Setting a Baseline: A Clearer View of Mosquito Resistance to Insecticides

Culex pipiens

Managing the resistance to insecticides among mosquitoes such as Culex pipiens is critical to effectively reducing their ability to spread disease. A new study in the Journal of Medical Entomology found varying degrees of resistance among several mosquito populations to six common insecticides. (Photo credit: Ary Farajollahi, Bugwood.org.)

By Andrew Porterfield

Mosquito control professionals have made significant gains toward managing malaria worldwide, as well as defending against West Nile virus, chikungunya, dengue, St. Louis encephalitis, and, more recently, Zika. However, mosquito resistance to existing insecticides also has been making gains. According to the World Health Organization, 60 countries have reported some mosquito resistance to at least one class of insecticide.

Andrew Porterfield

One challenge to managing mosquito resistance has been a lack of reliable information on the nature of resistance, including a dearth of baseline information on which mosquitoes survive which insecticides. A few studies have evaluated a few populations of mosquitoes for a few active ingredients, but no such studies examine several populations for several active ingredients from several different regions. A team of researchers, led by Stephanie Richards at East Carolina University, looked to change that, conducting the first large-scale baseline study in the United States on mosquito resistance, and they found degrees of resistance among two major mosquito genera to six common insecticide active ingredients.

Their study, published today in the Journal of Medical Entomology, showed that mosquitoes of the Aedes genus were less likely to show resistance than those in genus Culex, but both genera showed varying resistance to active ingredients.

Using WHO criteria to classify susceptibility (something that only a few states or even countries do) as “susceptible” (98-100 percent mortality), “possibly resistant” (80-97 percent mortality) and “resistant” (less than 80 percent mortality), the researchers found mosquito resistance to these common active ingredients:

  • Malathion: All Culex mosquito populations that were tested were resistant (4-64 percent mortality), while only Aedes triseriatus from Minnesota showed no resistance.
  • Etofenprox: All Culex were resistant, while three Aedes albopictus populations were susceptible, and 10 other species showed at least possible resistance.
  • Bifenthrin: All Culex were resistant, as were nine Aedes populations, while four Aedes and four Culex populations showed possible resistance.
  • Permethrin: Nine Aedes populations were susceptible, while the remaining four Aedes showed possible resistance. No Culex populations were susceptible.
  • Phenothrin: Twelve Aedes populations were susceptible, and one showed possible resistance, while no Culex were susceptible.
  • Deltamethrin: Eleven Aedes and five Culex populations were susceptible, while three Culex populations showed possible resistance and six showed resistance. One Aedes population (A. albopictus) from Florida was resistant.

Overall, Culex mosquitoes were 15 times more likely to show resistance than Aedes. Differences in resistance were more significant between genera than between regions where the mosquitoes lived. This was not a surprising finding itself, said Richards, because Culex mosquitoes are more active from dusk until dawn, while Aedes are more active during the day and thus were exposed to very different insecticide pressures due to exposure to sub-lethal doses of pesticides. “We did see that pyrethroid resistance is increasing due to increased usage, and it was interesting to observe the differences between the active ingredients.”

Large scale, baseline-setting studies like this one are key to understanding the dynamics and causes of mosquito resistance to pesticides. The U.S. Centers for Disease Control and Prevention is now encouraging pesticide control managers to evaluate resistance, including the issuance of guidance documents with protocols and bottle bioassay procedures. From these studies, local agencies can more easily analyze resistance and design more effective abatement programs.

“Only insecticides that are effective at killing mosquitoes should be used,” Richards said in an email. “Efficacy must be evaluated on a regular basis. These insecticides should only be used in a targeted surveillance-based manner, against potentially dangerous mosquitoes. Unfortunately, most spraying is used as a routine, reactive control,” which increases the risk of using pesticides that mosquitoes have evolved to resist.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.

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