Deer flies (family Tabanidae) have been particularly bothersome in many areas of Minnesota this year. These flies are about 1/4 - 3/8 inch long and are stout-bodied. They have yellow or black colored bodies with dark colored markings on their wings.
The larvae live in aquatic or semiaquatic areas, like marshy areas, streams and ponds. Adults are found near these breeding grounds, especially along the edge of woodlands but they are strong fliers and can be found miles away from these breeding areas. Watch out for deer flies especially on sunny, calm days. They have a tendency to wait in shady areas for hosts and ambush them as they move past. Deer flies primarily use sight to find a host and seem to be particularly attracted to moving, dark shapes.
They go for the head and neck when biting people. They inflict a painful bite as they use knife-like mouthparts to slice a wound in the skin and feed on the resulting blood. Fortunately, deer flies do not vector disease in Minnesota, although some people can suffer allergic reactions to the bites. In addition to humans, these biting flies also attack many different animals, including deer, horses, and cattle. Deer flies are most common in June and July, although can persist throughout the summer.
Unfortunately, we have very limited options when it comes to preventing deer flies from biting us. It is not practical to control immature deer flies by eliminating breeding sites, i.e. marshes, streams, and ponds. There are just too many potential sites to treat and the risk of environmental harm is too great. It is also prohibitive and impractical to treat adult flies in yards, parks and others areas with insecticide applications.
Photo 2: Deer fly taking a blood meal. Knife-like mouth parts usually results in a painful bite.
There are also a variety of devices that purport to protect people from deer flies. One method involves placing sticky patches on the back of hats. In theory deer flies land and stick to the patch before they can bite you. Another device is the trolling deer fly trap. You use a blue cup covered with glue. You mount it either to hats or caps or machinery, such as lawn mowers. The idea is the deer flies are attracted to the cup, land and get stuck on the glue, preventing them from biting you. It is advertised to be most effective when it is moving.