Evaluating large-scale effects of Bacillus thuringiensis var. israelensis on non-biting midges (Chironomidae) in a eutrophic urban lake

1. There is continued interest in controlling non-biting midges (chironomids) in some freshwaters because of the potential nuisance caused by emergent adults. One option is to apply Bacillus thuringiensis var. israelensis (Bti), primarily used against mosquitoes and blackflies, to control benthic chironomid larvae. Chironomids are also at risk of collateral effects where Bti is used on other targets, and such instances may increase if climate change alters the incidence of dipteran-borne diseases. However, most research on Bti effects on chironomids is available from mesocosms or ponds and might not scale-up to larger waterbodies. 2. We present results on the effects of Bti on larval chironomids from eight experimental treatments, over 3 years, on a newly created eutrophic, urban lake of 200 ha, Cardiff Bay. 3. The first two experimental years provided limited evidence of Bti effects, with chironomid densities reduced by up to 14%. Increased scale of application and altered experimental design in the third year revealed reductions in chironomid larval densities of around 35% following Bti treatment, with suppression lasting several months. 4. These large-scale Bti experiments - among the largest ever undertaken on chironomids - complement previous smaller-scale experiments in illustrating how field conditions and application methods influence Bti effects on chironomid densities. Specifically, the work reveals how near-neutral buoyancy formulations of Bti can reduce chironomid numbers in large lakes exceeding 3 m depth, but only where treatment methods avoid over-dispersion. We advocate further evaluation to assess whether chironomids can be suppressed over longer periods using whole-lake application without long-term ecological implications or excessive cost. We also suggest further consideration be given to non-buoyant Bti formulations for use in deeper lakes.