Yellow flies, biting insects belonging to the Tabanidae family, are a persistent nuisance in humid regions across the southeastern United States and Central America. The female yellow fly delivers a painful, slicing bite because she requires a blood meal to develop her eggs. Understanding the sensory mechanisms that draw these insects to a host is key to mitigating their effect on outdoor activities and livestock. Their host-seeking behavior is governed by visual, chemical, and thermal cues that operate at different distances.
Primary Visual Cues
Yellow flies rely heavily on sight to locate a host once they are in proximity. They are strongly drawn to large, dark, moving objects, which they perceive as potential sources of blood. This visual preference explains why dark clothing or the body of a large mammal, such as a horse or human, becomes an immediate target.
The specific color that attracts them tends to be dark blue or black, which contrasts sharply against a bright sky or background, making the object highly visible. Yellow flies are also particularly attracted to objects that have a glossy or shiny surface. This sheen may mimic the wet, reflective hide of a sweating animal, signaling a desirable host.
Movement of the object is another potent visual trigger for the yellow fly. Trapping studies have shown that dark, spherical objects that swing or rotate in the wind are significantly more effective at attracting flies than stationary ones. This motion simulates the movement of a breathing, walking host, compelling the fly to approach and land for a blood meal.
Chemical and Olfactory Triggers
Chemical signals are the primary long-range mechanism yellow flies use to detect a potential host. The most powerful attractant is carbon dioxide (\(text{CO}_2\)), which all vertebrates exhale as a byproduct of respiration. A plume of \(text{CO}_2\) acts as a homing beacon, signaling to the fly that a warm-blooded host is nearby.
Closer to the host, the flies detect volatile organic compounds (VOCs) emitted from the skin and sweat. L-lactic acid, a compound produced by muscles during exertion, is a significant component of human sweat and a documented attractant for blood-feeding insects. Other compounds, such as 1-octen-3-ol (octenol), also attract Tabanidae and other biting flies.
These chemical attractants often work synergistically; the combination of \(text{CO}_2\), octenol, and sweat components is more powerful than any single element alone. The flies are also guided by thermal signals, sensing the body heat and moisture emanating from the host. This combination of chemical and thermal input allows the female fly to pinpoint a precise location for landing and feeding.
Leveraging Attraction for Control
Understanding the yellow fly’s sensory preferences is directly applied to effective control and avoidance strategies. One successful method involves exploiting the fly’s attraction to dark, moving objects by using traps that mimic a host animal. Commercial and do-it-yourself traps often consist of black or dark blue objects, such as a beach ball or bucket, coated in a sticky adhesive.
The dark color and movement visually attract the flies, which become stuck on the adhesive surface. Chemical lures are also incorporated into some trapping systems, using \(text{CO}_2\) and octenol to enhance effectiveness and extend the attractive range. These traps are useful for reducing local populations in specific areas, such as a yard or campsite.
Simple avoidance measures involve reducing the visual and chemical signals a person emits. Wearing light-colored clothing, especially white, minimizes the dark contrast that yellow flies are drawn to, making the wearer less conspicuous. Using insect repellents containing DEET can also mask the chemical signals from sweat and breath, offering temporary protection.