In places where malaria is rampant, a common defense is to sleep under a insecticide-laden mosquito net. The nocturnal mosquito of the genus Anopheles can't penetrate the netting, and will die from the insecticide if they try. But malaria rates did not plummet as expected. Did the bed nets fail? No, the mosquitos changed -they stopped being strictly nocturnal and started feeding at dawn, after their human buffets got out of bed!
First, we don’t know yet if this was an evolutionary (i.e., genetic) change or a purely behavioral change. It is possible that there was quite a lot of genetic variation in timing of activity in the population a few years ago and that the bed nets provided a selective regimen that skewed the population to consist mainly of late night and dawn-active individuals. It is also possible that there is sufficient behavioral plasticity in the mosquito allowing it to learn the new best time of day to go out foraging. I’d love to see the mosquitoes placed in isolation chambers to monitor purely genetic patterns of circadian rhythms of activity.
If the change is genetic, meaning the circadian rhythm is inborn and the adaptive behavior is passed to future generations, malarial mosquitos may be much harder to defend against. If this is a learned behavior, well, we have a ready-made horror movie plot. How scary are mosquitos that can learn? Link -via Ed Yong