In the midst of the EEE crisis, a new mosquito is quietly making a home in the southeast part of the state. The Asian tiger mosquito can spread tropical diseases like Zika and dengue fever, and scientists say a warming climate is clearing the mosquito's path north.

About two years ago, Todd Duval got a call from a New Bedford homeowner. The woman’s children were being attacked by relentlessly aggressive mosquitoes in her yard. The kids had sustained so many bites that the woman took them to the doctor to make sure they’d be OK.

Duval, an entomologist with the Bristol County Mosquito Control Project, visited the home. He found plastic toys scattered throughout the yard, corrugated downspout extensions on the home’s gutters and buckets for collecting rainwater that the woman used to water her tomatoes.

The containers were an ideal breeding ground for a species of mosquito that has been threatening to colonize Massachusetts for a few years: the Aedes albopictus, or Asian tiger mosquito.

The first Asian tiger mosquito in Massachusetts was found in New Bedford in 2009. Over the next few years, Duval and other mosquito trackers found the tiny day-biters buzzing around traps in increasing numbers, mostly in New Bedford, Fairhaven and, in some years, a neighborhood in Dartmouth.

They’d started to reproduce.

In 2010, Bristol County mosquito control personnel usually found one of the black-and-white mosquitoes in each of its traps - $300 lobster trap-like devices loaded with bait that smells like sweaty gym socks and body odor. But by 2016, that number had jumped to an average of 23 per trap, an increase of more than 2,000% in just six years.

“It just kept creeping up and creeping up,” Duval said. “It was almost an exponential curve starting off until about 2017.”


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In 2017, the number of Asian tiger mosquitoes found per trap dropped to about seven. That’s because, for what is likely the first time in Massachusetts history, the state began chemically treating select locations with the goal of killing Asian tiger mosquito larvae.


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At a rough rate of 37 miles per year, the Asian tiger mosquito has steadily crept north, hitching rides from warmer climates in family cars, shipping containers, 18-wheelers and boats.

Interstate 195 has long been a sort of dividing line, representing an impenetrable wall of air too cool for the Asian tiger mosquito, infamous for spreading debilitating, sometimes deadly diseases - including Zika, dengue fever and chikungunya - to make a home.

But the climate is changing, with scientists predicting that by the 2080s, climates in Northeast cities “will tend to feel more like the humid sub-tropical climates typical of parts of the Midwest or southeastern U.S. today.”

The fact that the Asian tiger mosquito has breached the Massachusetts border means the state’s climate — at least in some areas — is already becoming more amenable to the species.

The question is how fast it will continue its northward march.

Dr. Catherine Brown, an epidemiologist for the state Department of Public Health, said climate change will play a role in how quickly the species spreads.

“We do know that it has been gaining the capacity through evolution to be able to survive cooler and cooler temperatures,” Brown said. “If you combine the fact that it can survive cooler temperatures with the fact that we have this warming climate, I think in 20, 25 years things could look very different.”


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Mosquitoes are extremely sensitive to rainfall and temperature, with higher temperatures and more rain causing the insect’s life cycle to speed up. Duval, Bristol County’s entomologist, said the EEE crisis currently rocking the state is an example of how warm and wet weather conditions can send mosquito populations soaring.

“I was expecting this to be a rough year, based on just the type of weather that we had,” Duval said. “We had an awful lot of rainfall in the fall. It was more of a mild winter than we usually see. When it came to the spring, there was an awful lot of rainfall in the spring again, which re-floods the swamps and allows the mosquito larvae to move around.”

 

The EEE virus is currently being spread by native mosquitoes. And while the Asian tiger mosquito is unlikely to pick up EEE because of its feeding habits, lab studies have shown it has the ability to carry it. Now regularly found across southeastern and lower Midwestern states, the Asian tiger mosquito can carry “pretty much anything,” Duval said.

Every Monday morning during mosquito season, Bristol County mosquito control staff drive containers full of mosquitoes to the Massachusetts Department of Public Health laboratory in Jamaica Plain to be stored in freezers. Duval sends every Asian tiger mosquito he catches.


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Brown, who works at the lab, said none of the Asian tiger mosquitoes trapped in the state has been tested for disease yet because, at least in Massachusetts, they aren’t carrying the tropical viruses the species are known for.

A lot would have to change for Massachusetts’ Asian tiger mosquitoes to spread diseases, Brown said.

“We would have to have a large influx of infected people all at the same time, and we would have to put them in the same geographic area as these mosquitoes,” Brown said. “Or we would have to have more of these mosquitoes, larger populations, in order for local transmission to occur.”

In the short and medium term, those are “very unlikely” scenarios, Brown said.

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Still, Brown is keeping an eye out. If a Massachusetts resident is infected with a tropical disease, like Zika or dengue fever, and he or she hasn’t traveled to a country where those diseases are widespread, the Aedes albopictus will come out of the freezers in the state laboratory for testing, she said.

In the future, Brown thinks the state will have to re-evaluate where the Asian tiger mosquito ranks in terms of public health priorities.


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“What I think about when I think about Aedes albopictus is what is this going to mean down the road?” she said. “As circumstances change, we will need to reconsider what we're doing with Aedes albopictus and be able to ramp up our response to it.”