At What Time Of Year Do Deer Flies And Horse Flies Become Most Problematic For Maine Residents, And Why Do Repellents Fail To Prevent Their Bites?

Many residents of Maine are well aware of the pest issues caused by deer flies and horse flies, and a significant amount of residents have likely sustained at least one deer or horse fly bite in the past. Deer flies and horse flies comprise numerous species that feed on mammalian blood, including human blood, and compared to other areas of the US, the northeastern states tend to see the greatest amount of deer and horse fly disturbances in residential regions and on beaches. Each year during the late summer, deer and horse flies become abundant in the northeastern coastal states, and it is not uncommon for swarms to suddenly descend upon unsuspecting residents, as these fierce flies are exceptionally fast flyers. Not only do these flies often inflict bites that draw blood and leave scars, but the most powerful insect repellents do not seem to deter deer and horse flies from attacking humans.

According to James Dill, a pest management specialist with the University of Maine Cooperative Extension, deer flies and horse flies possess large mouthparts that resemble blades, and their saliva prevents blood from clotting. To be more specific, deer and horse fly saliva contains anticoagulant compounds that promote bleeding, allowing these flies to suck large amounts of blood from their human and animal hosts. The terms “deer flies” and “horse flies” are sometimes used interchangeably to refer to any large-bodied fly specimen that travels at fast speeds and inflicts bites to humans. Given their distinctive appearance, most residents can easily identify fly specimens as being deer or horse flies. Around 350 deer and horse fly species have been documented in the United States and Canada, but the number of species inhabiting Maine is unknown. This is why Dill is documenting as many deer and horse fly species as he can within the state.

Dill claims that around 20 deer and horse fly species can maintain a presence in residential areas of Maine during August and September, and just about all species bite humans. Many people insist that powerful insect repellents that contain DEET fail to prevent deer and horse fly bites. However, Dill believes that repellents only seem ineffective at preventing deer and horse fly bites because, unlike mosquitoes, these flies generally land on people’s heads where repellents are not typically applied. These flies are well-known for dive-bombing into people’s heads at fast speeds, and they prefer to land on heads in order to  inflict bites to the scalp. So unless a person applies repellent to their face and scalp, bites may occur. Repellents are useful for masking the human odors that mosquitoes and most biting insect pests use to pinpoint human hosts, but deer and horse flies do not locate hosts by scent. Therefore, a person dripping in DEET-based repellent is no more hidden from deer and horse flies than a person not wearing repellent.

Have you ever sustained a deer or horse fly bite?