Where they hang out: These biting horseflies develop in coastal marshes in July and August and are especially problematic on Cape Cod and the
Jersey Shore, where black wooden box traps are installed on certain beaches to snare them.
What attracts them: They like large, dark objects, probably because their natural food sources are cattle and deer. (But they will bite you when they can’t find Bambi.) Many of their cousins, such as blackflies, have similar feeding habits.
Their kryptonite: The best defense against greenheads is permethrin, which can be found in products such as Permanone. It’s wise to save this for intolerable cases, as it can give skin a rash; spray it on clothes only.
What else you need to know: Greenheads attack during the day. Because they are larger than most insects, greenheads are often not bothered by a normal amount of insect repellent on skin, even if it contains DEET.