Horse flies are common biting insects that can be a significant nuisance to humans and animals, particularly during warmer months. This article will explore how horse flies interact with various light sources, detail other factors that attract them, and outline practical strategies for deterring these persistent pests.
How Horse Flies Interact with Light
Horse flies are not generally attracted to broad-spectrum white light in the same way some nocturnal insects are. Instead, their visual system is highly specialized, drawing them to specific characteristics of light, particularly polarized light. These insects are polarotactic, attracted to linearly polarized light, which helps them locate water sources and potential hosts. Horizontally polarized light, often reflected from water surfaces, helps horse flies locate water for drinking, bathing, and mating. Females also use it for egg-laying or resting near water, waiting for hosts.
Beyond water, female horse flies, which require blood meals for reproduction, are drawn to polarized light reflected from dark, shiny surfaces, such as animal fur or dark vehicles. This polarized reflection helps them differentiate potential hosts from the surrounding visual environment, even against patchy vegetation. Their vision is especially effective in the blue spectral range, which can enhance their ability to identify hosts. Some species of horse flies are also attracted to ultraviolet (UV) light, which they may use for navigation or to identify hosts with UV-reflective patches.
Other Attractants
Beyond specific light characteristics, horse flies are drawn to a combination of other cues that help them locate hosts. A primary attractant is carbon dioxide (CO2) exhaled by humans and animals. The more active a person or animal, the more CO2 they release, increasing their attractiveness.
Horse flies also rely on thermal cues, being attracted to body heat. Warm-blooded animals emit heat signatures that horse flies can detect, making them inviting targets, especially when exercising or in direct sunlight. Movement is another attractant; horse flies are highly sensitive to moving objects and often pursue active individuals or livestock.
Horse flies show a preference for dark, contrasting colors, particularly black and blue. These colors are more visible to their compound eyes and associated with warm, moving objects, which are prime blood meal sources. The absorption of heat by dark colors also contributes to their allure. Certain odors, such as those from sweat, lactic acid, and manure, also play a role in attracting horse flies.
Strategies for Deterrence
Understanding what attracts horse flies can inform effective deterrence strategies. Wearing light-colored clothing (white, yellow, or pastels) can make individuals less visible and attractive to horse flies, as these colors absorb less heat. Conversely, avoiding dark colors, especially blue and black, can reduce attraction. Loose-fitting, long-sleeved shirts and pants can also provide a physical barrier, though horse flies can sometimes bite through thinner fabrics.
Managing the environment is another approach. Since horse flies breed in moist breeding areas, draining standing water or clearing overgrown vegetation can reduce their numbers. For livestock areas, regular manure removal and proper hygiene can help control fly populations. Fans can create air currents that deter horse flies from settling, both in outdoor spaces and stables.
While general insect repellents containing DEET, picaridin, or lemon-eucalyptus offer some protection, they are often less effective against horse flies compared to other biting insects like mosquitoes. Some specialized repellents, including permethrin-based products, can be applied to clothing for longer-lasting protection but should not be applied directly to skin. Traps designed to exploit horse flies’ attraction to dark, warm objects, such as black balls that absorb heat and guide flies into a collection bin, can reduce local populations.