The greenhead fly, a common summertime nuisance along coastal regions, is a type of horse fly known for its persistent and painful bite. These insects can significantly interfere with outdoor activities, particularly in beach and marsh environments where they are most abundant. Understanding the biology and behavior of this aggressive pest helps individuals manage and avoid encounters during its peak season.
Physical Description and Classification
The greenhead fly is a member of the horse fly family, Tabanidae, specifically the species Tabanus nigrovittatus along the Atlantic coast. This classification places it among a group of flies recognized for their blood-feeding habits. The adult greenhead is noticeably larger than a common housefly, typically measuring around three-quarters of an inch in length.
The fly’s most distinctive feature is its large, compound eyes, which give the insect its common name. These eyes exhibit a striking iridescent appearance, often displaying shades of metallic green or gold. The body is generally dark, and its size and loud buzzing make it a recognizable visitor to coastal areas.
Habitat and Seasonal Activity
Greenhead flies are adapted to the salt marsh environment. The larvae develop within the mud and wet organic matter of these salt marshes and estuarine areas, remaining there for one to two years before pupating. This extended larval stage ties populations directly to the presence of these wetland habitats.
Adult greenhead flies emerge in late spring, reaching peak annoyance levels during the summer. Activity is most intense from late June through August, when warm temperatures and high humidity promote their flight. They are diurnal, meaning they are active during the day, with aggressive biting behavior observed during the sunniest hours.
Why Greenheads Bite
The biting behavior of the greenhead fly is exclusively a reproductive necessity carried out only by the females. The female requires a blood meal to obtain the protein necessary for developing subsequent masses of eggs. While the protein for their initial batch of eggs is derived from their larval diet, they must consume blood to continue reproduction.
The bite is immediately painful compared to a mosquito bite because the female greenhead uses stout, scissor-like mouthparts instead of a slender, piercing proboscis. These structures slash the skin, creating a small wound from which blood pools. The fly then laps up the blood, injecting anticoagulant saliva to maintain the flow, which causes the sharp, painful reaction.
Methods for Prevention and Control
Personal prevention involves understanding what attracts the female flies. They are visually drawn to large, dark moving objects and detect hosts through carbon dioxide and scent. Wearing light-colored clothing contrasts less with the sky and absorbs less heat, making the wearer a less obvious target.
Chemical repellents like DEET or picaridin offer some protection, but their effectiveness against this determined fly is often limited. Avoiding marsh edges during peak hours, such as mid-day and early afternoon, also reduces encounters. Community control often involves placing specialized black box traps within the marsh. These traps mimic the silhouette of a large animal and successfully capture large numbers of blood-seeking females.