Mitigating the effects of the green alga Scenedesmus quadricauda on the efficacy of Bacillus thuringiensis var. israelensis against larval black flies.

The green alga Scenedesmus quadricauda has been shown to decrease the efficacy of Bacillus thuringiensis var. israelensis (Bti) to black fly larvae in laboratory experiments. In this study, 2 approaches to mitigating the effects of S. quadricauda were tested, increasing the dose of Bti exposed to the larval black flies and increasing the length of the exposure period. Results showed that increasing the dose as well as increasing the exposure time were effective methods for mitigating the effects of S. quadricauda on Bti efficacy. Assessment of larval mortality at different exposure periods to Bti indicate that the manifestation of toxicity or acquisition of toxic doses of the Bti formulation takes place approximately 2-3 times slower when S. quadricauda is present. These results provide information that might be useful in developing a plan of action to combat this complex situation in field operations.