Greenheads | Learn about Greenhead Flies, the Beasts of the Northern Wild
Over the next few hours, I get a crash course in greenheads: Tabanids 101, if you will. For instance, I learn that the males don’t bite. Only the female is out for blood, but when she first takes flight, she’s actually a vegetarian. It’s a little like hearing that sharks prefer salad. (The seeds of bloodlust are sown early, though; the larvae are carnivorous and cannibalistic, and if there’s nothing else handy, like an earthworm or some other type of larva, they’ll eat each other.)
“Before she’s laid her eggs, you can even let her take a walk on you,” Dr. Stoffolano insists. “When the males and females emerge from the pupa stage, they feed on carbohydrates, like nectar or honeydew, for energy, for flight. That’s their gasoline. But once the female lays her eggs, her whole behavior changes. She becomes extremely aggressive. In this second cycle she’s seeking a host–cow, deer, human. She needs to have a blood meal if she’s going to lay another batch of eggs.”
Then he shows me the photos. And now I understand why it hurts.
Unlike the delicate mosquito, which is a vessel or capillary feeder, drawing blood much like a syringe does, tabanids are pool feeders, meaning they sever lots of capillaries at once, in order to create a gaping wound so that the blood can pour out. Close-ups of the greenhead’s tiny mandibles reveal an unsettling similarity to a pair of deadly, razor-sharp scissors, able to slice through skin like a knife through mayo. Tiny temperature receptors on the insect’s antennae have already detected whether or not the host is warm-blooded. Now that she’s attached, two other mouthparts pull her head deeper and deeper. She’ll keep on sucking until you smack her away. Then she’ll come back for more. Over and over and over.
At first, according to Dr. Stoffolano, it doesn’t hurt. “I’ve watched them go all the way down into my hand,” he says matter-of-factly. “They probe the skin for temperature, then they pierce the skin, and then they salivate. The saliva contains an anticoagulant so that the blood doesn’t clot. As soon as they release saliva, which is a foreign protein, our bodies react to it and we sense pain. That’s when we swat ’em.”
Pain is the greenhead’s great enemy. The very thing she does so well keeps her forever hungry: The viciousness of her bite practically guarantees that it will be difficult for this insect to get a good meal. But here’s where Nature’s brilliant adaptability comes through, letting the greenhead make the best of a bad deal.
“There’s not a lot of blood out on the salt marsh,” Dr. Stoffolano says. “They probably never get a good blood meal. So they’ve evolved a strategy of being able to lay the first egg batch without taking a blood meal. And that’s one of the major problems in controlling this fly. You’ve seen all the traps, right?” I nod my head. Anyone who’s driven the East Coast during the summer months can’t miss the black boxes strewn about the salt marshes up and down the Atlantic. “All those females that go into the traps have already laid a batch of eggs first. So you’re not cutting down on the population. What you’re cutting down on is the biting frequency and the nuisance factor. The traps are nuisance control.”
Which is still better than nothing, he acknowledges. Greenhead adults live three to four weeks, so the population builds up. They’re strong fliers, too, so if they can’t find a meal at the beach, they’ll fly a couple of miles inland. And although on a good day a greenhead can lay up to 200 eggs in what may be her first and last reproductive act, these dark, heavy traps–painted black to mimic large animals, like cows–can attract and capture up to 1,000 greenheads per hour. That’s good news for beachgoers, boaters, and anyone who lives nearby.
Frankly, short of spraying the entire East Coast, there aren’t a lot of other options. The insects themselves are too big to eliminate without using massive amounts of pesticides, which would damage the delicate salt-marsh habitat. Besides, greenheads play a couple of important roles within that fragile ecosystem. First, as an indicator of marsh health. And second, as a food source: Their spring larvae are consumed by shorebirds and fish, while adult greenheads are protein for purple martins and tree swallows. Eat or be eaten.
“The other problem we face with these greenheads is the vastness of the salt marsh,” Dr. Stoffolano adds. “It goes from Texas all the way around Florida, all the way up the East Coast, up into Nova Scotia. It’s a huge area. How do you control such a vast area?”