Impact of environmental factors on the toxicity of Bacillus thuringiensis var. israelensis IPS82 to Chironomus kiiensis

To determine the feasibility of microbiological control of chironomid larvae in water sources for the city of Shenzhen, China, the toxicity characteristics of Bacillus thuringiensis var. israelensis (Bti) IPS82 on Chironomus kiiensis Tokunaga were studied. Tests on fermentation products of IPS82 showed good correlations between toxicity, cell density, dissolved oxygen, and the spore-forming phase. In this study, bioassays were carried out with different stage larvae. Results showed that the LC50s (24 h postexposure) were 8.2, 15.2, 24.7, and 38.6 mg/liter for the 1st, 2nd, 3rd, and 4th instars, respectively. Tests on environmental factors influencing toxicity of Bti to C. kiiensis showed that sunlight is the most important factor, shortening the half-life of Bti from 21 days in dark to 10 days under sunlight. Temperature variations (15-30 degrees C) caused no impact on toxicity, but a 16% increase in larval mortality was observed at 35 degrees C. The toxicity of IPS82 was greatest at a pH of 7. In field trials, dosages above 100 mg/liter were effective in the control of C. kiiensis. Our study indicated that it is feasible to use Bti to control C. kiiensis in city source water.