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Every other Friday, the Outside/In team answers a listener question about the natural world. Got a question of your own? The Outside/In team is here to answer your questions. Call 844-GO-OTTER to leave us a message.

The Outside/In[box]: What Are Those Blue Boxes?

a picture of a blue box on stilts in a marsh
Photo by Donald Childs
The blue color is attractive to greenhead flies, who fly up under the box and are trapped inside.

Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.

This week, Ingrid from Newmarket asks: "We've been driving around in the Seacoast area recently and seen blue boxes through all of the salt marshes. I'm wondering what those blue boxes are?" 

The Unwelcome Visitor

The cobalt blue boxes are easy to see across the flat salt marshes. They look like big wooden flat-topped mailboxes, and they have to do with a very unwelcome beach visitor: greenhead flies.

Greenhead flies, along with deer flies and horse flies, are all biting insects of similar size that are often mistaken for one another. Greenhead flies are a species of horse fly, in the family Tabanidae, and they are easily identified by their large, iridescent green eyes.

Gabrielle Sakolsky is the superintendent of the Cape Cod greenhead control district, which manages greenhead fly populations for all of Barnstable County in Massachusetts. “You find them all along the coast from Maine down through Georgia. You see greenhead flies anywhere you have salt marshes.”

Greenheads lay their eggs in the mud of the salt marsh. “So when the greenhead flies first emerge as adults, they go out, they find some sugar source to feed on, for energy, for flight, for anything they're doing, “ says Sakolsky. “They go and they lay the first set of eggs. [Then] the females start to look for a blood meal. She needs the protein in your blood to develop [another set of] eggs.”

The blood meal? That's you, or a cow or a deer.

The Nuisance Factor

The blue boxes are attractive to the greenhead flies due to their color and design.

“Greenhead flies are going to go to the [underside] of a deer or cow and they're going to bite the belly. So they fly under that box and they see the light, the sunlight, and they end up flying up. The bottom of the box is an inverted screen V that is sort of like a fish trap where they get caught in the trap and can't get back out,” says Sakolsky.

Because the greenhead has already laid its eggs before going after humans, Sakolsky says the boxes aren’t doing anything for population control. “What they're really doing is cutting down on the nuisance factor."

Photo by Ed Hoopes

And if you've ever been bitten by a greenhead, well, you know.

“Oh my gosh. It totally hurts,” says Sakolsky.

“They have a different type of mouthparts than something like a mosquito that's a little more stealthy. The greenhead fly kind of cuts a little hole in your skin and then she lowers her mouthparts. She spits her saliva into it with an anticoagulant; it's something to keep your blood from clotting so she can suck it up, so she can get that protein for her little babies that she wants to develop. And it's actually your body's reaction to her chemicals in that anticoagulant that [causes] the pain.”

So What Are They Good For?

“Oh, they're nice pellets of protein for the birds that are out there on the marshes. The birds love to feed on them,” says Sakolsky.

Greenheads do not spread disease, unlike other blood-sucking insects like ticks or mosquitos.

The greenhead fly season is dependent on weather and tides, but the adult stage is generally about six weeks. But that also happens to coincide with the six most popular weeks to be at the beach, from the end of June to mid-August, and into September as you go south. And during that season, Sakolsky says, the greenhead flies are hunting you down, looking for movement and the scent of carbon dioxide being exhaled, as they seek a blood meal.

So, how can you prevent being bitten by greenhead flies that have ignored the blue boxes? Like mosquitos, they have a hard time on windy days. And, Sakolsky says, they are a bit slow, so if they land on you, you have a good chance of slapping them off. And, she suggests choosing your beach accessories with the blue boxes in mind: Avoid a blue bathing suit, blue towel or blue umbrella.

Got A Question?
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Jessica previously worked as a producer for NHPR's The Exchange, wedging in as many discussions as possible about the environment, wildlife, and the outdoors. You can hear her occasionally as a substitute host on NHPR's Morning Edition and All Things Considered.

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