The greenhead fly, a type of horse fly, is a painful summertime nuisance in coastal areas. They are problematic along the Eastern Seaboard, swarming beaches and marshlands during the warmest months. Greenheads interfere with outdoor activities, prompting many to seek ways to identify and avoid this aggressive pest. Understanding the fly’s biology is the first step in protecting yourself from its painful bite.
Identifying the Greenhead Fly and Its Environment
The greenhead fly is the salt marsh horse fly (Tabanus nigrovittatus), belonging to the family Tabanidae (horse flies and deer flies). They get their common name from their most distinct physical feature: large, iridescent compound eyes that appear brilliant green or greenish-gold.
The greenhead fly measures about 10 to 14 millimeters in length, similar to a common housefly. Its presence is tied to the coastal salt marsh ecosystem, its primary breeding ground. Females lay egg masses (100 to 200 eggs) on salt marsh grass blades.
After hatching, the larvae drop into the muddy, intertidal environment where they live for up to two years before pupating. Greenhead flies are localized pests, emerging as adults from late June to September, with peak activity in July and early August. Adults then fly inland to search for a blood meal.
Understanding the Painful Bite
Only the female greenhead fly bites people and animals, requiring a blood meal to produce egg masses. Male flies feed only on sugary substances like nectar for energy. Female greenheads use specialized, scissor-like mouthparts to slash or tear the skin’s surface.
This creates a small, open wound from which the fly laps up the pooling blood, unlike mosquitoes that pierce the skin. The bite causes immediate, sharp pain, followed by intense itching, swelling, and a visible red welt. The fly injects saliva containing an anticoagulant chemical into the wound to prevent the blood from clotting while feeding.
The irritation is the body’s localized reaction to this chemical. Due to the severity of the initial cut, bites can occasionally lead to secondary infections if scratched excessively. Although a nuisance, greenhead flies are not considered a major health problem, as they have not been found to transmit human diseases.
Effective Strategies for Prevention and Relief
Avoiding greenhead flies requires understanding their behavior and peak activity times. They are most active during the hottest part of the day, from mid-morning until dusk, and thrive in hot, sunny conditions. Greenheads are weak fliers and tend to avoid strong breezes or high winds.
Wearing appropriate clothing reduces the chance of being bitten, as the flies cannot bite through fabric. Female flies are attracted to dark objects, so choosing light-colored clothing, long sleeves, and pants is effective. They are also attracted to carbon dioxide, movement, and strong scents, so toweling off after swimming is helpful.
Insect repellents offer defense, though effectiveness varies. Repellents containing DEET or picaridin are the most effective options for biting flies. Picaridin, a synthetic compound, works well and is often preferable as it does not damage plastics like DEET.
If a bite occurs, prompt treatment minimizes discomfort and swelling. Cleaning the bite area with soap and water prevents secondary infection. Applying a cool compress or ice pack reduces swelling and irritation, while over-the-counter anti-itch creams or oral antihistamines manage the itching.