How Many Mosquitoes Are Killed by Insecticides?

Indoor residual spray of insecticides and insecticidal nets are the two most important malaria vector control tools. But both tools face an imminent threat: mosquito resistance. Science and industry are rapidly developing and testing a range of new compounds and products to win this arms race, at least for the time being.

The level of resistance in natural mosquito populations and the efficacy of new mosquitocidal compounds and products are both assessed at 25-27°C, as recommended by the WHO Pesticide Evaluation Scheme (WHOPES). But malaria mosquitoes are mostly active during the evening and night when temperatures are considerably lower. Could this mean that the efficacy of a bednet inside a dwelling is different than laboratory tests would lead us to believe? Will one mosquito species be affected differentially than another under the same environmental conditions? Is the proportion of a mosquito population that is still susceptible to a chemical sprayed on a wall different from the resistance level detected under standard laboratory conditions?  We believe the answer to all these questions is ‘yes’.

Insecticides interfere with the nervous system of a mosquito. The nervous system and the metabolic activity of a mosquito, which is involved in the breakdown of insecticides, are temperature dependent. Hodjati and Curtis showed in experiments with Anopheles gambiae and An. stephensi that the toxicity of permethrin is affected by temperature. The WHO even highlighted their findings in its report “Test procedures for insecticide resistance monitoring in malaria vectors, bio-efficacy and persistence of insecticide on treated surfaces” and acknowledged that temperature is a critical variable in insecticide resistance monitoring. Yet it continued to recommend that the ideal temperature for testing is 25°C.

The fact that local environmental conditions could impact the efficiency of a bednet or a chemical sprayed on the walls is worrisome, to say the least. Especially since the ability of these tools to kill mosquitoes can decrease rapidly after deployment. Temperature could work in our favor when more mosquitoes are killed/fewer mosquitoes are resistant under cooler nighttime conditions. But imagine that temperature works against us in our fight against malaria vectors, that we are underestimating true levels of resistance. That would add a significant pothole on the road to malaria elimination.

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Local Climate Alters the Effectiveness of Malaria Vector Control Tools