Mosquitoes push back the northern limits with control capsule eggs to survive winters

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When the Asian tiger mosquito (Aedes albopictus) arrived in the United States in the 1980s, it took only a year for the invasive blood-sucker to spread from Houston to St. Louis. A new study from the Washington University in St. Louis shows that mosquitoes at the northern edge of their current range are successfully using egg-cape-like eggs to survive in colder conditions than those in their territory. native.

Northern mosquitoes have adapted to colder winters than their southern counterparts. This new evidence of rapid local adaptation could have consequences for efforts to control the spread of this invasive species, considered a "competent vector" for many human-relevant pathogens, including Zika viruses. , Chikungunya and dengue. The work is published in the August 21 issue of the Journal of Applied Ecology.

"All this has happened in the space of 30 years," said biologist Kim Medley, director of the Tyson Research Center and first author of the new study. "This disease vector has evolved rapidly to adapt to the US The fact that this occurred at a range limit suggests that the species may continue to move. further north. "

Mosquitoes respond to shorter days marking the beginning of winter by laying diapause eggs – literally, delayed-development eggs. These special eggs contain a fertilized embryo in the state of near-hibernation and whose metabolism is very slow. The result is almost a time capsule against mosquitoes.

The ability to produce eggs that can wait to hatch is not new. This technique helps mosquitoes survive winter cold, but it also works in dry weather. All mosquitoes lay eggs in or near stagnant water, and the larvae hatch in standing water. But they can survive drying out between the two.

However, diapause eggs are different from ordinary eggs. Previous research had shown that northern mosquitoes lay more diapause eggs than their southern cousins. What the researchers did not know was the actual performance of these eggs in the conditions in which they were prepared.

For this new field experience, Medley and her team, including Katie M. Westby, Postdoctoral Fellow at the Tyson Research Center, collected eggs and larvae of live mosquitoes in towns near the center of the habitat. they have invaded (Huntsville, Alab., Macon, Georgia, Beaufort, South Carolina) and about the northern limit of their US range (Peoria, Illinois, Columbus, Ohio, and Harrisburg, Pennsylvania). The researchers hatched and raised these mosquitoes and their subsequent generations in the laboratory.

Then it was time to get cold. The researchers exposed mosquitoes to shortened light periods to signal the onset of winter. They collected the diapause eggs produced by the mosquitoes, then sent batches of eggs that experienced true winters at four different locations: on the ground, at the northern limit and at the core of their current range; in a climate-controlled laboratory site that represented the "optimal" winter conditions in the area of ​​origin of mosquitoes in Japan; and in a site very north of Wisconsin, significantly outside the current range of mosquitoes.

Once the real winter was over, the researchers took the eggs back to the lab and hatched them.

"We counted all the eggs to see how many survived the winter in all these places," Medley said. "What we have learned is that northern mosquito diapause eggs have far better survived the northern winters than the southern mosquito eggs.

"Everyone is doing well in the southern winter," she said. "They played pretty much the same thing." The same goes for the members of the room at optimal conditions. As for Wisconsin? Well …

"Nobody has survived the winter of Wisconsin," Medley said.

Although the conditions in Wisconsin are too difficult for these mosquitoes – at least for the moment – Medley is particularly interested in the changes that she is observing at the very edge of what can survive.

"These northern mosquitoes produce a lot more diapause eggs," said Medley. "We now know that these eggs are also much better in the winter."

What Medley and his team have learned is important not only for this species, but also for ecologists who are studying how animals adapt to the new conditions and pushing the boundaries of their historical ranges.

"Based on the theory, we expect populations at the edge of the range to be small, fragmented and with low genetic diversity," she said. "It is thought that these populations will not have the demographic and genetic robustness to adapt, so they remain in this state of maladjustment.

"This may not be the case with this species," Medley said.

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