Impacts of a mosquito selective pesticide, Bti, on the macroinvertebrates of a subtropical stream in Hong Kong

To test the hypothesis that Bti was specific to mosquito larvae, the granular form of Bti was tested on commonly found Hong Kong species from four naturally occuring aquatic insect orders and one species of decapod crustacean (Neocardina serra). Because data on Bti impacts on species in tropical and subtropical countries is relatively rare, the present study was conducted in Hong Kong's New Territories. Using static acute toxicity bioassays, all mosquito larvae exposed to Bti at the recommended dosage were dead except for some dark pigmented fourth instar individuals that had stopped feeding prior to emergence and as a result, did not ingest the Bti toxin. The only non-target species killed by the Bti were some chironomid species. In 1998 Bti inoculated pools along the Tai Tan River in the New Territories of Hong Kong had significantly fewer chironomids and mosquito larvae than control pools. By January, 1999, these same Bti inoculated pools still had no mosquito larvae in them, however, chironomids were no longer significantly rarer when compared to the control ponds. Thus, it would appear, that during the 1998-99 study in which Bti was added to the study sites at weekly intervals, Bti resistant chironomid species replaced Bti sensitive species in the Bti inoculated pools.