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We declare mosquito season is finally over in the Washington region

Rising temperatures and humidity have lengthened D.C.’s mosquito season by about 10 days since the 1980s.

A yellow fever mosquito feeds on Kevin Ambrose’s arm. (Kevin Ambrose for The Washington Post)

Did you notice that mosquitoes seemed more numerous than usual this fall? Even this past weekend, when grilling or mowing the lawn, we were attacked by these little blood suckers. We can’t recall mosquitoes biting us so deep into October in past years.

In recent weeks, the insects have seemed to be everywhere. Even when we weren’t outside, they were sneaking into our homes and cars and having a feast. Fortunately, conditions have finally sufficiently cooled to end their reign of terror, according to experts we consulted. We’ve also noticed they’ve been far fewer in the past couple of days.

Yes, we can declare mosquito season over, for the most part.

The late-season mosquito onslaught arose from warm and humid conditions that lasted much longer than normal. Low temperatures were above 60 degrees on 13 straight days in the first half of October, the longest such streak on record. It was like summer never ended. Not until Sunday did the mercury sink below 54 degrees, the latest in the season on record.

On Monday and Tuesday, the temperature slipped below an important threshold: 50 degrees. Mosquitoes do not function well when temperatures drop below this.

“Temps in the 40s will surely slow them down a bit,” said Michael J. Raupp, emeritus professor of entomology at the University of Maryland, in an email.

Paul T. Leisnham, also a professor at the University of Maryland, agreed. “The mosquito season is firmly coming to a close,” he wrote in an email.

Both Leisnham and Raupp, however, remarked that although mosquito activity is waning, the insects have not been eliminated.

“People may encounter the occasion adult mosquito if we experience warmer conditions in late October,” said Leisnham, who studies environmental science.

Raupp mentioned that the mosquito species Culex pipiens, common house mosquitoes that are well-known carriers of West Nile virus, can remain active all year. “Even on warm days in late autumn or winter in D.C., there could be blood,” Raupp said. “These devils live and breed year round in basements, parking garages, and tunnels under cities. They can bubble out and feed when it is warm enough.”

Mosquito questions and answers

If you felt that this past mosquito season was particularly bad, you may be on to something. Mosquitoes thrive in warm, wet weather, and the June-through-August period was the 11th-wettest and eighth-warmest on record in Washington.

“We expect mosquito densities to be greater when summer temperatures are higher,” Leisnham said. “But rainfall is another critical environmental condition. More rain usually means more aquatic habitat, especially for urban mosquitoes that tend to lay their eggs and develop as larvae in small containers that we find around suburban yards (e.g., buckets, child toys, garbage cans, trash, etc.).”

Leisnham’s lab did not sample mosquito numbers over the summer, “but we anecdotally noticed greater mosquito activity,” he wrote.

According to an analysis from Climate Central, a nonprofit climate communication group, we probably should get used to longer mosquito seasons. Weather favorable for longer mosquito survival is increasing on average across the United States, including in the D.C. region.

Climate change has increased humidity in D.C., making it feel even hotter

Rising temperatures and humidity levels from human-caused climate change extended the mosquito season at 64 percent of 239 sites Climate Central analyzed. In D.C., the length of the mosquito season has grown by about 10 days since the 1980s.

Setting aside the lone mosquito here or there through the winter, we can enjoy the bug hiatus that began this week.

When will they be back? “The [mosquito] eggs generally won’t hatch until they experience a period of cold winter conditions followed by warmer temperatures and longer day lengths next spring or early summer,” Leisnham said.

Every season except summer is getting shorter, a sign of trouble for people and the environment

Thus, we should be relatively mosquito-free until April, May or June. After all, this is the season for pumpkins, apple cider and wood fires, not bothersome bugs. Good riddance, mosquitoes!