Decreased Toxicity of Bacillus thuringiensis subsp. israelensis to Mosquito Larvae after Contact with Leaf Litter

Bacillus thuringiensis subsp.israelensis is a bacterium producing crystals containing Cry and Cyt proteins, which are toxic for mosquito larvae. Nothing is known about the interaction between crystal toxins and decaying leaf litter, which is a major component of several mosquito breeding sites and represents an important food source. In the present work, we investigated the behavior ofB. thuringiensis subsp.israelensis toxic crystals sprayed on leaf litter. In the presence of leaf litter, a 60% decrease in the amount of Cyt toxin detectable by immunology (enzyme-linked immunosorbent assays [ELISAs]) was observed, while the respective proportions of Cry toxins were not affected. The toxicity of Cry toxins towardAedes aegypti larvae was not affected by leaf litter, while the synergistic effect of Cyt toxins on allB. thuringiensis subsp.israelensis Cry toxins was decreased by about 20% when mixed with leaf litter. The toxicity of two commercialB. thuringiensis subsp.israelensis strains (VectoBac WG and VectoBac 12AS) and a laboratory-producedB. thuringiensis subsp.israelensis strain decreased by about 70% when mixed with leaf litter. Taken together, these results suggest that Cyt toxins interact with leaf litter, resulting in a decreased toxicity ofB. thuringiensis subsp.israelensis in litter-rich environments and thereby dramatically reducing the efficiency of mosquitocidal treatments.