Climate Change Could Mean More Mosquitoes…or Fewer

by Katharina Buchholz

Stegomyia aegypti mosquito biting a human. (Photo/USDA)

Climate change can mean a higher risk of mosquito-borne disease due to increased precipitation. But increasingly hot temperatures could reach a threshold that will actually diminish mosquito populations.

These are the findings of Andrew Comrie’s Dynamic Mosquito Simulation Model. In a recent talk at CU Boulder, entitled Catching Climate Fever: Diagnosing the Changing Environment of Infectious Disease, Comrie stressed that the link between climate change and mosquito populations must be evaluated on a case-to-case basis.

“Climate is changing, most definitely, no question about it,” said Comrie, the dean of the graduate college at The University of Arizona. “But climate change is hard to understand. The world warms three degrees–what does that mean? We just warmed five degrees walking into this room.”

Comrie told the 35-member audience that even if mosquito populations and mosquito-borne diseases are associated with a certain high temperature pattern, the two don’t necessarily move in tandem.  High temperatures may move into formerly temperate regions, but the diseases will not necessarily move with them.

“It is completely unrealistic ecologically,” he added.

Comrie’s model uses variables from actual mosquito habitat—temperature, precipitation, daylight, evaporation and land versus water surface—to predict the response of mosquito populations to climate change.

“We start with one theoretical egg,” Comrie said. “We do the math every day. Is it warm enough? Is it wet enough? We take all the logical decisions and put them down on virtual paper.”

According to Comrie’s model, climate change will make some places more hospitable to mosquitoes and other places less so. For example, if the average temperature in the U.S. increased by 1.5 degrees Celsius, causing drier winters and wetter summers, his model predicts an earlier mosquito season in Florida.

“In April, even though it’s drier, there are more mosquitoes. The temperature is winning over the precip(itation),” Comrie said.

The situation would be reversed in California. “We have fewer mosquitoes, especially in the Central Plains. Even though the weather in California would have 20 to 30 percent more rainfall in July, it would not offset the fact it is just too hot for those mosquitoes,” Comrie said.

Another advantage of the model is its ability to incorporate spikes of extreme rainfall.

“The average is important, certainly, ecologically, over a long period of time, but for health, often it’s not just the averages, but the extremes that count, ” Comrie said. Comparing precipitation and mosquito population charts of Tucson, Ariz., from the 50s, he noted that in 1954 there were almost no mosquitoes, but populations exploded in 1955. ”The total (precipitation for both years) might have been the same, but the timing was totally different,” Comrie said.

The model predicts the influence of environmental factors by using data collected from mosquitoes in a laboratory. Comrie tested his theoretical mosquitoes against real mosquito populations counted in DeSoto County, Fla. Even though some spikes on the graph didn’t match, the model was able to predict mosquito seasons and the severity of mosquito population increases with relative accuracy.

Comrie said both methods of measuring mosquito populations are estimates, not actual numbers. “We have something that does the basic job of a good theoretical mosquito. This is theory; let’s go and do practice.”

Audience member Stacy Smith was more critical about the possibility of implementation. “It is a predictive model, so it can give communities some guidelines,” said the geography major. “But the way it is right now, it can’t be given to the public.”

Geographer Petra Norlund said that policy and methodology for climate change studies are still two separate camps. “It is hard to bridge the gap. Policy makers are not up to date. Methodology makers don’t really branch out into policy,” she said.

The doctoral candidate added that Comrie’s work marked a rare case of applied, case-based climate research. “A lot of climate change research has not been done right,” she said.

The University of Colorado Boulder’s Geography Department hosted the event as part of its colloquium, Global Climate Change: A Geographic Perspective.

Katharina Buchholz earned her undergraduate degree in American studies and sociology from the University of Hamburg, Germany. After completing assorted internships in print and public relations, she is now pursuing a graduate degree in print journalism at the University of Colorado Boulder. Katharina likes to explore the surroundings of her new home, Boulder, on foot, by bike or, most recently, on skis, which she considers an accomplishment for a flatlander like herself.