Deer Flies: Good Bug or Bad Bug

By Kathy Conner Cornell VCE Southside Master Gardener When my husband and I first moved to Southside, we were amazed at the amount and diversity of insects. My husband would tease that we have a new bug every week. Since I know the majority of insects are either beneficial or innocuous and that birds need a variety of insects to feed their young, I wasn’t concerned. However, my husband found one particular insect extremely annoying and we learned that was the deer fly. Deer flies are relatively small, 1/4 to 1/3 inches. The wings are clear with mottled specks throughout. Deer flies prefer to bite humans and mostly go after moving targets. Deer flies are in the genus Chrysops, they share the same family,Tabanidae, with horse flies. Both the deer fly and the horse fly inflict a painful bite to open the skin to suck out blood. Just like mosquitoes, it is only the female that bites. The saliva from the bite can cause a swelling that will go away in a day or two. However, if scratched, could lead to infection.

“No one would ever call them a ‘dear’ fly,” quips Eric Day, manager of the insect identification lab at Virginia Tech’s department of entomology. This female deer fly is the biter. Males feed on nectar. Picture credit University of Georgia The process of feeding on one animal or human and then going to another means that deer flies could be carrying diseases. According to Ohio State Extension, “There is evidence that a deer fly in the western U.S. is involved in the transmission of a bacterium that causes the disease tularemia, which is known as deer fly fever and rabbit fever. The role of deer flies in transmission is minor, however, compared to transmission by ticks and via contact with infected small game animals, especially rabbits”. We know that mosquitoes are the most dangerous insect in the world when it comes to disease transmission. One aspect that makes deer flies difficult to control is its life cycle. It is a true fly so has a complete life cycle including egg, larvae, pupa and adult. Per University of Kentucky Department of Agriculture, “The larvae of horse fly and deer fly species develop in the mud along pond edges or stream banks, wetlands, or seepage areas. Some are aquatic and a few develop in relatively dry soil. Females lay batches of 25 to 1,000 eggs on vegetation that stand over water or wet sites. The larvae that hatch from these eggs fall to the ground and feed upon decaying organic matter or small organisms in the soil or water. The larvae stage usually lasts from one to three years, depending on the species. Mature larvae crawl to drier areas to pupate and ultimately emerge as adults”. Generally, insects are best controlled in their larval state. However, because they are residing in sensitive aquatic environments, insecticides are not an option. The deer flies were especially aggravating to my husband when he was mowing the lawn. His way of coping was to strap one of those battery operated bug zappers, that look like tennis rackets, to the front of his lawn tractor. I’d laugh listening to the sizzle, sizzle as he rode along. Are there any good aspects of the deer fly? I found one. Male deer flies feed on nectar, therefore, they are pollinating plants. “Natural predators of deer flies include frogs, toads, spiders, wasps and hornets, dragonflies, and birds such as the killdeer” says the University of Maine. Therefore, limit or eliminate your use of pesticides and synthetic fertilizers to keep these critters around. Whether deer flies are a good bug or bad bug is a matter of personal opinion. For some reason, deer flies tend to attack some people more often than others. When we would take our daily walk, my husband was always swatting at them but I was rarely bothered. Of course, I think my husband is much sweeter than I! We were married in June and in 2019 celebrated our 25th anniversary. We decided against having a celebration in June because of the deer flies and went with October instead. So, in our mind, deer flies are a bad bug. Fortunately, deer flies are only prevalent in Southside from May through early July. The VCE Halifax Extension Office is now open. Masks and social distancing are required. If you have gardening questions, you can continue to reach an Extension Master Gardener or Extension staff member by sending an email to wmccaleb@vt.edu or calling the Halifax Extension Master Gardener Help Desk at (434) 830-3383. Be sure to give us your first and last name, telephone number and the nature of the call. The Help Desk phone is routinely checked, Monday - Friday. Someone will get back to you, although it may be from a different telephone number.