Evaluation of esterases hydrolyzing α-naphthyl acetate as markers for malathion resistance in Culex quinquefasciatus.

Early detection of insecticide resistance is essential to develop resistance countermeasures and depends on accurate and rapid biological and biochemical tests to monitor resistance and detect associated mechanisms. Many such studies have measured activities of esterases, enzymes associated with resistance to ester- containing insecticides, using the model substrate, α-naphthyl acetate (α-NA). However, in the field, pests are exposed to ester-containing insecticides such as malathion, that are structurally distinct from α-NA. In the current study, malathion resistance in C. quinquefasciatus (3.2- to 10.4-fold) was highly associated with esterase activity measured with either α-NA (R2 = 0.92) or malathion (R2 = 0.90). In addition, genes encoding two esterases (i.e., EST-2 and EST-3) were over-expressed in field- collected strains, but only one (EST-3) was correlated with malathion hydrolysis (R2 = 0.94) and resistance (Rs = 0.96). These results suggest that, in the strains studied, α-NA is a valid surrogate for measuring malathion hydrolysis, and that heightened expression of an esterase gene is not necessarily associated with metabolic resistance to insecticidal esters.

Impact of Nonconventional Selection by Insecticides on Susceptibility of the Southern House Mosquito, Culex quinquefasciatus.

Mosquitoes are an important target of pest control as they vector pathogens that are associated with many debilitating human diseases. Given that mosquitoes have been selected with insecticides for over 100 years, much is known about the development of insecticide resistance associated with targeted application against populations of these insects. However, off-target selection by applications of insecticides in agricultural and residential sites also impacts development of insecticide resistance and is understudied. Similarly, the impact of selecting one life stage of mosquitoes on the insecticide susceptibility of different life stages is largely unknown. Here, we show that susceptibility to chlorantraniliprole, which is applied in rice (Oryza sativa) and sugarcane (Saccharum officinarum) fields in Louisiana, decreased (5.7- to 12-fold) in populations of Culex quinquefasciatus collected from near these fields compared with a reference field strain. In addition, application of bifenthrin by commercial application on an individual residence increased the resistance frequencies to bifenthrin in 5 nearby residential sites. These increased frequencies of resistance, as measured with diagnostic concentrations of bifenthrin, were highly correlated (R2 = 0.92) between larvae and adults, suggesting that selection of adults also confers resistance to larvae. Finally, esterase activities and bifenthrin susceptibilities were moderately correlated (R2 = 0.4 for larvae and 0.52 for adults), suggesting that multiple mechanisms (including metabolism by esterases) were associated with the observed resistance. Results from this study suggest that nonconventional selection by insecticides is a variable to consider when developing management strategies for populations of Cx. quinquefasciatus.

Effects of truck-mounted, ultra low volume mosquito adulticides on honey bees (Apis mellifera) in a suburban field setting.

Few studies have examined the impact of mosquito adulticides on honey bees under conditions that reflect actual field exposure. Whereas several studies have evaluated the toxicity of mosquito control products on honey bees, most have been laboratory based and have focused solely on acute mortality as a measure of impact. The goal of this study was to determine effects of routine applications of truck-based ultra-low volume (ULV) mosquito adulticides (i.e., Scourge, Duet, and Deltagard) on honey bees in a suburban setting. The mosquito adulticides used in this study were pyrethroids with active ingredients resmethrin (Scourge), prallethrin and sumithrin (Duet), and deltamethrin (Deltagard), in which resmethrin, prallethrin, and sumithrin were synergized with piperonyl butoxide. We measured and compared mortality and detoxification enzyme activities (esterase and glutathione S-transferase) from sentinel beehives within and outside of mosquito control areas. Concurrently, colony health (i.e., number of adult bees, brood quantity and brood quality) was compared throughout the study period. No significant differences were observed in honey bee mortality, colony health or detoxification enzyme activities between treated (five sprayed areas each received one to three insecticide treatment) and control sites (four unsprayed areas that did not receive insecticide treatment) over the seven week study period. However, our laboratory study showed that exposure to resmethrin, the active ingredient in Scourge, caused significant inhibition of esterase activity compared with the control group. Our findings suggest that proper application of truck based insecticides for mosquito control results in little or no exposure and therefore minimal effects on domestic honey bees.