State inspectors say mosquito counts have tripled in parts of Baltimore County since heavy rains from Hurricane Irene and Tropical Storm Lee hit the area.
"The mosquitoes on the wing now are quite annoying," said Mike Cantwell, chief of mosquito control at the Maryland Department of Agriculture.
They're mostly fresh floodwater breeders, especially our native Aedes vexans. "They are persistent biters, and go after mammals and birds," he said.
"They are very active in the periods after sundown and, in shaded areas, they won't even wait until sundown," he said. "They're less active in the dead of night, but in those couple of hours before dawn their activity increases. You might have the greatest amount of activity during that period of the day."
Aedes vexans are also capable of transmitting the West Nile virus. For now, they're probably not, Cantwell said, "But in the next couple of weeks, they could be."
That's because they're hatchlings, and it will take them a few weeks to pick up the virus by biting infected birds, incubate it in their systems, and then turn on humans.
The immediate problem appears to be concentrated in southern and eastern portions of Baltimore County.
"Our local supervisor is saying they've gotten a threefold increase in the number of mosquitoes they collect using these portable light traps," Cantwell said. "We're probably talking about traps that generally average a dozen to two dozen mosquitoes a night. Now that's increased threefold."
No such increase has been reported in Baltimore City, or in the hillier, northern part of the county where drainage is better. The big increases were noted outside the city line, south of Route 40 and east to the Gunpowder River.
These fresh floodwater species typically respond best to heavy, late-season rains, he said. They lay their eggs in depressions in woods and grassy areas. And when the rain comes, the depressions fill with water just long enough to hatch the eggs.
Other species — the ones that lay eggs in permanent water bodies, such as Culex and Anopheles — did less well because heavy rain tends to flush out their habitats and wash the mosquito eggs away.
"We do expect to see their numbers drop," Cantwell said.
As always, mosquito control experts urge people to wear loose clothing with long sleeves and light colors. Use repellants with DEET or Picaridin, and stay indoors if possible when and where mosquitoes are active.
"And, if they want, report unusually heavy mosquito activity to the Maryland Department of Agriculture," he said.