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Tolerance to individual and joint effects of arsenic and Bacillus thuringiensis subsp. israelensis or Lysinibacillus sphaericus in Culex mosquitoes


Arsenic contamination of global water supplies has come to the forefront in policy decisions in recent decades. However, the effects of arsenic on lower trophic levels of insects inhabiting contaminated ecosystems are not well understood. One approach to document both acute and sublethal effects of toxicants like arsenic is to assay them in combination with microbial pathogens to evaluate shifts in survival curves of the test organisms. Larvae of Culex quinquefasciatus and Culex tarsalis were reared in water containing 0 or 1 000 μg/L of arsenate or arsenite. Fourth instars were then exposed to a range of doses of Bacillus thuringiensis subsp. israelensis (Bti) or Lysinibacillus sphaericus (Ls), with shifts in lethal concentrations determined. Arsenic accumulation in 4th instars was also quantified, and a relative growth index (RGI) calculated for the treatments and compared to controls. Larvae of both species accumulated between 4 447 ± 169 ng As/g and 6 983 ± 367 ng As/g, though RGI values indicated accumulation did not affect growth and development. In all cases, the LC50 's and LC90 's of Cx. quinquefasciatus exposed jointly with arsenic and Bti/Ls were higher than Cx. tarsalis. Cx. tarsalis reared in arsenite showed a significant reduction in their Bti LC90 values compared to the control, indicating a sublethal effect of Bti. When exposed jointly with Ls, arsenite was more toxic than arsenate in Cx. tarsalis. Overall, these results indicate tolerance of these Culex species to arsenic exposures, and why this may occur is discussed.

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