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Mosquitoes arrive earlier, stay later in Philly region because of climate change, data suggest

Communities across the Philadelphia region are seeing more mosquito-friendly days, as they warm because of climate change, according to data compiled by Climate Central. Philly has gone from 153 such days in the 1980s to 163 now. Allentown has gone from 128 to 142.

In this Aug. 26, 2019 file photo, a biologist examines a mosquito.
In this Aug. 26, 2019 file photo, a biologist examines a mosquito.Read moreRick Bowmer / AP

Mosquitoes in Philly and elsewhere are feasting on flesh earlier in the summer and prompting thigh-slapping swats later in the season than before, new data suggest.

In some areas of Pennsylvania, New Jersey, and Delaware, mosquitoes are around weeks longer than in previous decades because of the changing climate, according to Climate Central.

The nonprofit group of scientists and journalists calculated the number of mosquito-friendly days in the Philadelphia region and found they are growing.

The insect survives best at temperatures between 50 and 95 degrees Fahrenheit with relative humidity of 42% or more, according to studies by the National Institutes of Health. Climate Central analyzed weather data from 1979 on, and filtered for days meeting those conditions.

Philadelphia went from 153 such days in the 1980s to 163 in the 2010s, according to the data. Allentown went from 128 to 142. And Atlantic City went from 142 to 155.

State College, Pa., has seen nearly a month of additional mosquito-friendly days.

Climate Central found the number of mosquito days has increased in 64% of the 239 U.S. locations it examined for the two periods, 1980-89 and 2010-19. The biggest increases were in the Northeast, Pacific Coast, and Ohio Valley.

The average temperature in July 2020 for Philadelphia was 81.9 degrees, which is 3.8 degrees above normal. It was Philadelphia’s third-hottest July since the government began keeping score in 1871 and tied a record for 90-plus temperatures.

And it’s part of a trend. Philly’s average summer temperature has risen about 2.8 degrees from 1970 to 2019, according to another analysis by Climate Central.

But even mosquitoes have their limits. The South is seeing fewer mosquito-friendly days because it is getting too hot in some locations for the insects to thrive, according to the Climate Central analysis.

The data suggest that more weather conducive to mosquitoes’ survival can mean an increased risk of diseases they transmit, such as West Nile virus.

West Nile is the leading cause of mosquito-borne disease in the United States, according to the U.S. Centers for Disease Control and Prevention. The mosquitoes get infected from feeding on birds carrying the disease, then transmit it to humans.

Though there are no vaccines to prevent the disease or medications to treat it, most of those infected do not feel sick. Those who do can develop a fever. The disease does prove serious or fatal in about 1 out of 150 cases.

Matt Helwig, a West Nile specialist for the Pennsylvania Department of Environmental Protection, cautioned that more mosquitoes doesn’t automatically translate into more cases of the disease.

For example, Helwig said, 2018 saw a big outbreak of the disease. But 2019 and, so far, 2020, have proven minimal for West Nile, because birds that transmit the virus to mosquitoes developed an immunity after that outbreak. Only one probable human case has been reported this year in Pennsylvania.

“It’s going to take a bit of time to ramp back up,” Helwig said.

The DEP has detected West Nile virus-infected mosquitoes in five counties. Non-human infections have been reported in New Jersey as well. None have been reported in Delaware.

The Asian tiger mosquito, Aedes aegypti, poses another concern. The invasive species is an aggressive biter that’s pushed north into Pennsylvania as the weather has become warmer. The mosquito is more typically found in tropical areas, such as South America, Central America, and Southeast Asia and can carry the Zika virus.

The chances of Zika transmission are likely lower now because of the coronavirus pandemic, Helwig said. That’s because most Zika cases in the U.S. occur when a mosquito bites a traveler infected in another country, and the pandemic has canceled most travel. So far, no local transmissions have occurred in Pennsylvania.

Nate Wardle, a spokesperson for the state Department of Health, said that warmer temperatures also mean other pests that pose a danger to human health can thrive.

He said the blacklegged tick (deer tick) that transmits Lyme disease has begun to survive in the winter months due to milder weather. And the Lone Star tick, once confined to the South, is now present in Pennsylvania. It can cause ehrlichiosis and alpha-gal allergy, a severe immune response that affects people after they have eaten red meat.

Though mosquitoes can bite at any time, they are most active at dawn and dusk. Experts recommend that people use a repellent containing DEET, cover exposed skin with lightweight clothing, and make sure window screens are in good condition.

The mosquitoes that transmit West Nile breed in areas with standing and stagnant water, such as catch basins, clogged gutters, old tires, poorly maintained swimming pools, bird baths, flower pots and other types of containers. Any of these located near your home should be eliminated.

Where that’s not possible, officials recommend treatment with Bti products, sold at most home improvement stores. Bti is a natural product that kills mosquito larvae but is safe for people, pets, aquatic life and plants.