Climate change may allow mosquitoes, rodents to proliferate

Photo Courtesy: Wikimedia Commons

By Lindsey Vickers
BU News Service

BOSTON — With the leaves falling, the first frost behind us, and the National Oceanic and Atmospheric Administration predicting a mild, but wet winter for the Northeast, citizens across Massachusetts are likely looking forward to pumpkin spice lattes, cozy sweaters, and a break from mosquito bites and ticks.

But, the question of how pest populations will respond to climate change in Massachusetts, and around the world, lingers.

Pests have been a longstanding issue for the commonwealth. This summer and fall, mosquitoes were particularly troublesome, with 12 human cases of the mosquito-transmitted eastern equine encephalitis virus.

While it is impossible to determine exactly why Massachusetts experienced such a large EEE outbreak this year, climate-related factors, including substantial rainfall and warmer temperatures last fall and winter, may have played a role, according to Todd Duval, an entomologist with the Bristol County Mosquito Control Project.

Temperatures across the contiguous United States have risen: Last year’s nationwide average was 67.53 F, or 3.68 F warmer than the mean 20th-century temperature of 64.86 F, according to data from NOAA.

Massachusetts is no exception. Data on average annual temperatures across Massachusetts show a gradual increase. Last year Boston’s average temperature was 49.8 F, or 2.88 F warmer than the mean 20th-century temperature of 46.9 F, according to data from NOAA.

“Annual air temperatures in the Northeast have been warming at an average of 0.5 degrees Fahrenheit per decade since 1970. Winter temperatures have been rising at a faster rate of 0.9 degrees per decade on average,” according to a guide on climate change published by Massachusetts government in 2017.

Climate changes, like the mild winters Massachusetts may experience in the future, will affect not only humans, but also pests. Cities and states across the Northeast have already reported problems attributed to climate change, with Cape Cod seeing faster rates of coastal erosion, and New York City reporting an increase in rat complaints.

Mosquitoes inhabiting the commonwealth have already changed. In recent years, a new species was found in Massachusetts. Unlike many others, the Asian tiger mosquito is active during the day, rather than around dusk, and is known for being an aggressive biter.

The species was introduced to the U.S. in the mid-1980s by way of shipments from Asia, according to the Center for Invasive Species Research at University of California Riverside. Since then, the mosquito has spread to 26 states.

Just a few years ago, the Asian tiger mosquito’s northernmost established populations were in New Jersey, southern New York, and Pennsylvania, according to a research paper published in 2013. Since then the species has crept into Massachusetts, with increasing numbers reported and trapped since 2009, according to Duval.

The Asian tiger mosquito is “very portable.” In fact, Duval said it’s possible that the species could be introduced to new habitats via car, escaping through the window, or an open door. The question then is “whether or not they’ll actually survive and successfully reproduce.”

According to recent data from the CDC, the Asian tiger mosquito is classified as “very likely” to be able to survive and reproduce in areas along and near the Massachusetts coast. Throughout the rest of the state, it’s viability is “unlikely.”

Fortunately, this species is unlikely to start rapidly spreading disease in Massachusetts. This is due to limited habitat overlap with the wild bird population, a primary carrier of EEE, West Nile virus, and other mosquito-borne diseases, Duval said. Asian tiger mosquitoes tend to prefer urban environments and like small water containers for laying eggs, sometimes in something as small as a bottle cap.

“While it isn’t much of a disease risk, it is an incredible nuisance,” Duval said. “Its aggression, combined with the fact that it is active throughout the day rather than just at dusk, makes it is difficult to live with, to say the least.”

Duval said that this year Massachusetts has had significantly less rain over the summer and fall, which improves the mosquito outlook for next year.

“August and September is when [mosquitoes] start laying down overwintering eggs.” This is true of the cattail mosquito, a key EEE transmitter, that generally survives winter as larvae, he said.

“As the swamps dry up, which they are pretty dry right now, the amount of available habitat for larvae shrinks. This time last year, we probably had 10 more inches of rain than we did this year, so all these little woodland pools and swamps were quite full,” Duval said.

Mosquitoes are always affected by annual weather cycles and patterns, so if projections for Massachusetts hold true, and winters become milder and wetter, mosquitoes may have more space and opportunities to overwinter and survive.

Another multi-legged fiend Massachusetts residents worry about is the blacklegged deer tick.

Unlike mosquitoes, evidence for climate change’s impact on tick populations and lifecycles is dubious, at best, according to Stephen Rich, a professor of microbiology at University of Massachusetts at Amherst. His research focuses on diseases that are maintained in animals, and sometimes transmitted to humans, such as Lyme disease and EEE.

While mosquitoes react to short-term climate swings, such as precipitation increases or decreases, changes in tick population and tick-borne diseases will be less affected by seasonal and interannual shifts, according to a paper published by the Trends in Pathology magazine in 2016.

This is in part due to the tick’s longer, predetermined life cycle.

“It’s not like [the tick’s life cycle] is necessarily driven by seasonality, it’s that they have this sort of innate life cycle trajectory,” Rich said.

Increases in Lyme disease cases over the last 40 years can be explained by other factors, he said.

“There are lots of things independent of climate change that are much more plausible explanations for why there’s been an explosion of Lyme disease,” including landscaping, he said.

Aesthetics may come at a cost, in terms of ticks and disease. “Just the fact that we like our manicured lawns … and here in New England, there’s a tendency to have nice stone walls. All those things create a great habitat for mice, which are the primary reservoirs of the pathogens,” he said.

All of this is compounded by how difficult it is to measure tick populations.

“We don’t really know the drivers of tick density … it can vary greatly from one side of a driveway to another … and it’s really tough to measure,” Rich said.

Rats, like ticks, are difficult to count, especially in cities. While New York residents have reported more rodents in the last few years, likely due to warmer winters, people in Massachusetts shouldn’t rush to their nearest Home Depot to load up on rat poison and traps.

Rodent experts rely on “crude measures to estimate their populations, [for example], the number of complaints from people calling 311,” said Michael Parsons, a professor at Fordham University who studies urban ecology and rodents. Unfortunately, these measures aren’t perfect for estimating population.

“Those numbers tend to be biased because people who previously reported rats, but to no avail, no longer bother reporting them, because they received no benefit from doing so.”

The Boston 311 line has seen a decrease in calls about rodents over the past three years, dropping from 3,735 calls in 2017 to 3,382 in 2018, according to Brittany Silva, who works in the Boston Inspectional Services Department. As of Oct. 25, they had received just over 3,000.

Though it is always difficult to determine which environmental factors may influence rodent population, rats seem to fare better during warm winters. This could be due to increased human outdoor activity leading to more food waste and littering, Parsons said.

In addition, warmer winters would allow rats to spend more time outside their underground burrows, offering them additional opportunities to mate and reproduce.

Though ticks may be a wild card, climate change bodes well for rodents and mosquitoes, and by contrast, not so well for people trying to live without these pests.

If carbon emissions remain high, by 2080, states across the Northeast “will tend to feel more like the humid subtropical climates typical of parts of the Midwest or southeastern U.S. today,” according to a paper by Matthew Fitzpatrick, a scientist at the University of Maryland’s Center for Environment Science, and Robert Dunn, a professor of applied ecology at North Carolina State University.

The paper matches the projected future climates of specific cities to a city with current climate conditions that are representative, a technique called “climate analog mapping.”

The University of Maryland’s Center for Environment Science’s interactive map predicts Boston will follow this projection. With high emissions, Boston’s climate in 2080 will likely be similar to that of a city several hundred miles to the south, Rosedale, Maryland, where the season is generally warmer and wetter.

Questions about climate change and pest populations will always be followed by caveats, and information about rodents, ticks, and mosquitoes will become clearer with time. It is hard to predict precisely what will happen as Massachusetts’ climates change, but for now, don’t drop too much money on rat traps.

In the meantime, one of the best things people can do is take precautions when they might be exposed to tick or mosquito-borne diseases, by wearing bug repellents, and long clothing and being mindful of potential tick habitats.

This article was originally published in the Worcester Telegram.

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