Synergism of DL-methionine and Bacillus thuringiensis subspecies israelensis as a combined larvicide against Anopheles quadrimaculatus

Mosquitoes vector many pathogens that cause human and other animal diseases worldwide. The application of mosquito larvicides reduces populations at the source, with the potential to reduce biting and pathogen transmission. In many parts of the world the most utilized mosquito larvicide is the biopesticide Bacillus thuringiensis subspecies israelensis (BTI). Unfortunately, resistance to BT toxins has been demonstrated in mosquitoes. Therefore, there is a need for new active ingredients (AIs) that can be used in combination and alongside currently available products. The toxicity of an essential amino acid, methionine, has been demonstrated against mosquito larvae in laboratory bioassays. This study’s objective was to test for a synergistic relationship between BTI and DL-methionine. This was done by exposing mosquitoes to test solutions in glass jars for 48 hours before recording mortality. First, the lethal concentration values (LCs) of the two AIs were determined when used against larval Anopheles quadrimaculatus Say and Aedes aegypti Linnaeus (Diptera: Culicidae). The individual AI LC values were then used to calculate the concentrations needed to conduct bioassays with a BTI/DL-methionine combination. The results showed that BTI and DL-methionine had synergistic properties when used against An. quadrimaculatus, but only at high concentrations. In contrast, the AIs were antagonistic in their effect against Ae. aegypti. The results of our study indicate that for Anopheles species management a BTI/methionine combination larvicide could provide enhanced control.