Bacillus thuringiensis Berliner var. kurstaki (Bt) has long been used in gypsy moth
suppression programs because of its specificity for Lepidoptera (moths and butterflies). The short
residual toxicity of Bt observed for vegetable crop pests has minimized concerns raised about
the impact of Bt on nontarget species. Only those Lepidoptera present in the caterpillar stage
during suppression sprays have been considered to be at risk. Researchers in Michigan have
found residual toxicities of Bt to some forest Lepidoptera may be longer than expected. Shaded
foliage treated with Bt was found to be toxic to tiger swallowtail caterpillars (Papilio glaucus)
for more than 30 days after the spray was applied. The authors attribute this long residual toxicity
to the high sensitivity of tiger swallowtails to Bt, and to the persistence of Bt residues on shaded
foliage where it is somewhat protected from degradation by UV light and rainfall. Tiger
swallowtail caterpillars were found to be over 100 times more susceptible than gypsy moth to Bt.
Survivorship of tiger swallowtail caterpillars fed Bt treated foliage was higher when foliage was
in full sunlight than when in shade. These results and other reports of reduced lepidopteran
diversity after Bt sprays question the safety of Bt for non-target Lepidoptera in gypsy moth
suppression programs. The authors are careful to note however, that decisions about gypsy moth
suppression programs must also consider suppressed lepidopteran diversity that can be caused by
widespread gypsy moth defoliation in the forest.
Source: Johnson, K. S., J. M. Scriber, J. K. Nitao, and D. R. Smitley. 1995. Toxicity of
Bacillus
thuringiensis var. kurstaki to three nontarget Lepidoptera in field studies. Environ. Entomol.
24:288-297.
