Decreasing Bacillus thuringiensis israelensis sensitivity of Chironomus riparius larvae with age indicates potential environmental risk for mosquito control

Mosquito control based on the use of Bacillus thuringiensis israelensis (Bti) is regarded as an environmental friendly method. However, Bti also affects non-target chironomid midges that are recognized as a central resource in wetland food webs. To evaluate the risk for different larval stages of Chironomus riparius we performed a test series of daily acute toxicity laboratory tests following OECD guideline 235 over the entire aquatic life cycle of 28 days. Our study is the first approach that performs an OECD approved test design with Bti and C. riparius as a standard organism in ecotoxicological testing. First-instar larvae of Chironomus riparius show an increased sensitivity towards Bti which is two orders of magnitude higher than for fourth instar larvae. Most EC50 values described in the literature are based on acute toxicity tests using third and fourth instar larvae. The risk for chironomids is underestimated when applying the criteria of the biocide regulation EU 528/2012 to our data and therefore the existing assessment approval is not protective. Possible impacts of Bti induced changes in chironomid abundances and community composition may additionally affect organisms at higher trophic levels, especially in spring when chironomid midges represent a key food source for reproducing vertebrates.