Date Published: October 3, 2013
Publisher: Public Library of Science
Author(s): Katey D. Glunt, Justine I. Blanford, Krijn P. Paaijmans, Glenn F. Rall.
Malaria vector control currently relies almost exclusively on killing adult mosquitoes with chemical insecticides. Insecticide-treated nets (ITNs), long-lasting insecticide-treated nets (LLINs), and indoor residual sprays (IRS) aim to repel, disable, and/or kill mosquitoes on contact. While these tools have proven to be extremely successful in reducing disease incidence and mortality , insecticide resistance is on the rise and a resurgence of malaria is feared . To mitigate the effects of resistance, the development of new insecticides and formulations for use in LLINs and for IRS remains a research priority . In this paper we argue that, to increase the effectiveness of the chemical arsenal available, we need to consider the relevant microclimatic conditions in which these tools are deployed. We will discuss how temperature in particular can interact with the conventional use of chemicals within houses, and broaden our discussion to consider its potential influence on the use of semiochemicals to lure mosquitoes to traps.
The World Health Organization Pesticide Evaluation Scheme (WHOPES), which promotes and coordinates the testing and evaluation of pesticides for public health, specifies laboratory conditions in their guidelines for testing mosquitocidal compounds and products. The recommended temperatures for phase I trials are 25±2°C for testing of LLINs  and 27±2°C for IRS and treated bednets .
The insecticides used in public health for vector control kill mosquitoes by interfering with nervous system function. But metabolic activity , which is involved in degradation of insecticides, and nervous system sensitivity  are highly temperature-dependent. As mosquito body temperature changes with its surroundings, environmental temperature has the potential to influence the toxicity of insecticides. This effect is quantified by measuring the temperature coefficient (TC) of an insecticide (Figure 2). A positive TC indicates that an insecticide becomes more toxic as temperature increases; insecticides with a negative TC kill more insects at lower temperatures. Pyrethroids, the dominant insecticide class currently used for malaria control, and DDT, the only organochlorine permitted for IRS, commonly exhibit a negative temperature coefficient. Therefore, in theory, they should perform better under cooler nighttime conditions. On the other hand, carbamates and organophosphates (two and three out of the 12 recommended compounds for IRS, respectively) generally have a positive TC, and may be less efficient under these conditions.
Insecticide resistance is one of the greatest threats to the success of malaria control and elimination campaigns. The WHO currently recommends that the level of resistance in mosquito populations be evaluated at 25±2°C . As with susceptible insects, the mortality of resistant insects can increase or decrease with temperature (e.g., , ). Hodjati and Curtis  showed that resistant An. stephensi mosquitoes were more susceptible to permethrin at 16 and 37°C, compared to 22 and 28°C, where nearly all mosquitoes survived the exposure. In resistant An. gambiae, as in the susceptible strain, susceptibility increased with temperature. This suggests that quantifying resistance under relatively high temperature conditions in the laboratory will not necessarily inform us to what extent a chemical intervention is still effective in the field.
There is growing evidence that the widespread use of LLINs and IRS is reducing mosquito activity indoors and can drive vector-species composition changes or host-species switching behavior to increase outdoor biting . Alternative interventions that specifically target outdoor biting are needed. One approach is to use chemical compounds to trap or repel mosquitoes, thereby reducing the number of mosquito bites to human hosts. There are reasons to expect that the effectiveness of such odor-baited traps could be affected by environmental temperature.
Chemicals are powerful tools in the control of malaria and other vector-borne diseases such as dengue, leishmaniasis, and Chagas disease . Given that temperature has the potential to affect the toxicity of chemicals used for ITNs, LLINs, and IRS, as well as to alter chemical release from and mosquito response to odor-baited traps, candidate chemicals need to be evaluated under relevant climatic conditions. For the initial development of chemicals to be used in the fight against malaria, we suggest that testing recommendations, currently at 25 to 27±2°C, should include a range of temperatures: 15, 20, 25, and 30°C. Such a change would provide valuable information about how mosquitoes and chemicals will interact under natural field conditions, therefore allowing us to develop more effective tools in the laboratory and to select the tools most likely to be effective in a given local environment. As insecticide resistance monitoring in the field is frequently carried out in areas where malaria is endemic (or epidemic), and these areas are often low-income countries, we suggest adding one additional temperature for these tests: 20°C. This change will give us a better understanding of how well the chemicals currently being used are working to control night-biting vectors. In areas where insecticide resistance has been detected in the mosquito population, such knowledge could be especially valuable. By applying a mixture of chemicals, which may also counter or postpone the development of insecticide resistance in mosquito populations to chemicals used on ITNs, LLINs, and in IRS , a given regimen could be efficient across different thermal environments, or in environments with a wide thermal envelope . We believe that considering the temperature coefficient of chemicals from the outset of testing will increase the effectiveness of the chemical toolbox for malaria vector control.