With a worldwide distribution and a documented presence in every U.S. state except Hawaii, the deer fly (common name) comprises several different species of true flies of the family Tabanidae and genus Chrysops. Deer flies are biting flies and are therefore regarded as pest insects. Deer flies often produce large, local populations that have the potential to disrupt outdoor activities and cause physical harm to humans.
The insect is also a frequent irritant to cattle, dogs, horses, and other common household animals.
More importantly, deer fly bites can spread tularemia, a rare but sometimes serious disease.
Appearance & Identification
What Does a Deer fly Look Like?
Often compared to and grouped with horse flies, deer flies are smaller than their similarly named counterparts.
Adults are typically brownish, yellowish, black, or a combination of those colors and measure between six and 10 millimeters in length.
Like all true flies, deer flies possess a single pair of wings. The wings of an adult deer fly feature distinctive dark patches that help distinguish the biting insect from other species of flies.
Large and colored with green or gold patterns, the eyes of deer flies meet contiguously at the top of the head on males but remain clearly separated on females.
Difference Between Deer Flies & Horse Flies
Deer flies and horse flies are in the family Tabanidae. They are both biting flies. Scientists have classified most of the horse flies in the genus Tabanus and most of the deer flies in the genus Chrysops.
Deer flies are about the same size as houseflies – about ¼” to 0.5″ in length. They may be black or brownish in color. There are several physical differences between deer flies and horse flies, but most of them are hard to see without a magnifying glass.
The most obvious differences are in the wings and the eyes. Deer flies have dark spots on their wings while horse flies have clear wings. Many deer flies also have spots on their compound eyes.
Where Do They Live?
Deer flies generally reside in and around ideal breeding sites such as the marshes, ponds, swamps, damp woodlands, and other wetland areas.
Deer flies require aquatic environments during larval stages of their life cycle. Active primarily from May to September, adult deer flies typically emerge to search for host animals and feed during the daylight hours of the warmest part of the day.
Noticeably less active on cloudy or overcast days, the deer fly shows an attraction to shiny surfaces as well as dark, moving objects.
Deer flies are good fliers. They feed during the daytime and they can fly a long distance from the breeding site in search of a meal.
Deer flies occasionally enter buildings, but after they get inside, they fly toward the windows and keep flying there until they die.
Experts recommend securing the screens on windows and doors to prevent deer flies from entering.
Several species of deer flies are common in the United States. Chrysops callidus (Osten Sacken) and C. carbonarius (Walker) are widespread throughout the United States. C. discalis (Williston) is chiefly found in the western U.S, while C. flavidus (Weidemann) is more common in the east.
What Do They Eat?
Nectar and pollen from flowering plants make up the majority of the deer fly diet.
However, adult females also require blood meals to facilitate the development of eggs. Like mosquitoes, only female deer flies attack other animals for the purpose of ingesting blood, which the biting insects obtain by using specialized mouthparts that function like blades and slice open the skin of the host (and subsequently spongefeed on the blood).
During the day, deer flies usually hide under bushes, leaves, tall grass, or trees and wait for suitable host animals to pass.
The pest insects are also strong fliers and can travel for miles to find sources of food and blood meals.
Life Cycle & Reproduction
After mating and consuming the requisite blood meal, female deer flies lay eggs in batches of 25 to 1,000. Initially white in color, deer fly eggs darken soon after the female deposits them on rocks, soil, or vegetation located just above the surface of a body of water.
Larvae hatch from the eggs about five to seven days later and drop to the water beneath. Classified as hydrobionts, deer fly larvae develop in aquatic surroundings and feed on organic matter found in the water. Each larva possesses a breathing tube at the posterior end of a segmented and tapered body. Just before entering the pupal stage of the life cycle, larvae move to drier ground.
The larvae of most species are scavengers. They feed on decaying organic material. They molt several times as they grow. When they are full grown, the larvae make a case where they change into an adult fly.
Pupae are brown in color, with visible leg and wing cases. Three months to two years after hatching, deer flies emerge from the pupal stage as adults and begin mating.
Adult deer flies feed on nectar from flowers and honeydew. Females also seek a blood meal in order to produce eggs. They deposit their eggs in water or damp soil.
Adults typically live for 30 to 60 days.
Problems Caused by Deer Flies
Female deer flies in search of blood meals often inflict painful bites on humans and other animals. Frequently likened to a bee sting, deer fly bites occur when the pest insect identifies a suitable host, uses specialized mandibles like scissors to cut a hole in the skin of the host animal, and feeds on the resulting pool of blood.
Unlike mosquitoes, deer flies do not pierce the skin of the host. The female deer fly uses her mouthparts to tear the skin of the host. She drinks the blood as it flows from the wound.
Reactions to Bites
The female deer fly injects some saliva that prevents the blood from clotting during feeding. Many people have an allergic reaction to the saliva.
People have experienced lesions and high fever after deer fly bites. Many people have increasing allergic reactions after repeated bites. In some cases, people experienced swelling of the arms or legs and red streaks in the affected area.
Interrupt Outdoor Activities
Most prevalent during the summer months, local populations of deer flies can wreak havoc on residents and tourists alike and often disrupt outdoor activities such as construction work, fishing, logging, sunbathing, and yard work.
In addition to biting humans on a regular basis, the pests often attack cattle, horses, and other livestock, resulting in lower productivity and potentially significant loss of blood over time.
They are attracted to movement and dark colors.
Deer fly bites are also known to facilitate the spread of tularemia.
One species of deer fly, C. discalis, has been known to transmit tularemia from infected rabbits and rodents to humans.
Signs of Infestation
Most people notice deer fly infestations only after the insects reach adulthood and are capable of flying and biting.
In severely infested areas, the insects often pursue and swarm around host animals in large groups. However, because adult deer flies sometimes travel several miles to find food and blood meals, pinpointing the source of an infestation can prove challenging.
However, the presence of deer fly larvae in damp soil or other moist areas near water generally signifies an infestation problem.
Few, if any, methods exist to prevent or manage deer fly infestations. The migratory tendencies of deer flies renders pesticides ineffective, while the propensity of the insects for breeding in wetland areas makes the use of toxic chemicals impractical and harmful to the environment.
Rather than futilely attempting to manage populations, residents should strive to prevent the occurrence of deer fly bites by wearing light-colored clothing, using DEET- or permethrin-based repellents, and covering as much exposed skin as possible when venturing outdoors.
In cases of severe infestations that negatively affect everyday life, contact a pest control specialist for professional assistance in managing and alleviating the problem.
It is probably not practical to try to eliminate deer flies outdoors. However, experts recommend wearing light-colored clothing and using insect repellants during outdoor activities. The local pharmacy may be able to recommend a repellant that works well in the local area.