Deer Flies Hunting for a Meal

— Written By Michael Waldvogel

If you’ve been outdoors working or  jogging or horseback riding along tree-lined trails, you’ve probably had the misfortune of being harassed by deer flies or horse flies searching for a meal.

Adult deer flies are yellow with dark stripes.

Adult deer flies are yellow with dark stripes. Photograph by Matt Bertone (2014).

Deer flies range in length from 1/3″ to 4/10″. Horse flies are slightly larger and can be nearly 1″ in length. Deer flies are yellowish and have dark stripes on their thorax and abdomen. Their wings have dark patches.

Deer flies pass the winter at larvae and mature in the spring. Adult flies show up primarily in May-August (depending on the weather). Similar to mosquitoes, only female deer flies and horse flies bite animals to obtain a blood meal which they use to produce eggs. Males feed on plant nectar. The females attack animals along hiking trails and the edges of wooded areas. They have mouthparts equipped with a pair of blades which make the incision in your skin. They then use the sponge-like part to soak up the blood that pools on the skin surface. Eggs are laid on foliage in wet-to-flooded (and often shaded) areas. They hatch in about a week and the larvae drop to the ground where they prey on small invertebrates and on organic matter.

People ask about what they can spray for the flies. Unfortunately, trying to spray adults isn’t effective because they are a moving target and it’s too difficult to identify and spray landing/resting areas on shrubs and other vegetation as we do for mosquitoes. Targeting breeding sites is equally difficult (and often environmentally complicated). Adults can be trapped using shiny black balls (resembling bowling balls) or using a dark plastic cup covered with glue. These traps are hung and must be moving with wind currents to attract the flies. (see

The best option for people spending time outdoors is to use repellents but keep in mind if you’re out there for work or exercise, you’re also sweating which reduces the length of time that the repellent is effective and you’ll need to reapply it.

For more information about deer flies and horse flies, visit:

Picture of deer fly from Matt Bertone’s Flickr site:  (