One might think that they are only driven by simple bloodlust – but mosquitoes are also capable of “clever” behaviors, as a study shows: If they have just escaped death from an insecticide, the insects then avoid the smell of it experiments. The ability to learn could thus play a role in the resilience of dangerous disease vectors to control measures, say the scientists.
As is well known, they are not only hated pests – mosquitoes are considered to be the most dangerous animals in the world: some species spread diseases such as malaria, dengue and yellow fever, from which at least half a million people die every year. Various strategies are currently being worked on to contain the problem. However, an important weapon in the fight against mosquitoes is still the chemical club: different active ingredients are used in different ways. An important strategy is to treat mosquito nets or surfaces with the active substances on which the bloodsuckers settle. Through contact, they then absorb the toxins and die – that’s the concept.
Because of simple bloodsuckers
However, the use of these methods has already led to the development of resistance in mosquitoes: they form stronger protective layers or produce active substances that break down the toxins. So far, however, less attention has been paid to another possible aspect that could limit treatment effectiveness: It seems possible that mosquitoes can learn to avoid insecticides at the individual level. Because it is already generally known that mosquitoes can use certain experiences for themselves. For example, they can learn to associate previously unfamiliar scents with a chance at a blood meal. As part of their study, the researchers led by Seynabou Sougoufara from Keele University (UK) have now explored the opposite effect: do mosquitoes learn to associate certain odors with sublethal poisoning?
They conducted experiments with two notorious mosquito species: the yellow fever mosquito Aedes aegypti and the southern house mosquito Culex quinquefasciatus. They placed some of the animals in paper-striped jars containing non-lethal doses of the common anti-mosquito pesticides malathion, propoxur, deltamethrin, permethrin and lambda-cyhalothrin. Although the animals apparently survived this unscathed, it can be assumed that they felt the poison – they probably felt bad. Mosquitoes, which had to stay in the same vessels, served as controls – but without poison. Instead, the paper there was saturated with harmless oil. The researchers then conducted experiments to investigate the extent to which the treated mosquitoes exhibited abnormal behavior when confronted with the pesticides in different ways.
Survival advantage through clever behavioral adaptation
The investigations showed that the mosquitoes that had previously been exposed to one of the insecticides were remarkably often reluctant to cross a hole in a net treated with these agents in order to reach a tempting source of blood: the majority of the untreated test animals squeezed themselves through the opening, while only around 15 percent of the “experienced” mosquitoes dared to do so. Subsequent investigations made it clear how sensible the “wise” decision had been: during the passage, the mosquitoes came into contact with the poisoned net and often died from it. The bottom line is that the treated mosquitoes benefited from their avoidance behavior through a high survival rate.
The researchers also found that the pre-exposed mosquitoes generally avoided the smell of the pesticides. In these experiments, the animals were given a choice of two containers for resting, one smelling of pesticide and the other smelling of a control substance. The evaluations showed that 76 percent of the A. aegypti and 83 percent of the C. quinquefasciatus that had previously been exposed to a pesticide were resting in the pesticide-free container. The inexperienced controls, on the other hand, did not show this preference and equally sought out the container smelling of insecticide.
Significance for control strategies
According to the researchers, this shows that the behavioral adaptation experienced by mosquitoes is of considerable importance: they are more likely to look for food sources and resting places, which increases their chances of survival and reproduction. As the scientists explain, it can also be assumed that non-fatal pesticide experiences occur very frequently in practice. Because if surfaces are not continuously wetted, the concentrations can easily fall below the lethal level. In this case, the toxins can then make the mosquitoes smart in a way that is problematic for us.
“The results underscore the role of mosquito cognitive abilities in pesticide resistance in chemically controlled mosquito populations,” the scientists write. They are therefore now calling for more detailed research into the basics of behavioral flexibility in these insects. “Ultimately, a better understanding could lead to more effective control strategies that circumvent the mosquito behavioral resistance aspect,” Sougoufara and his colleagues say.