Pesticides are often used to control the spread of mosquito-transmitted diseases, such as malaria or Dengue fever. However, pesticide-resistance among mosquitoes has significantly increased in recent decades, making scientists worry that some of the most successful ways of controlling many severe infectious diseases may stop functioning in the near future.
According to a new study led by Frédéric Tripet, the director of the Center for Applied Entomology and Parasitology (CAEP) at Keele University in the United Kingdom, female mosquitoes are able to learn to avoid pesticides after a single exposure.
Professor Tripet and his colleagues exposed female Aedes aegypti and Culex quinquefasciastus mosquitoes (which transmit severe illnesses such as Dengue, Zika, or West Nile fevers) to non-lethal doses of some of the most frequently used anti-mosquito pesticides, including malathion, propoxur, deltamethrin, permethrin, and lambda-cyhalothrin. Afterward, they tested whether subsequent exposures of these mosquitoes to the same pesticides deterred them from feeding and resting in the proximity of these substances.
The scientists discovered that mosquitoes which were pre-exposed to these pesticides avoided passing through a net treated with the same pesticide in order to reach food sources. Only 15.4 percent of A. aegypti and 12.1 percent of C. quinquefasciastus which were pre-exposed passed through the net in search of food, compared to 57.7 percent of A. aegypti and 54.4 percent of C. quinquefasciastus from a control group. Subsequently, the survival rate of pre-exposed mosquitoes was more than double that of mosquitoes in the control group.
The researchers also found that pre-exposed mosquitoes were more likely to rest in a container that smelt of a different substance, rather than in one that smelt of the pesticide to which they have been exposed, with 75.7 percent of A. aegypti and 83.1 percent of C. quinquefasciastus choosing to rest in the pesticide-free container (compared to 50.2 percent of A. aegypti and 50.4 percent of C. quinquefasciastus from the control group).
These findings suggest that mosquitoes that have been already exposed to non-lethal doses of pesticides learn to avoid such substances in the future – a behavior which increases their survival and reproduction rates, and makes them even more dangerous vectors of disease.
The study is published in the journal Nature Scientific Reports.