Biting flies are one of the worst aspects of visiting coastal areas in the northeast. One of the most dreaded fly pests in this region is T. lineola, or the greenhead fly, as it is more commonly known. These insects belong to the Tabanidae family of flies, which includes horse flies and deer flies. Well over 2,500 Tabanidae species have been described, and all but a very small minority suck blood from mammals. In fact, this insect family consumes more mammalian blood than any other family of insects. These insects prefer to feed on livestock and horses, but several species feed on the blood of humans as well.

Tabanidae flies are notable for their aggressive pursuit of mammalian blood, as the sudden pain of a bite causes both humans and animals to make a sudden attempt to remove the fly from the skin after it sinks it sharp mouthparts into flesh. After successful removal, these flies will continue to bite until they obtain a blood meal or die. Greenhead flies are particularly large in size, as adult body length averages at .6 of an inch. Greenheads weigh around 50 milligrams, and the blood these flies consume account for 20 percent of their weight. Greenhead fly activity is influenced by climatic factors, which allows humans to avoid bites and swarms by avoiding outdoor activities on days when the weather forecast favors greenhead fly activity.

The flight activity of greenhead flies depends largely on outside temperature. On days when temperatures are high and cloud cover is minimal, greenhead swarming activity becomes heavy, and bites can be expected in coastal areas on these days. This is why greenhead fly bites and nuisance swarms become most problematic during the month of July when temperatures reach their peak for the year. A sudden drop in temperature, or sudden cloud cover, will cause greenhead fly activity to cease, making greenhead flies disappear during bouts of rainfall. Unfortunately, the best days to visit the beach are also the days when greenhead fly activity is at its most significant. While several studies have demonstrated how climate influences greenhead fly activity, little is known about the sensory organs that allow greenheads to perceive subtle climate changes.

Have you ever encountered greenhead flies during cloudy or cooler days?