Abstract
BackgroundInsecticide resistance monitoring in malaria mosquitoes is essential for guiding the rational use of insecticides in vector control programs. Resistance bioassay is the first step for insecticide monitoring and it lays an important foundation for molecular examination of resistance mechanisms. In the literature, various mosquito sample collection and preparation methods have been used, but how mosquito sample collection and preparation methods affect insecticide susceptibility bioassay results is largely unknown. The objectives of this study were to determine whether mosquito sample collection and preparation methods affected bioassay results, which may cause incorrect classification of mosquito resistance status.MethodsThe study was conducted in Anopheles sinensis mosquitoes in two study sites in central China. Three mosquito sample collection and preparation methods were compared for insecticide susceptibility, kdr frequencies and metabolic enzyme activities: 1) adult mosquitoes collected from the field; 2) F1 adults from field collected, blood-fed mosquitoes; and 3) adult mosquitoes reared from field collected larvae.ResultsMosquito sample collection and preparation methods significantly affected mortality rates in the standard WHO tube resistance bioassay. Mortality rate of field-collected female adults was 10-15% higher than in mosquitoes reared from field-collected larvae and F1 adults from field collected blood-fed females. This pattern was consistent in mosquitoes from the two study sites. High kdr mutation frequency (85-95%) with L1014F allele as the predominant mutation was found in our study populations. Field-collected female adults consistently exhibited the highest monooxygenase and GST activities. The higher mortality rate observed in the field-collected female mosquitoes may have been caused by a mixture of mosquitoes of different ages, as older mosquitoes were more susceptible to deltamethrin than younger mosquitoes.ConclusionsFemale adults reared from field-collected larvae in resistance bioassays are recommended to minimize the effect of confounding factors such as mosquito age and blood feeding status so that more reliable and reproducible mortality may be obtained.
Highlights
Insecticide resistance monitoring in malaria mosquitoes is essential for guiding the rational use of insecticides in vector control programs
In the Anhui study site, a mortality rate of 47.6% was detected for field-collected adults, significantly higher than the adults reared from field-collected larvae (31.7%) and F1 adults from field collected bloodfed females (32.9%)
Similar patterns were observed in the Hunan study site (Table 1), the mortality rates for mosquitoes in all three preparation methods were lower than the Anhui study site, suggesting the Hunan site exhibited higher resistance than the Anhui site
Summary
Insecticide resistance monitoring in malaria mosquitoes is essential for guiding the rational use of insecticides in vector control programs. Resistance bioassay is the first step for insecticide monitoring and it lays an important foundation for molecular examination of resistance mechanisms. The objectives of this study were to determine whether mosquito sample collection and preparation methods affected bioassay results, which may cause incorrect classification of mosquito resistance status. One important tool to prevent and control malaria is vector control, especially using long-lasting insecticidetreated bed nets (LLINs) and indoor residual spraying (IRS) [1]. The use of insecticides for agricultural purposes exerts selection pressure for resistance in mosquitoes because mosquito larvae breed in agricultural fields and are directly exposed to insecticides. Residual insecticides from agricultural pest control may be leaked into mosquito breeding sites and expose mosquito larvae to insecticides.
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