Research reveals mischievous mosquitoes learn to avoid pesticides

By Mark Waghorn via SWNS

Pesky mosquitoes learn to dodge pesticides, according to new research.

They recognize the smell and steer clear - making them ineffective, say scientists.

The clever critters are known for their itchy bite - and carry malaria and other deadly diseases.

Pesticides are used to limit the spread. They pick up the scent after a single exposure.

The study, published in the journal Scientific Reports, shows the insects are more intelligent than previously thought.

Corresponding author Professor Frederic Tripet, of Keele University, Staffordshire, said: "Understanding learning and memory is important for insects that transmit human diseases.

"Behaviour such as host finding and blood-feeding have critical implications for disease transmission."

Vector-borne diseases represent 17% of the global burden of communicable illnesses - and 80% of the world's population is at risk.

Prof Tripet said: "Among them, mosquito-transmitted diseases such as malaria, chikungunya, dengue, yellow fever, Zika and West Nile fever cause millions of cases every year and tropical and subtropical regions are particularly affected."

The international team exposed female Aedes aegypti and Culex quinquefasciastus mosquitoes - which transmit dengue, Zika and West Nile fever - to non-lethal doses of five pesticides.

They included malathion, propoxur, deltamethrin, permethrin and lambda-cyhalothrin.

In tests, subsequent exposure to the same one deterred mosquitoes from feeding and resting - increasing survival.

Prof Tripet said: "Insects use a range of senses such as sight, smell, taste and hearing to navigate their environment.

"Under challenging variable conditions, the sensory and cognitive abilities of insects enable them to modulate their behavior to ensure their survival and reproduction.

"Learning plays a key role in behavioral plasticity by allowing sensory stimuli to become associated with positive or negative experiences, thereby enabling novel adaptive responses.

"Different insect studies have shown that traits such as foraging, feeding, oviposition and mating all involved associative learning."

Mosquitoes that had been pre-exposed to a pesticide avoided a pesticide-treated net in order to reach a food source.

Only 15% of A. aegypti and 12% of C. quinquefasciastus that had been pre-exposed went through - compared to 58 and 55 percent of respective peers.

Subsequently, the survival rate of pre-exposed mosquitoes nearly trebled.
almost four in ten (38%) of A. aegypti and a third (32%) of C. quinquefasciastus.

This was compared to 11 and 13 percent of their respective peers.

They were also more likely to rest in a container that smelt of a control substance, rather than a pesticide.

This applied to Three quarters of A. aegypti and 83 percent of C. quinquefasciastus compared to 50 percent of the others.

Prof Tripet said: "The most important mosquito vectors are Anopheles, Aedes and Culex species that feed preferentially on humans inside
and around their dwellings.

"In such setting, chemical vector control measures in the form of insecticide treated bed nets (ITNs), indoor residual spraying (IRS) and outdoor space spraying have proved the most effective in reducing mosquito populations and disease transmission."

All the experiments were carried out on females - as only they drink blood.

Prof Tripet said: "These results showed that, following a single
exposure, mosquitoes can associate the olfactory stimulus of pesticides with their detrimental effects and subsequently avoid pesticide contact.

"Findings highlight the importance of mosquito cognition as
determinants of pesticide resistance in mosquito populations targeted by chemical control."

He added: "This study provides the first comprehensive experimental evidence of learned pesticide avoidance in females of two major vector species, A. aegypti and C. quinquefasciatus."