Horse flies (Tabanidae family) are relentlessly aggressive, large, fast-flying insects. They inflict a painful bite and are a public health concern due to potential disease transmission. Many people use DEET, the most common insect repellent, seeking a protective shield. Determining if this chemical is an adequate defense requires understanding how the repellent functions.
How DEET Works to Deter Insects
DEET (N,N-Diethyl-meta-toluamide) is the most widely used synthetic insect repellent, developed by the U.S. Army in 1946. Its primary action is to confuse the insect’s ability to locate a host by interfering with its olfactory system. The chemical creates a scent barrier that makes the host undetectable or unattractive, rather than killing the insects.
The mechanism involves the repellent blocking or activating the insect’s odorant receptors, which detect host-seeking cues like carbon dioxide and lactic acid. DEET acts as a “confusant” that scrambles the neural signals the insect receives, preventing accurate target location. This disruption is highly effective against insects like mosquitoes, which rely heavily on scent for host detection.
Specific Effectiveness Against Horse Flies
The aggressive behavior of horse flies presents a unique challenge that often bypasses DEET’s odor-blocking mechanism. Unlike mosquitoes, which are guided by chemical plumes, horse flies rely heavily on visual and thermal cues to find their next meal. They are strongly attracted to movement, large size, dark colors, and heat signatures, which override the repellent’s scent barrier.
Studies show that while DEET offers a temporary reduction in horse fly landings, its efficacy is short-lived and inconsistent compared to its performance against mosquitoes. Even high concentrations of DEET (15-17%) provided protection for only up to four hours in field tests. The persistence of a horse fly, which lands and bites immediately upon visual confirmation, often overcomes the localized chemical defense. Relying solely on DEET for horse fly protection is an inadequate strategy.
Other Methods for Horse Fly Protection
Since DEET’s scent-based defense is frequently overcome by horse flies, a multi-modal approach is necessary for effective protection.
Alternative Chemical Repellents
One alternative chemical repellent that shows greater effectiveness against biting flies is Picaridin (Icaridin). Picaridin works similarly to DEET by interfering with an insect’s chemoreceptors. It is often preferred for its less greasy feel and lack of damaging effect on plastics and synthetic materials.
Clothing and Gear Treatment
For clothing and gear, Permethrin is a highly recommended option that works as a contact insecticide, killing insects that land on the treated fabric. This chemical should never be applied directly to the skin. When sprayed on outerwear and allowed to dry, it bonds to the fibers and remains effective through several washings.
Non-Chemical Strategies
Non-chemical strategies are also invaluable. These include wearing thick, light-colored clothing, as dark colors attract more horse flies. Specialized traps designed to mimic the visual and thermal cues of a large animal can be used to reduce the local horse fly population. These traps often use dark-colored objects suspended and heated by the sun.