Flies land on humans because they are searching for sustenance, primarily moisture and organic residues. These insects are efficient scavengers, motivated by the biological need to find salts, proteins, and liquids. A fly landing is essentially a quick taste test on your skin’s surface, prospecting for a nutritional snack.
The Chemical Signature Flies Seek
Flies use a sophisticated array of chemoreceptors located on their antennae and feet to detect subtle chemical plumes in the environment. When a housefly lands on your skin, it is tasting the surface using specialized sensory hairs on its tarsi. This action allows them to instantly determine if the surface contains something worth investigating further, such as a sugar source or a protein residue.
Sweat provides a rich cocktail of attractants. Sweat contains lactic acid and salts, which are byproducts of exertion that flies are keenly interested in. The ammonia produced when the body breaks down protein is another potent lure that flies use to locate a potential meal source.
A large part of your personal scent profile comes from the skin microbiome, the bacteria living on your skin’s surface. These microbes consume the organic compounds in sweat and oils, producing a unique blend of volatile odorants. These bacterial byproducts, which include carboxylic acids, act as powerful signals that flies use for long-range attraction.
Flies possess highly sensitive CO2 receptors, typically located on their antennae, which they use to track the plume of air exhaled. This carbon dioxide stream signals the presence of a large, warm-blooded animal, which represents a rich source of moisture, heat, or a potential blood meal for certain fly species. The combination of CO2 and other skin volatiles creates a synergistic effect, making the host much more detectable than either cue alone. External scents like sweet perfumes or heavily scented hair products can also mimic food sources, overriding natural human odors and attracting flies.
Visual and Thermal Triggers
Beyond scent, flies rely heavily on visual and thermal cues to pinpoint a final landing spot. The speed at which a fly perceives movement is much faster than human perception. For a common housefly, the world can appear up to four times slower than it does to a human, giving them ample time to dodge a swatting hand.
Flies are attracted to visual contrast and specific color wavelengths. While they are highly sensitive to ultraviolet light, many species are drawn to colors like blue and white, which resemble flowers or water sources. Darker colors, such as black or navy, also attract flies because these surfaces absorb heat and create a sharp contrast against a lighter environment.
Flies can also detect the infrared radiation, or heat, emitted by a warm body. This thermal signature serves as a final, close-range signal confirming the presence of a host. This combination of a heat signature and a contrasting visual target makes a sweating person particularly noticeable.
Reducing Your Attractiveness to Flies
Minimizing chemical attractants begins with maintaining hygiene to reduce lactic acid and ammonia. Frequent showering, especially after physical activity, removes the residual salt and organic compounds before they can be metabolized by skin bacteria. This limits the strength of the attractive odor plume emanating from the body.
Strategic use of clothing can reduce both thermal and visual attractiveness. Choosing light-colored fabrics, such as white or light yellow, helps reflect sunlight and reduces the heat signature that draws flies. Avoiding dark colors like black or dark blue minimizes the high-contrast targets that flies are drawn to for landing.
The application of certain repellents can disrupt the fly’s ability to locate you by masking human scent.
Repellent Options
Picaridin is a highly effective synthetic repellent that works well against flies and is often preferred over DEET for this purpose. Natural essential oils can also deter flies due to their strong aromas, which overwhelm the fly’s olfactory receptors. These include:
- Peppermint
- Lemongrass
- Citronella
Environmental control measures are highly effective in creating a fly-free zone. Placing fans on a medium or high setting creates an air current that is too turbulent for flies to navigate or land in. The constant breeze also rapidly disperses the plume of carbon dioxide and other body odorants, preventing flies from locking onto the scent trail.