Effects of Bacillus thuringiensis israelensis (Bti) and methoprene on nontarget macroinvertebrates in Minnesota wetlands

We studied the effects of the mosquito larvicides methoprene and Bacillus thuringiensis israelensis (Bti) on the benthic macroinvertebrate communities of 27 wetland ecosystems in Wright County, Minnesota. These larvicides are generally considered safe for nontarget species. After 3 yr of preliminary investigations, including 2 yr of intensive sampling, larvicide treatments were applied during 1991-1993. Nine of the wetlands were experimentally treated with methoprene, which disrupts insect development; an additional set of nine wetlands were treated with Bti, a microbial larvicide; and nine wetlands were left untreated to serve as a control treatment. In general, insecticide treatment had minimal effects on nontarget groups during the first treatment year. However, during 1992, highly significant reductions due to both methoprene and Bti were observed in several insect groups. Predatory insects were reduced on methoprene-treated sites but not Bti-treated sites in 1992. In 1993, treatment with both larvicides resulted in wetland communities that were depauperate in most insects. Although effects were observed broadly across insect taxa, Diptera, which comprised 79% of the insects, were affected most strongly, especially the dipteran suborder Nematocera, which included 71% of total insects and was dominated by Chironomidae. Minimal effects on noninsect macroinvertebrates were observed. Bti- and methoprene-treated sites also showed a reduction in richness of insect genera and an increased tendency to be dominated by one or a few genera. Pretreatment data, collected under drought conditions from the same wetlands, showed that the benthic macroinvertebrate fauna was dominated by mollusks during the drought but became increasingly dominated by insects during the wetter years. On the treated sites, insects remained at low density, very similar to the drought conditions, but the noninsect macroinvertebrates declined on treated sites in the same pattern as on control sites. Both indirect effects and direct toxicity likely contributed to the observed differences. Bti is likely to be directly toxic only to nematoceran Diptera; thus effects of Bti on other insect groups may have resulted from disruption of the invertebrate food web. Methoprene is more broadly toxic; thus observed methoprene effects on nonnematoceran groups may have been due to either direct toxicity or food web effects, or both. The 2-3 yr lag time in response of nontarget insects to larvicide treatment demonstrates the need for long-term studies in wetland ecosystems, and the need to reconsider the conclusions based on previous short-term studies that these larvicides are environmentally safe.