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Aedes albopictus
Aedes albopictus
Photo: James Gathany/Centers for Disease Control and Prevention (AP)

Mosquitoes didn’t become the most prolific animal killer of humanity by being lazy. A new study this week suggests that a common disease-causing species in the U.S. has learned how to lay dormant eggs that can survive harsher winters in the North.

The Asian tiger mosquito, or Aedes albopictus, is thought to have first invaded the U.S. in the mid-1980s, establishing itself in Texas. It rapidly expanded its range and is now found throughout much of the Southeastern and Central U.S. While its related cousin Ae. aegypti is more likely to cause disease, Ae. albopictus is still capable of spreading Zika, dengue, West Nile, and other brain-infecting viruses.

One strategy that has allowed the Asian tiger mosquito to survive in more temperate weather involves its eggs. When the mosquitoes “sense” rough conditions ahead, meaning the longer nights caused by winter, they lay more eggs capable of going into dormancy. Once the days are longer and conditions are again ripe for growth, these eggs wake from their slumber and hatch as normal. As a release from Washington University in St. Louis put it, the eggs are like mosquito time capsules.

The authors of this new study, published in the Journal of Applied Ecology, wanted to see how durable these dormant eggs really were, and whether there were any key differences between populations living in the warmer South and those living on the edge of survival in the North.

They scooped up eggs and larvae from mosquitoes living in the North and South, raised them for several generations, then left new egg batches before the winter in various spots throughout the country. Once winter was over, they were collected so the researchers could see how many hatched successfully.

The current range of Aedes albopictus, but who knows for how long? The circles represent where the mosquitoes in the study were first collected; the squares where the new batches were left for the winter.
The current range of Aedes albopictus, but who knows for how long? The circles represent where the mosquitoes in the study were first collected; the squares where the new batches were left for the winter.
Illustration: Medley, et al. (Journal of Applied Ecology)

In what might be the only good news, the eggs weren’t able to survive as far north as Wisconsin, no matter where they first came from. Northern and Southern mosquito eggs survived equally as well in warmer areas near the core of their current range in the U.S., as well as in a lab that simulated the typical winter in Asia where the species originated. But around three-quarters of Northern eggs were able to outlast the winter in Pennsylvania, the northernmost range where they live regularly—that’s nearly double the percentage of Southern eggs that survived.

What the findings suggest, according to the authors, is that Northern tiger mosquitoes have acclimated much better to their new homes than expected. And they’ve managed to accomplish this feat in an incredibly fast amount of time, evolutionarily speaking.

“This all happened within a period of 30 years,” said lead author Kim Medley, director of the Tyson Research Center at the Washington University in St. Louis, in a release from the university. “This disease vector has evolved rapidly to adapt to the United States. The fact that this has occurred at a range limit may suggest that there is potential for the species to continue to creep farther northward.”

Medley and her team speculate that the continued adaptation by these mosquitoes “coupled with warming climate could increase the risk of disease exposure for human populations.” So while northern states might be safe for now, that could soon change. And it seems like the mosquitoes may be a lot better at adapting to a changing world than we are.


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