Mosquito numbers in Maryland are up 3 times their normal numbers in early summer because of recent storms and flooding.
First came the rains. Now come the mosquitoes.
Populations of the itch-inducing insects have multiplied across Maryland — in many areas up to three times their normal early summer numbers — because of recent storms and flooding that have given them an abundance of water to breed in.
That means the ankle-biters are even more of a nuisance than normal and, potentially, a bigger public health threat, too.
Health officials regularly test the region’s mosquitoes for signs of West Nile virus, Zika and other illnesses that can be carried from person to person through the pest’s bite.
These viruses can cause vomiting, rash, nausea and, in the worst cases, death. The Zika virus can be transferred from pregnant women to their babies and cause severe birth defects. There are no vaccines for these diseases.
“The mosquito-borne illnesses can cause a range of health issues, including infection that can spread to the brain,” said Baltimore City Health Commissioner Dr. Leana S. Wen.
“These are illnesses that can be fatal,” Wen said, “but are preventable” through practices such as wearing long sleeves at peak times of the day, using repellent and getting rid of standing water.
The Zika virus has only ever been transmitted to Marylanders who have traveled to other countries.
Illnesses from mosquito, tick and flea bites tripled in the U.S. from 2004 through 2016, according to the Centers for Disease Control and Prevention.
There are no reports this year of mosquito-borne diseases in Maryland, but that doesn’t mean they’re not coming, said David Crum, public health veterinarian at the Maryland Department of Health. Mosquito season runs through October.
“I do worry a lot because it has been a very wet, warm year,” Crum said. “Mosquitoes tend to breed well in this environment, so it may result in a higher population, but it doesn’t always correlate with higher incidence of disease. We will have to wait and see.”
Many of the mosquito larvae that began emerging last month have even been waiting years for this spring’s floodwaters, said Brian Prendergast, manager of the mosquito control program at the Maryland Department of Agriculture.
Their arrival wasn’t a surprise to Prendergast after Maryland’s wettest May on record and Baltimore’s wettest June in three years. Some varieties of mosquito eggs need such wet conditions to hatch.
The conditions have crews working overtime to spray neighborhood streets across the Baltimore region with pesticide, and to dust the Eastern Shore with mosquito killer from the air a couple of months earlier than normal.
“The rains hit in mid-May, and so about a week after that is when we started seeing the mosquito numbers going way up,” Prendergast said. “We expected a lot of mosquitoes, and that’s indeed what we found.”
The result is a lot of itchy mosquito bites. But those bites are, in a way, just the start of the surging insect population’s life cycle.
It is only female mosquitoes that bite because they need the protein found in blood to produce their eggs. And of about 60 mosquito species in Maryland, only about half feed on humans.
Of those, many are “floodwater mosquitoes” — varieties that lay hundreds of eggs at a time in soil that is prone to flooding. The eggs can sit dormant for years, Prendergast said, just waiting for a surge of rainwater.
That’s what came this spring to hatch them.
More than 8 inches of rain fell across the region in May, and in areas such as Ellicott City and Catonsville, 15 inches of rain or more within a monthlong period, causing devastating flooding. An additional 5 inches fell in June at Baltimore-Washington International Thurgood Marshall Airport.
State officials monitor mosquito populations using cylindrical traps that lure the insects with a light and then suck them in with a fan. They leave the trap out overnight, and if there are at least 24 female mosquitoes inside by morning, the mosquito control program gets to work spraying the area with pesticide.
In some areas, as many as 150 females have crowded traps in a single night, Prendergast said. Most mosquitoes are active from about two hours before dusk until dawn.
Luckily, he said, the booming numbers don’t include too many of the most notorious of mosquito species, the Asian tiger, known for its persistence and aggression, even during the day.
Asian tiger mosquitoes breed most commonly in plastic garden containers, buckets, cans and flower vases. So while the increased precipitation means more of those objects hold puddles for the mosquitoes to breed in, it does not have as much of an impact on their numbers as it does on floodwater varieties whose eggs might collect for years before hatching.
“Whatever is collecting rainfall in people’s yards can, and is, flooded in every single year,” Prendergast said. “Maybe it’s two times as bad. But it’s always an issue.”
To reduce mosquito numbers, the state program sprays a pesticide known as permethrin from the back of a pickup truck or, on the Eastern Shore, from the air. It kills mosquitoes on contact and thus must be deployed at night, when mosquitoes are active, and only when their populations boom.
The state budget for mosquito control is $2.7 million.
Pesticide is sprayed at a rate of 0.8 of an ounce per acre, which Prendergast called “a very low amount of material.” According to the National Pesticide Information Center, permethrin is not known to cause cancer or affect hormone systems. It can remain on plants for several weeks, and if it gets into lakes or streams, it can remain in sediment for more than a year. It is considered highly toxic to fish and bees, but low in toxicity to birds, according to the center.
The spraying is, in part, aimed at reducing the nuisance of pesky insects, but also to prevent the spread of disease.
“It’s a very robust nuisance control program, but it’s not strictly a nuisance control program,” Prendergast said. “It’s much less likely someone is going to contract a disease if they live in an area that’s regularly sprayed.”
Even amid the spraying, officials urge residents to use any of four insect repellents found to be safe and effective on mosquitoes: DEET, oil of lemon eucalyptus, Picaridin and IR3535.
Prendergast emphasized the importance of emptying any yard containers in which Asian tiger mosquitoes breed.
“It’s something that people cannot be told too often,” he said.