Mosquito Adapting to Global Warming, Study Finds

Bijal P. Trivedi
National Geographic Today
November 5, 2001
Scientists have found a mosquito that appears to have evolved and adapted to climatic changes induced by global warming— the first documented case of a genetic change in response to the apparent heating up of the planet.

Even more surprising, said evolutionary biologist William Bradshaw, of the University of Oregon, in Eugene, who led the study, is that this evolutionary change can occur in as little as five years.

Mosquitoes use the length of day to anticipate the oncoming winter and to plan hibernation. But with the onset of warmer winters mosquitoes are reproducing later in the year and postponing dormancy; instead of beginning hibernation in late summer when the days are still long, these mosquitoes are using fewer hours of daylight later in the year as their cue to go to sleep.

Bradshaw and colleague Christina Holzapfel found that compared with 30 years ago northern populations of the mosquito Wyeomyia smithii have adapted to milder winters and become dormant later in the annual cycle.

The mosquito, which lives at the base of the carnivorous pitcher plant, is found in North America. It is not a mosquito that sucks the blood of mammals but rather one that lives off its host plant.

In 1972 W. smithii mosquitoes from locations 50 degrees north (close to Sioux in western Ontario) began hibernation when the length of daylight was 15.79 hours. In 1996 W. smithii would have entered dormancy when the day length was 15.19 hours—this corresponds to about nine days later in the fall.

These results are published in the November 6 issue of the Proceedings of the National Academy of Sciences.

"This is not the type of experiment you can plan for," explained Bradshaw.

Bradshaw and Holzapfel have been collecting mosquitoes from 31 locations in North America for about 30 years.

Their original intention was to understand how the hours of daylight affect the length of the mosquitoes breeding season.

In Manitoba, Canada, for example, cool weather arrives early in August and mosquitoes begin hibernating in July when the days are still long. By contrast, mosquitoes in Florida don't begin hibernation until November when the days are short, as cooler weather does not strike till December.

The two populations of mosquitoes are genetically distinct; Manitoba mosquitoes, if brought to Florida, will not become dormant later in response to the warmer weather.

Response to day-length, or photoperiod, is a gene-based trait that affects the mosquitoes seasonal life cycle. Bradshaw and Holzapfel find that these northern mosquitoes are becoming more like their southern cousins suggesting that a genetic change has occurred.

Other populations also seem to be adapting to the longer growing season brought on by global warming. British birds began egg laying nearly nine days earlier in 1995 than in 1971. British frogs began spawning almost ten days earlier in 1994 than 1978.

The greatest consequence of altering the breeding seasons of insects such as W. smithii is a "discordance between predator and prey" says Bradshaw. Altering the breeding season affects all the creatures that rely on W. smithii as a source of food.

The great tit, for example, feeds on the mosquito larvae. However, if the mosquitoes have already hatched the chicks will go hungry.

Bradshaw anticipates that other species are also evolving and adapting to the extended growing season. Bradshaw and Holzapfel hope to identify the gene associated with response to daylight and the genetic changes responsible for delaying dormancy.

If there is some good news, it is that W. smithii doesn't bite.

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