The greenhead fly, often Tabanus nigrovittatus, is an aggressive summer pest that severely impacts outdoor activities, particularly along coastal areas. These robust flies belong to the horse fly and deer fly family and deliver a painful bite, far more irritating than a mosquito’s. They are prevalent near salt marshes and wetlands where their larvae develop. This guide offers practical strategies for reducing local populations and protecting yourself from these insects.
Identifying the Biting Fly
The greenhead fly is often Tabanus nigrovittatus, a large member of the family Tabanidae. These flies are identifiable by their size, typically a half-inch to an inch in length, and their signature iridescent green or gold eyes. When at rest, they often exhibit a dark, striped pattern on their abdomen, distinguishing them from other horse fly relatives.
Only the female greenhead fly bites, requiring a blood meal to produce eggs. Unlike mosquitoes, the female greenhead fly uses blade-like mouthparts to slash the skin and lap up the pooling blood. This cutting action makes their bite instantly painful. Their flight is fast and direct, often attacking without the preliminary buzzing or circling behavior seen in other flies, making them difficult to swat.
Environmental Management and Prevention
Greenhead flies are obligate breeders in moist, often saline environments, linking their populations directly to coastal salt marshes and low-lying wet fields. The larvae develop in the mud or wet soil, feeding on organic matter and small invertebrates for up to a year before emerging. Because their primary breeding grounds are expansive coastal wetlands, large-scale source reduction by individual property owners is not feasible.
Property owners can implement management strategies to reduce the attractiveness of areas immediately surrounding structures. Focus on improving drainage to eliminate standing water or overly saturated ground near homes, especially after heavy rains or high tides. While this will not stop flies emerging from distant marshes, it limits secondary breeding opportunities on the property.
Managing vegetation density can also help, as tall grasses and dense shrubs provide resting spots for adult flies waiting to ambush a host. Keeping lawns mowed and vegetation trimmed back from pathways and entry points reduces the available habitat for resting adults. These measures aim to make the immediate home environment less hospitable, pushing the attack zone farther away from human activity.
Active Control Methods: Traps and Physical Barriers
Since adult greenhead flies are strong fliers and difficult to control with conventional insecticides, trapping methods offer the most effective active control. The most successful device is the specialized box trap, often called the New Jersey or NZI trap, which exploits the fly’s visual and behavioral cues. This trap utilizes a large, dark object, often a black beach ball or rubber sphere, suspended beneath a translucent cone or canopy.
The dark object absorbs solar radiation, heating up and mimicking the appearance and warmth of a large mammal. Flies are attracted to the dark mass, land on the sphere, and then attempt to fly upwards toward the light when they fail to obtain a blood meal. This upward flight path directs them into the cone and then into a collection chamber from which they cannot escape.
Trap placement is important; traps should be installed in open, sunny areas near known fly pathways, such as the edge of a property bordering a marsh or field. Multiple traps may be necessary, as proper placement can significantly reduce local populations by capturing hundreds of female flies daily during peak season.
Physical barriers offer another layer of protection, particularly around porches, windows, and outdoor living spaces. Utilizing fine-mesh screening, often a 20-mesh size, can successfully exclude the large flies from entering screened areas. Sticky traps coated with a high-tack adhesive can also be strategically placed near entry points, though they are less effective than the dedicated box traps for population reduction.
Personal Protection and Repellents
Protecting the individual requires strategies that address the fly’s attraction to movement and color, as chemical repellents have limited utility. Greenhead flies are less susceptible to standard topical repellents compared to mosquitoes. Most commercially available products offer only short-term or incomplete protection.
If a repellent is used, products containing high concentrations of DEET (25% to 50% range) provide the best available defense, though frequent reapplication is necessary. A more effective measure involves treating clothing with Permethrin, an insecticide that binds tightly to fabric and repels or kills insects on contact. Permethrin should only be applied to clothing, never directly to the skin.
Wearing appropriate clothing is the most effective personal defense strategy. These flies are strongly attracted to large, dark, moving objects, so choosing light-colored clothing, such as white or tan, is an important countermeasure. Long sleeves, pants, and hats should be worn to create a physical barrier, as the fly must land on the skin to bite.