Ecological effects of mosquito control with Bti: evidence for shifts in the trophic structure of soil- and ground-based food webs

Abstract

The microbial control agent Bacillus thuringiensis var. israelensis (Bti) has been successfully used worldwide to reduce abundances of biting Nematocera (Diptera), often with little direct impact on non-target organisms observed. However, the potential for additional indirect efects on other ecosystem properties, including on trophic linkages within food webs, is poorly known. We investigated the efects of multiple-year mosquito control treatments using the Bti product VectoBac®-G on the stable isotope composition of epigeal and soil-based consumers inhabiting replicate foodplains along the River Daläl ven, Sweden. We observed signifcant changes in the isotopic composition of detritivores feeding at the base of foodplain food webs. Enchytraeid worms were characterised by 3.5% higher δ13C values in treated foodplains, suggesting increased consumption of δ13C-enriched food. The overall range of community-wide δ15N values was 56% greater in the treated food plains, whilst δ15N values of oribatid mites were elevated by 97%. These results suggest extra fractionation in the transfer of nitrogen through foodplain food chains. We conjecture that the ecological mechanisms driving these food web shifts are (1) the mass mortality of high δ13C A. sticticus larvae, which leaves high concentrations of dead mosquito biomass deposited on soils at local scales, after the foodwaters have receded and (2) incorporation of the very high δ13C-enriched corn particles comprising the bulk of the VectoBac®-G product into foodplain food webs. Our results suggest that repeated applications of Bti might have wider, still largely unknown implications for nutrient and energy cycles within foodplain ecosystems.