Black flies are small, biting insects that can make outdoor activities unbearable during their peak season. While the males of the species feed on nectar, it is the female black fly that is a nuisance to humans and animals. Understanding the signals these insects use to locate a meal is the first step in avoiding their painful, persistent bites. This process involves a complex sequence of long-range chemical detection followed by short-range physical confirmation of a host.
Chemical Signatures from the Host
The search for a host begins with the detection of chemical signals, specifically volatile compounds. The most significant long-range attractant is carbon dioxide (\(\text{CO}_2\)), which is exhaled by all mammals and birds. Black flies possess highly sensitive chemoreceptors that allow them to track the plume of \(\text{CO}_2\) emanating from a host, effectively using it as a general beacon for a potential blood meal.
As the flies get closer, they sense a blend of volatile organic compounds (VOCs) present in the host’s breath and sweat. These compounds include lactic acid, produced in higher concentrations during physical exertion, as well as various aldehydes and carboxylic acids. The specific mix of these VOCs varies between individuals, which helps explain why some people are targeted more frequently than others. The combination of \(\text{CO}_2\) and these body odors creates a powerful, synergistic attractant that guides the fly to its final destination.
Physical and Visual Cues
Once a black fly has entered the immediate vicinity of a host, it switches from relying primarily on chemical detection to using physical and visual cues for confirmation. Body temperature is a significant short-range signal, as the warmth radiating from the skin indicates the presence of a living, blood-filled target. The fly’s sensory organs detect this heat signature, which triggers the final approach behavior.
Moisture and humidity from the host’s breath and sweat also serve as close-range attractants. Visually, black flies are drawn to movement and color contrast, seeking out large, dark objects against the horizon or sky. Darker colors absorb more heat and create a stronger silhouette, making them more attractive targets than light colors.
The Reproductive Need for Blood
The entire process of host-seeking is driven by the biological requirement for reproduction in the female black fly. Unlike their male counterparts, female black flies need a blood meal to obtain the protein necessary for the maturation of their eggs. This reproductive strategy is known as anautogeny, where a high-protein meal, typically vertebrate blood, is required for the production of viable eggs.
While some species are autogenous and can produce an initial batch of eggs without blood, most black flies are anautogenous and must feed to complete their gonotrophic cycle. This biological imperative explains the persistent, focused biting behavior observed during the peak black fly season. The act of seeking and feeding on blood directly fuels the next generation of the species.
Minimizing Your Allure
Translating the science of attraction into practical avoidance strategies centers on disrupting the chemical and physical signals that black flies use. Since \(\text{CO}_2\) is a primary long-range attractant, avoiding strenuous activity in black fly-prone areas can reduce the volume of exhaled gas and sweat. By limiting heavy exercise, you decrease the output of both \(\text{CO}_2\) and lactic acid, effectively making your chemical signature less pronounced.
Choosing light-colored clothing is an effective way to minimize visual and thermal attraction. Light colors absorb less heat and create less visual contrast, making you a less obvious target for approaching flies. Wearing long sleeves and pants, and tucking clothing in, physically blocks access to the skin, particularly around the neck and ears where skin is thinner and the flies often congregate. Additionally, avoiding scented hygiene products like perfumes or strongly fragranced soaps can eliminate sweet smells that may also draw these insects closer.