Abstract
BackgroundCurrent efforts are underway to quantify the chemical concentration in a treated air space that elicits a spatial repellent (deterrent) response in a vector population. Such information will facilitate identifying the optimum active ingredient (AI) dosage and intervention coverage important for the development of spatial repellent tools – one of several novel strategies being evaluated for vector-borne disease control. This study reports initial findings from air sampling experiments conducted under field conditions to describe the relationship between air concentrations of repellent AIs and deterrent behavior in the dengue vector, Aedes aegypti.MethodsAir samples were taken inside and outdoors of experimental huts located in Pu Tuey Village, Kanchanaburi Province, Thailand in conjunction with mosquito behavioral evaluations. A mark-release-recapture study design using interception traps was used to measure deterrency of Ae. aegypti against 0.00625% metofluthrin coils and DDT-treated fabric (2g/m2) within separate experimental trials. Sentinel mosquito cohorts were positioned adjacent to air sampling locations to monitor knock down responses to AI within the treated air space. Air samples were analyzed using two techniques: the U.S. Environmental Protection Agency (USEPA) Compendium Method TO-10A and thermal desorption (TD).ResultsBoth the USEPA TO-10A and TD air sampling methods were able to detect and quantify volatized AIs under field conditions. Air samples indicated concentrations of both repellent chemicals below thresholds required for toxic responses (mortality) in mosquitoes. These concentrations elicited up to a 58% and 70% reduction in Ae. aegypti entry (i.e., deterrency) into treated experimental huts using metofluthrin coils and DDT-treated fabric, respectively. Minimal knock down was observed in sentinel mosquito cohorts positioned adjacent to air sampling locations during both chemical evaluations.ConclusionsThis study is the first to describe two air sampling methodologies that are appropriate for detecting and quantifying repellent chemicals within a treated air space during mosquito behavior evaluations. Results demonstrate that the quantity of AI detected by the mosquito vector, Ae. aegypti, that elicits repellency is far lower than that needed for toxicity. These findings have important implications for evaluation and optimization of new vector control tools that function through mosquito behavior modification as opposed to mortality.
Highlights
Current efforts are underway to quantify the chemical concentration in a treated air space that elicits a spatial repellent response in a vector population
Mosquito behavior Overall, 0.00625% metofluthrin coils elicited a 58% (20/48) decrease in Ae. aegypti entry in the treatment hut compared to the negative control hut which had a total of 48 marked mosquitoes entering (Table 1)
Percent knock down (KD) observed in Ae. aegypti sentinel cohorts inside the hut containing 0.00625% metofluthrin coil was higher (1.7%; 4/240) than both the positive (0.8%; 2/240) and negative controls (0%; 0/60); minimal KD occurred overall during the evaluation with a total of only 6 and 5 observations from indoor and outdoor cohort positions, respectively (Table 1)
Summary
Current efforts are underway to quantify the chemical concentration in a treated air space that elicits a spatial repellent (deterrent) response in a vector population Such information will facilitate identifying the optimum active ingredient (AI) dosage and intervention coverage important for the development of spatial repellent tools – one of several novel strategies being evaluated for vector-borne disease control. A thorough evaluation of the role of supplementary (or stand-alone) tools targeting those vectors not affected by either LLINs or IRS is needed This is true for outdoor disease transmission settings as well as for day biting vectors, perhaps most evident in the case of dengue. The key characteristic that separates spatial repellents from current chemical adult control tools – LLINs and IRS – are that they provide protection at concentrations much lower than recommended field application rates [3] This is because the outcome measure is not toxicity but rather behavior modification. Direct benefits of such a tool would be: 1) the delayed onset for selection of insecticide resistance (minimal selection pressure due to reduced toxicity) thereby extending the products effective duration, 2) reduced mammalian toxicity due to application at low chemical concentrations, 3) delivery formats that eliminate the need for direct vector contact to be effective allowing for point source application which can facilitate product distribution
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