Synergistic activity of Bacillus thuringiensis toxins against Simulium spp. larvae.

Species of Simulium spread diseases in humans and animals such as onchocerciasis and mansonelosis, causing health problems and economic loses. One alternative for controlling these insects is the use of Bacillus thuringiensis serovar israelensis (Bti). This bacterium produces different dipteran-active Cry and Cyt toxins and has been widely used in blackfly biological control programs worldwide. Studies on other insect targets have revealed the role of individual Cry and Cyt proteins in toxicity and demonstrated a synergistic effect among them. However, the insecticidal activity and interactions of these proteins against Simulium larvae have not been reported. In this study we demonstrate that Cry4Ba is the most effective toxin followed by Cry4Aa and Cry11Aa. Cry10Aa and Cyt1Aa were not toxic when administered alone but both were able to synergise the activity of Cry4B and Cry11Aa toxins. Cyt1Aa is also able to synergise with Cry4Aa. The mixture of all toxin-producing strains showed the greatest level of synergism, but still lower than the Bti parental strain.