Greenhead boxes are a common sight along local marshes, such as these on Plum Island. Ben Laing/ Staff Photo

NEWBURYPORT — We don't want to think of them, but the Tabanus nigrovittatus, known locally as the greenhead, won't be ignored for much longer.

If you haven't already suffered through your first greenhead bite of the season, officials say it won't be much longer before the dreaded pest peaks in population as hungry female greenhead horseflies set their sights on their first blood meal of the summer.

But while it may be no solace to someone nursing a wound left by the greenhead, according to Northeast Mosquito Control Director Walter Montgomery, the persistent army of flies that has plagued coastal residents since Colonial days is actually diminishing in strength and ferocity.

Insect repellent and spraying regimens rarely dissuade them, but the simple traps laid out in the marshland from Cape Cod to Ipswich and down to Plum Island have done wonders to outsmart greenheads and beat them at their own game, as Montgomery describes it.

"It's basically a physical attraction," said Montgomery. "The trap is supposed to simulate a four-legged, warm-blooded mammal. Usually before a greenhead gets to us, they may see a deer on the marsh or a cow or something like that. And they like to bite the soft underbelly of cows or deer or any other farm animal."

While we humans see a wooden box atop four simple wooden legs, the greenhead sees a blood meal and flies straight to the underside of the legs, said Montgomery. Once there, the light shining through the top of the trap serves to attract the fly further.

"They go in and then it's the lobster trap principle — they don't have the intelligence to go back out the way they went in," said Montgomery.

To provide extra enticement, Montgomery said officials have gone the extra mile by baiting the traps with a man-made product meant to simulate the "breath of an ox."

"It's basically a strip or a vial — it's a liquid formula," he said. "It's in a vial with a wick on it. We just put it on the inside of the trap. The wick slowly but surely gives off a little bit of the stuff."

Local officials don't monitor the number of flies historically trapped in the makeshift boxes since 1968, when they were first placed in the marsh. But numbers tracked at Cape Cod give an indicator of how they've helped stem the number of bites endured by coastal inhabitants — including the bovine and equine friends who have historically suffered the brunt of greenhead attacks.

"Putting the traps out every year for a lot of years has significantly diminished the greenhead population," said Montgomery. "We used to put them in more places, but now we put them in Newburyport, Newbury and Ipswich — all in the salt marsh. There's approximately 500.

"We haven't really done any numbers, but Cape Cod Mosquito Control has done numbers," he added. "When traps were first put out, it was common to get about 5,000 flies per hour in a trap. Now it's down to a few hundred flies per day that you'll catch in a trap. There's also a lot of anecdotal evidence that people will tell you, who say it's just so much better than what it used to be."

Historical accounts from the region's marsh hay farmers whom Montgomery has come across tell stories of cows dropping dead from the sheer number of greenhead bites they endured while grazing in area fields.

Montgomery recalls when his office was located in Rowley some years ago, and trips to the post office were always memorable for the sheer number of greenheads that would swarm the windows in pursuit of human hosts.

"It was terrible," he said. "I remember going to the Rowley post office to pick up mail and the glass window on the inside of the post office, you wouldn't be able to see out the window from the number of greenheads trapped in there.

That just doesn't happen today, he said, though it's anyone's guess what would happen if trap use were decreased.

Nevertheless, Montgomery said though the flies are starting to show up in traps already this year, he's hoping for a light season.

"We're hoping we have another season like last year," he said. "Last year was very light and we're hoping this year will be light as well."

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