The greenhead fly (Tabanus nigrovittatus and related salt marsh horse flies) is a notorious biting pest of coastal regions during the summer months. These insects interfere with the enjoyment of beaches and marsh-adjacent areas due to their painful, persistent attacks. Understanding what draws these flies to a target is the first step in mitigating their effect. Attraction relies on a sequence of visual, thermal, and chemical signals that guide the fly to a potential host.
Understanding the Greenhead Fly
Greenhead flies are part of the Tabanidae family (horse flies) and are abundant along the Atlantic Coast’s salt marshes. Only the female bites, driven by the need for a protein-rich blood meal to develop eggs. This reproductive requirement transforms the female from a nectar feeder into an aggressive, host-seeking insect. Adult flies are most prevalent and actively biting during the peak summer months, typically from July through mid-August. They are strong fliers and can travel a significant distance from their marsh breeding grounds.
Visual Cues and Thermal Signatures
Long-range attraction relies heavily on the fly’s visual system, which detects objects that contrast sharply with the background. Greenhead flies strongly prefer dark colors, such as black, blue, and purple, which mimic the appearance of large animals against the bright sky or sand. Colors with low light reflectance (400 to 800 nanometers) are the most effective targets. Conversely, light colors like yellow and white are consistently unattractive to host-seeking females.
The perception of movement further refines the fly’s targeting, as motion signals a potential host. Once in close range, the fly uses sensitive sensors to detect the host’s body heat, or thermal signature. The flies prefer sunlit, warm targets, interpreting increased heat as an indication of a warm-blooded animal. This sensory input confirms the target is a viable source of blood, prompting the fly to land and bite.
Environmental Factors and Chemical Signals
Beyond sight and heat, greenhead flies are guided by potent chemical signals detected over long distances. Exhaled carbon dioxide (CO2) is a primary long-range attractant, acting as a plume that indicates the presence of a breathing animal. Studies show that CO2-baited traps increase collections by over 300% compared to unbaited traps, underscoring its role as a primary locator.
As the fly closes in, short-range chemical cues from the host’s skin become influential. The moisture and specific compounds in human perspiration, such as sweat and saltwater residue, are highly attractive. The chemical 1-octen-3-ol (octenol) is found in the breath and sweat of mammals, mimicking host odor. Traps baited with octenol collect substantially more greenhead flies than unbaited controls, demonstrating the flies’ sensitivity to these short-chain alcohols and sweat components.
Reducing Attraction Based on Known Triggers
To reduce the likelihood of a bite, minimize the visual and chemical signals that draw the female flies. Wearing light-colored clothing (white, light gray, or yellow) decreases visual contrast and makes the body less appealing. This counteracts the fly’s strong preference for dark, low-reflectance colors.
Limiting physical activity reduces the production of attractants, since strenuous exercise increases body heat, CO2, and sweat output. It is also beneficial to avoid marshy areas, where flies are concentrated, during their peak activity window (mid-morning until dusk). Commercial traps utilize these known triggers, typically featuring a dark, heat-absorbing shape and sometimes a chemical bait like octenol to lure female flies away from people.