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

Abstract

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 of B. 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 toward Aedes aegypti larvae was not affected by leaf litter, while the synergistic effect of Cyt toxins on all B. thuringiensis subsp. israelensis Cry toxins was decreased by about 20% when mixed with leaf litter. The toxicity of two commercial B. thuringiensis subsp. israelensis strains (VectoBac WG and VectoBac 12AS) and a laboratory-produced B. 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 of B. thuringiensis subsp. israelensis in litter-rich environments and thereby dramatically reducing the efficiency of mosquitocidal treatments.

Fate of Bacillus thuringiensis subsp. israelensis in the Field: Evidence for Spore Recycling and Differential Persistence of Toxins in Leaf Litter

“…These samples were constituted mainly of Quercus sp. leaves and some Alnus glutinosa leaves (42). Litter samples were left to dry and were stored at room temperature until the toxins were added for the experiments.…”

“…israelensis (4Q2-81) transformed with plasmids pHT606, pHT618, pWF53, and pWF45 for Cry4Aa, Cry4Ba, Cry11Aa, and Cyt1Aa, respectively, was used to produce each protoxin separately (10,32,49). Spores and crystal suspensions were produced as previously described (41,42) and conserved in distilled water at Ϫ20°C until use.…”

“…FR1160365. This protocol is highly reproducible for each toxin and with a large range of concentrations, suggesting that the extraction procedure in itself has no impact on the integrity of the toxins (42). All the extractions were performed in duplicate.…”

“…israelensis toxins in field samples. Furthermore, most previous studies investigated the interaction between Cry toxins and soil, but there have been few studies on the behavior of Cry and Cyt toxins in leaf litter under aquatic conditions (42), which is one of the main constituents of several mosquito breeding sites and the main food source for larvae.…”

“…israelensis to determine the concentrations of the toxins and their relative proportions in several field-collected leaf litter samples (42). We then applied this test to leaf litter samples contaminated in the laboratory with different amounts of toxins in their crystal form to better understand how long toxins can persist in leaf litter and if they have different behaviors.…”