Longtime residents already have a vivid familiarity and healthy respect for the blood-sucking flies that hatch from the salt marshes in early July. But for ignorant newcomers expecting a pleasant day at the beach or exploring the Parker River Wildlife Refuge, the clouds of vicious insects make for an unpleasant surprise.
"These things pack a real wallop," said Bill Gette, director of the Massachusetts Audubon Education Center and Wildlife Sanctuary at Joppa Flats. "They're twice as big as a housefly - bigger than horseflies even. And they're not like a mosquito, sucking your blood through a syringe-like thing. They have jaws, and they bite a chunk out of you and lap at the blood. It hurts. It's bad."
Nancy Pau, a wildlife biologist at the refuge, said the flies have been out for about a week and a half, and they will stick around for the rest of the month.
"But it's going to get worse before it gets better," Pau cautioned. "Their maturation is timed to the tides, and with the high tide coming next week we could see a whole lot more."
After emerging from their pupal stage in the salt marsh, female greenheads mate with males to produce a batch of 200 to 300 eggs, which they lay under leaves and grasses in the marsh. But after the first batch, the females need fresh blood before they can lay any more eggs. Since the flies lay as many as six batches of eggs in the course of their one-month adult lives, there's a lot of blood to be drunk.
That's where you come in.
Pau said the flies seem to have a preference for the wet, exposed flesh of beach-goers, but birders in the refuge are also easy prey. Gette reeled off horror stories of returning to the center's van after a summer expedition near the marshes only to find he had left the windows open, and the cab was thrumming with hundreds of hungry flies.
Research suggests that development of the area may have only made the problem worse. Greenheads thrive in the drier marshes created by the same ditches dug in the 1930s and '40s to control the mosquito population, and as the trees and shrubs that once bordered the marsh have been cut down, the flies have a wider range to breed. That's no problem for an insect that can fly at 50 miles per hour and roams as far as 30 miles.
Officials at the refuge put signs up in July to warn visitors about the flies and set traps out in the marsh to help control the population, but they remain the bane of the unprotected visitor. Opinion differs on whether conventional insect repellent is effective against greenheads, but popular lore has it that Avon's "Skin-so-soft" bath oil keeps them at bay.
Gette tries to remind himself that the flies are an important part of the marsh's ecosystem, providing abundant and nutritious food for many bird species. But even for a conscientious naturalist, it's hard to love the greenhead.
"They have a role to play, it's true," he said. "But it doesn't mean I have to like them."