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

Abstract

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.

Pharmaceuticals and personal care products alter the holobiome and development of a medically important mosquito

“…This value of 10.20 ng/mL is much lower than previously reported acute LC 50 s determined in 24 h tests. For example an acute LC 50 value was reported at 140.0 ng/mL by Mogren et al (2013). Thus, although acute assays are faster to conduct, and can be compared against previously published reports, they overestimate the amounts of Bti needed to kill 50 percent of mosquito populations that are exposed over their entire life spans.…”

“…Initially, doses of Bti were chosen by dividing a 24-h acute LC 50 (123.0 ng/mL), determined by Mogren et al (2013), by ten. Based on the resulting mortalities, an eight-dose concentration range was developed to determine the chronic LC 50 covering the time span from second instar through adult eclosion.…”

“…israelensis (Bti), a bacterial insecticide commonly used to control larvae of mosquitoes. Joint exposure to a pollutant and a microbial larvicide can be used to detect sublethal physiological stress (Mogren et al, 2013). Further, since 1997, 1000s of hectares of floodwater mosquito breeding sites have been treated with Bti (Becker et al, 2003).…”