10 July 2023
This week at Cape Cod I learned how to avoid a biting fly I never see in Pittsburgh.
Greenhead horse-flies(Tabanus sp.) inhabit the salt marshes of North America’s Atlantic coast from Nova Scotia to Florida. For most of their lives they ignore humans, but during egg-laying season the females need a blood meal to produce their second sets of eggs. When they fly off the marsh in search of something to bite, human skin is very tempting. Ouch! It hurts immediately and raises a welt.
Greenheads are so annoying in July and August that the Cape Cod Greenhead Control Program sets up traps to reduce their numbers. Before I went birding at Water Street Marsh one of the gatekeepers at Seagull Beach told me “Don’t go near the blue boxes.”
The blue boxes trap only the female greenheads that are looking for a blood meal. Since the females search for mammals by smelling their breath, the traps are baited with artificial ox-breath, octenol, and are painted blue because it’s the bugs’ favorite color. According to Cape Cod Greenhead Control, up to 30,000 flies are found in each trap at the end of the summer.
In places without greenhead traps, such as Parker River National Wildlife Refuge in Essex County, Massachusetts, there are sometimes signs to warn visitors.
At Water Street Marsh I stayed on the gravel and left my blue backpack in the car.
Read about the Cape Cod Greenhead Control program in their Greenhead fly pamphlet.
(photos credits in the captions)