Black flies are small, biting insects found worldwide, especially in regions with flowing water. Only female black flies bite, as they require a blood meal to produce eggs. These insects can be a significant nuisance, particularly during certain seasons and in specific environments. Understanding the various factors that attract black flies to humans can help explain why some individuals might be more susceptible to their bites.
Universal Signals That Attract Black Flies
Black flies, like other blood-feeding insects, rely on universal cues to locate a host. Carbon dioxide (CO2) is a primary attractant, constantly released by human respiration. Black flies possess highly sensitive olfactory receptors on their antennae, detecting changes in CO2 concentration to track hosts from a distance. This explains why they often swarm around the head, where CO2 levels are highest.
Body heat is another important signal. Humans emit heat that these insects detect using specialized thermoreceptors. This thermal signature helps black flies pinpoint a host once they are within closer range. The temperature gradient created by a warm body against cooler surroundings serves as a clear target for these insects.
General body odors also play a role. The human body releases a complex mix of volatile organic compounds (VOCs) from the skin, forming a unique “scent signature.” These compounds, including lactic acid and ammonia, are broadly appealing. Black flies also appear attracted to dark colors, which may help them visually identify hosts.
Personal Biology and Black Fly Attraction
Beyond universal attractants, individual biological differences influence how appealing a person is to black flies. The composition of an individual’s sweat varies, leading to differences in lactic acid, ammonia, and other organic acids produced. People who produce higher concentrations of these compounds or sweat more heavily may be more attractive to black flies.
Metabolic rate also plays a part; individuals with higher metabolic rates produce more carbon dioxide and body heat. This increased output of primary attractants can make a person more conspicuous to black flies. This can also vary with activity level, as exercising increases CO2 exhalation and body temperature.
The unique microbial community on a person’s skin, known as the skin microbiome, influences body odor. These bacteria interact with sweat and skin secretions, producing a distinct fragrance that contributes to attractiveness. Variations in these microbial byproducts can make some individuals more appealing. While less understood, genetics can influence sweat composition and metabolic rate, contributing to these personal differences. Diet may also subtly impact the volatile compounds released through the skin, potentially affecting attraction.
Environmental Conditions for Encounters
The likelihood of encountering black flies is influenced by environmental factors. Black flies breed exclusively in flowing water, such as streams and rivers. Proximity to these breeding grounds significantly increases the chance of encountering adult black flies. Clean, healthy streams are often indicators of large black fly populations, as most species do not tolerate organic pollution.
Black flies are primarily active during the day, with peak activity often occurring mid-morning and late afternoon/early evening. They do not bite at night. Their populations also fluctuate seasonally, typically peaking from late April through June, depending on water temperature and flow rates.