Early in the morning of June 4, two twin-engine Beechcrafts lifted into the air on the Eastern Shore of Maryland.
Their goal: to head off swarms of Eastern Salt Marsh mosquitoes before they hatch. Their target: 200 acres of wetlands and salt marshes flooded by high tide and heavy rains. Their plan: to spray the area with a new larvicide, a pesticide targeted at the larval stage of insects, in hopes of reducing the potential population of adults, which bite fiercely and can carry diseases—including West Nile virus—up to 20 miles away.
"These marshes in the Fishing Bay area are very productive," said Michael Cantwell, chief of mosquito control for Maryland's Department of Agriculture.
Fishing Bay is a small bay to the south and east of the Chesapeake. This year it produced what the department's chief entomologist was calling "a monster brood," with as many as a hundred larvae in a pint of water.
Once the floodwater recedes, larvae are left in isolated depressions that retain enough water to allow them to transform into adult mosquitoes over 7 to 14 days—depending on the temperature—and swarm out of the marshes looking for a blood meal. That's the cycle Maryland mosquito control hoped to interrupt by using a very specific natural larvicide that has virtually no impact on other species.
Yes, it's that time of year again when whining mosquitoes zigzag and follow your breath until an annoying ritual takes place: They land, you swat, they die or fly away to lay eggs, you swell and itch.
Mosquito-control officials and other experts say it's almost impossible to forecast how good or bad a year will be in terms of mosquitoes. Too much depends on weather and timing.
While Maryland was targeting its monster brood, for instance, Texas mosquitoes have been mostly no-shows. Both are coastal states, which provide rich mosquito habitat—Texas has the most varieties of any state—though the worst infested areas in the United States are the Florida Everglades and the Arctic National Wildlife Refuge in Alaska.
"We really haven't found very much this season," said Richard Duhrkopf, a biologist and mosquito researcher at Baylor University in Waco. "We had a very cold and very, very dry winter, so there were fewer adults around to get the population up and running. In Texas we're about a month behind."
Mosquitoes are very adaptive, though, said Cantwell and Duhrkopf. A few species, including the Eastern Salt Marsh mosquito, lay their eggs in soil or mud, where they stay dormant until a high tide or enough rain comes along for them to hatch. Duhrkopf said he had heard speculation that the dormancy could last as long as five years.
What the experts do know is that a new species of mosquito has arrived in the U.S., a new mosquito-borne disease is headed here, and there are some promising new approaches to mosquito control.
The new mosquito is Aedes japonicas, a Japanese bug that entered the U.S. in lucky bamboo (a houseplant that is not actually bamboo) and in shipping containers, according to Joseph M. Conlon, a retired U.S. Navy entomologist who is a technical adviser and spokesman for the American Mosquito Control Association. Aedes japonicas, he said, has been associated with disease transmission in the Far East but so far hasn't been here.
The new mosquito-borne disease that has public-health officials and mosquito experts on alert is the chikungunya virus (CHIKV)—not fatal but a painful disease. The Centers for Disease Control and Prevention noted in December 2013 that cases had been confirmed in the Caribbean among residents of the French side of St. Martin.
The same mosquitoes that spread both CHIKV and dengue fever are also found in parts of the U.S., the CDC noted. One is Aedes aegypti, heavily concentrated in South Florida but also found along the Gulf Coast. The other, Aedes albopictus, lives in the eastern U.S. and as far north and west as Chicago.
"CDC experts have predicted and prepared for [CHIKV's] arrival for several years, and there are surveillance systems in place to help us trace it," said CDC Director Tom Frieden.
The name "chikungunya," according to the CDC, is from the Makonde language spoken in parts of Tanzania and Mozambique. It means "that which bends up," describing the acute pain associated with the disease.
One species of mosquito that spreads the disease is Aedes aegypti, which originated in Africa but spread to the U.S. from the Caribbean and South America. Aedes aegypti has been in retreat since the mid-1980s, when the other disease—spreading species—Aedes albopictus, aka the Asian tiger mosquito—arrived and displaced it.
"What we have noticed is that Aedes aegypti has started to reemerge and is fighting back," said Conlon.
The threat of mosquito-borne disease is less a function of global warming than it is of globalization, said Conlon. Trade and transportation, he explained, now link once faraway places—and their pests.
Mosquito control nowadays employs a battery of weapons, including habitat reduction (one example: dumping out water from containers such as wading pools where mosquitoes could breed), use of natural predators such as mosquito fish (which eat larvae), and natural larvicides.
In April, Maryland Mosquito Control targeted more than 4,000 acres of flooded woodlands to kill the larvae of an early spring hatching mosquito with Bacillus thuringiensis, a bacterium that occurs in soil, described as highly effective on the mosquito larvae and having minimal or no impact on other species. April's outing produced an unusually high 95 percent larvae kill rate, compared with normal rates ranging from 60 to 85 percent.
What You Can Do
Not all mosquitoes or larvae are as easily targeted. Some, such as the Asian tiger, may breed in a discarded potato-chip bag or even a soda bottle cap, said Conlon. That's why everyone involved in mosquito control would like you to police your yard and empty containers filled with water.
The U.S. military and private industry are now working to create new ways to reduce mosquitoes. That includes systems that push mosquitoes away from humans and into traps and combinations of sugar, which they feed on, laced with a toxin that entices them to dine and die.
For individuals seeking protection, DEET "is still the gold standard of repellents," said Conlon. After a 1998 safety reassessment, the U.S. Environmental Protection Agency concluded that "as long as consumers follow label directions and take proper precautions, insect repellents containing DEET do not present a health concern."
A few days after the most recent aerial spraying, Maryland technicians hadn't yet been able to check the kill rate in the marshes around Fishing Bay. But in a county where mosquito bites can tally as high as 40 to 100 a minute, any reduction was likely to be welcome.