“Deer fly” is a common name for any of 250 species of the genus Chrysops. Deer flies are found all over the world, and 45 deer fly species are found in Pennsylvania. Most Chrysops species prefer habitats that are fully to partly sunny with a sufficient source of water and prey species to support their reproduction. Deeply shaded areas (like dense woodlands and inside barns and stables) are typically avoided by deer flies.
Appearance Deer flies are between 5 and 12 mm long (larger than house flies but smaller than horse flies). They are black, yellow and black, or, sometimes, gray and often have “bee-like” abdominal stripes. Their wings are cross banded with black stripes, and their eyes are brightly colored (gold, green, orange, or blue). The genus name “Chrysops,” in fact, is derived from the Greek words “chrysos” (gold) and “ops” (eyes)
. Diet, Prey Species and Diseases Female deer flies require a blood meal from a vertebrate prey species in order to make viable eggs. Reproduction is not possible without this blood meal. Typically, females wait in shaded microhabitats for passing prey (which includes, but is not limited to, deer, cattle, horses, and people). Visual perception and/or carbon dioxide or body odor detection attracts the female fly to its prey. Moving objects (often darkly colored (blue seems to be the color that most strongly attracts deer flies)), lights in the night, body heat, and, in the case of one species, wood smoke all serve as attractants for deer flies. Most often the head and neck of the prey species is attacked. In general the hours around dawn and dusk are the times of maximum deer fly activity. Temperatures below 22 degrees C (72 degrees F) or above 32 degrees C (90 degrees F) and overcast skies prevent or inhibit deer fly activity.
Once a female deer fly has located its prey individual, it uses its mouthparts (mandibles and maxillae) like a scissor to cut open a wound. Anti-coagulants in the fly’s saliva prevent the blood from clotting (and may also serve as an antigen for a sometimes severe allergic reaction), and the fly laps up the flowing blood with its sponge-like labrum. The wound may continue to bleed after the fly has departed and may also form a large, painful welt. A number of diseases and parasites can be transmitted by the various species of deer fly. These include anthrax, tularemia, equine infectious anemia, anaplasmosis, hog cholera, filiariasis (including the Loa Loa worm), and Lyme disease. Deer flies can reduce milk production in dairy cows by 20 to 30% and can cause up to 100 pounds in weight loss in cattle. They can also have serve impacts on tourism!
Life Cycle Throughout the summer, eggs are laid by gravid females on clusters of vegetation (often cattails or sedges) in marshy or wet soil areas. Following a 5 to 12 day incubation period, emerging larvae drop into the aquatic or semi-aquatic habitat, burrow into the sediment or wet soil, and begin to broadly feed on organic materials, insects, annelids, and crustaceans. The larvae remain somewhat active through the winter and then pupate in the drier conditions of the late spring. Pupation lasts 6 to 12 days after which the adult flies emerge. Males emerge a few days before the females. Mating occurs in early summer after which the females begin their hunt for blood meals.