphoricarpos albus). Lepidopteran larvae were collected from common snowberry foliage at 24 urban parks and from Garry oak foliage at 28 oak-dominated habitats, representing 12 and 14 replicates, respectively, of two treatments: unsprayed (reference) and sprayed (treatment). Prespray data were collected from March 25 to May 6 for S. albus, and from April 26 to May 6 for Q. garryana. Postspray data were collected from May 10 through June 15 for S. albus and from May 10 to July 6 for Q. garryana. The 15 most abundant lepidopteran species were analyzed statistically. However, the majority of species were collected infrequently, and, therefore were pooled for statistical analysis. After the Btk spray applications, 11 of the individual species and groups of uncommon species were found to be significantly less abundant in the treatment sites than in the reference sites. The effects of sample date were statistically significant on almost all groups of Lepidoptera analyzed, both before and after Btk spray applications, indicating temporal variation in lepidopteran abundance. Significant variation in diversity of members of the Lepidoptera, as a result of Btk spray application, was not detected on S. albus or Q. garryana. However, results showed significant variation in lepidoptera richness and abundance on both host plant species.