Yellow flies (Diachlorus ferrugatus) are biting flies in the Tabanidae family, related to horse and deer flies. They are typically about 3/8 inches (1 cm) long, with yellow bodies and clear wings. They possess large, iridescent blue-green eyes.
They thrive in warm, humid conditions, commonly found in wooded, marshy areas and near water. Male yellow flies feed on nectar, but females are aggressive biters, requiring blood meals to produce eggs. Their bites are painful, often leading to itchy, swollen welts that can become infected. Yellow flies are a nuisance, particularly from April through June in southern regions, attacking both outdoors and indoors.
Factors That Draw Yellow Flies
Yellow flies are drawn to sensory cues indicating a potential blood meal. Carbon dioxide (CO2), exhaled by humans and animals, is a primary attractant. Their sensitive antennae detect CO2 plumes, allowing them to find a host from a distance. This chemical signal prompts them to investigate further for a blood meal.
Body heat also attracts yellow flies. They detect temperature differences from warm-blooded animals, guiding them toward a host. This thermal signature helps them pinpoint a target. The combination of CO2 and heat provides a powerful long-range signal for these biting insects.
Movement and dark, moving objects are strong visual attractants. Yellow flies are visually oriented hunters; a moving silhouette, especially one that contrasts with the background, mimics a large animal. Dark clothing, such as black or blue, makes individuals more conspicuous, providing a strong visual contrast. This visual cue helps them efficiently locate a host.
Moisture and chemical compounds in sweat, such as lactic acid, further attract yellow flies. Sweat indicates an active host, and the chemical profile of human perspiration provides an additional close-range cue. These chemical signals, along with body odor, guide the flies once they are in close proximity to a potential target.
Yellow flies also exhibit a strong attraction to specific colors. Blue and black are particularly attractive, a characteristic exploited in commercial traps. Dark colors like black resemble large animals, and blue has been found to be highly attractive. This color preference makes certain clothing or outdoor equipment more likely to attract their attention.
Minimizing Yellow Fly Presence
Understanding yellow fly attractants provides practical strategies for reducing their presence. Choose light-colored clothing, such as white, tan, or light yellow, to be less visually appealing. Dark colors, especially black and dark blue, create a strong contrast that attracts these visually driven flies, so avoid them. Wearing long sleeves and pants also provides a physical barrier against bites.
Minimizing vigorous movement in prevalent areas reduces visual cues. Rapid motion signals a potential host, drawing their attention. Staying still or moving slowly in infested environments can help you go unnoticed by these visual hunters.
Strategic location and timing can limit exposure. Yellow flies are most active during the day, especially in the late afternoon and on cloudy days, congregating in shaded, humid areas near water bodies. Avoid heavily wooded or marshy areas during peak activity hours; instead, choose open, sunny locations to reduce encounters.
Disrupting their ability to detect attractants offers some relief. Fans can disperse CO2 plumes and body heat, making it harder for flies to locate a host. Creating airflow around an area makes it less hospitable for yellow flies. Staying in air-conditioned environments also reduces body heat and sweat, diminishing attractants.
Knowledge of yellow fly attraction is applied in trap design. Commercial and DIY traps exploit their strong visual attraction to dark, moving objects, often using sticky black balls. These traps are strategically placed away from desired activity areas to lure flies from people. Some traps also incorporate blue colors, leveraging their visual preference.
Repellents containing DEET offer moderate protection by interfering with the flies’ ability to detect human attractants. Applying them to exposed skin creates a chemical barrier. Botanical ingredients like eucalyptus, lemongrass, and citronella also deter yellow flies.