Abstract
Outdoor residual spraying is proposed for the control of exophilic mosquitoes. However, the residual effect of insecticide mists applied to outdoor resting habitats of mosquitoes is not well characterized. The objective of this study was to assess the longevity of the residual insecticidal effect of three pyrethroid formulations applied to outdoor vegetation against the Southeast Asian malaria vector Anopheles dirus. Lambda-cyhalothrin capsule suspension, deltamethrin emulsifiable concentrate and bifenthrin wettable powder were sprayed on dense bamboo bushes on the Thailand-Myanmar border during the dry season 2018. The duration and magnitude of the residual insecticidal effect were assessed weekly with a standard cone assay, using freshly collected insecticide-treated bamboo leaves and a laboratory-adapted colony of Anopheles dirus sensu stricto susceptible to pyrethroids. The experiment was repeated during the rainy season to assess the persistence of the lambda-cyhalothrin formulation after natural rains and artificial washings. During the dry season (cumulative rainfall = 28 mm in 111 days), mortality and knockdown (KD) rates were >80% for 60 days with bifenthrin and 90 days with lambda-cyhalothrin and deltamethrin. The 50% knockdown time (TKD50) was <15 min with lambda-cyhalothrin and deltamethrin, and <30 min with bifenthrin. During the rainy season (cumulative rainfall = 465 mm in 51 days), mortality and KD rates were >80% for 42 days and TKD50 was <15 min with lambda-cyhalothrin. Additional artificial washing of the testing material with 10L of tap water before performing the cone tests had no significant effect on the residual insecticidal effect of this formulation. Long-lasting residual insecticidal effect can be obtained when spraying pyrethroid insecticides on the outdoor resting habitats of malaria vectors.
Highlights
The two broadly scalable interventions recommended by World Health Organization (WHO) for malaria vector-control are mass distribution of long-lasting insecticide-impregnated bed nets (LLINs) and, where appropriate, indoor residual spraying (IRS) [1]
Lambda-cyhalothrin gave the shortest KTD50s followed by deltamethrin and bifenthrin: 13/16, 9/16 and 1/16 data points with Knockdown time 50% (KDT50)
This study confirms that residual insecticidal effect compatible with malaria vectorcontrol operation can be obtained when applying an insecticide mist to outdoor vegetation, and that a lethal concentration of the active ingredient can persist for several weeks despite monsoon rains with the lambda-cyhalothrin formulation
Summary
The two broadly scalable interventions recommended by World Health Organization (WHO) for malaria vector-control are mass distribution of long-lasting insecticide-impregnated bed nets (LLINs) and, where appropriate, indoor residual spraying (IRS) [1]. LLINs protect against mosquitoes seeking for human blood, indoors and at a time when people are sleeping under a bed net. Indoor residual spraying is effective against mosquitoes resting indoors, before or after the blood meal. These stereotypical trophic behaviors apply only to a minority of the dominant malaria vectors worldwide [2, 3]. Outdoor residual spraying has been proposed for the control of exophilic mosquitoes, including relevant malaria vectors [9,10,11]
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