SUSCEPTIBILITY OF THE ENDANGERED KARNER BLUE BUTTERFLY (LEPIDOPTERA, LYCAENIDAE) TO BACILLUS-THURINGIENSIS VAR KURSTAKI USED FOR GYPSY-MOTHSUPPRESSION IN MICHIGAN

We investigated the phenological and physiological susceptibility of t he endangered Karner blue butterfly (Lycaeides melissa samuelis) to Ba cillus thuringiensis var. kurstaki (Bt), a product widely used for gyp sy moth (Lymantria dispar) suppression in Michigan and other infested states. We monitored phenology of the bivoltine Karner blue in two reg ions of Michigan from 1993 to 1995 to determine if larval stages overl apped temporally with the period of Bt application for gypsy moth supp ression. Karner blue larvae of the spring generation were found during the period that Bt was applied in nearby areas in 1993 only. However spring-generation adults or newly laid eggs were observed up to 11 day s before applications in 1994 and 1995. Since Karner blue eggs develop within one week, summer-generation larvae were most likely present du ring or shortly after 1994 and 1995 Bt application periods. These larv ae would have been at risk, assuming Bt persistence of 4 to 6 days. Ph ysiological susceptibility of Karner blue larvae to Bt was determined in a laboratory bioassay. Larvae were reared on wild lupine (Lupinus p erennis) foliage that was untreated, or sprayed with Bt formulations a t rates of 30-37 or 90 BIU/ha. A similar bioassay with second instar g ypsy moth larvae on similarly treated white oak (Quercus alba) foliage was conducted concurrently. Karner blue survival was 100%, 27% and 14 % on control, low and high Bt treatments, respectively. Early and late Karner blue instars were equally susceptible to Bt. Survival of gypsy moth was 80%, 33% and 5% on control, low and high Bt treatments, resp ectively, and did not differ significantly from Karner blue survival. We conclude that Karner blue is both phenologically and physiologicall y susceptible to Bt used for gypsy moth suppression although the larva l generation at risk and extent of phenological overlap may vary from year to year.