Effects of Bti on the diversity and community composition of three Chironomidae subfamilies across different micro-habitats

The mosquito control agent Bacillus thuringiensis subsp. israelensis (Bti) is considered environmentally friendly due to its highly specific mode of action. Nevertheless, adverse effects of Bti have been observed in non-biting midges of the family Chironomidae. In this study, we applied the maximum field rate of Bti three times from April to May to six out of twelve floodplain pond mesocosms. Chironomidae larvae were sampled two weeks after the third application in three different micro-habitats and DNA metabarcoding was used to identify the larvae. We observed Bti effects on the Chironomidae subfamily Chironominae, while Tanypodinae and Orthocladiinae remained unaffected. The interspecific diversity of Chironominae was significantly reduced by 27% in the Bti treatment. Although the interaction between treatment and habitat was not significant, a notable decrease in interspecific diversity of Chironominae between the control and Bti treatment in two out of three micro-habitats was detected (47% and 41%, respectively). We observed a significant habitat-dependent change in intraspecific diversity of Chironominae, with a 28% decline in one habitat and a 21% increase in another. The Chironominae community composition differed between the control and Bti treatment in two out of three habitats. These outcomes highlight the variability of Bti's impact on Chironomidae communities across subfamilies and micro-habitats, potentially elucidating discrepancies reported in prior studies and emphasising the necessity for comprehensive risk assessments that encompass diversity at various taxonomic levels and environmental variation at different spatial scales.