Horse Flies

By Mary Kay Ruble

Horse flies and deer flies, which fall into the same fly family, are the bane of many horse-people’s existence, with their biting, buzzing and usual annoying dive-bombing tactics. It seems that not only must we worry about ourselves and our horses being bitten, but we also worry about what our horses will do to evade the bite itself, sometimes putting us at risk of being kicked, bumped, trod over or stepped on.

Of the 2000 species of horse flies world-wide, several of those species need a blood meal to develop eggs, but of those, only the females will bite and draw blood from mammals and humans. With their knife-like mouth parts, and painful bites, the flies are static in their feeding methods, as a host animal will make evasive maneuvers once bitten, cutting short the fly’s snack time. The horse fly will infuse an anticoagulant to keep their meal free-flowing, and once it finishes, will fly away, leaving an open, blood-oozing wound for secondary feeders to dine on, and spread diseases thru. Along with the bite and draw of blood, horse flies can spread Equine Infectious Anemia and according to a 1990 issue of The New England Journal of Medicine can also transmit Lyme disease.

The less vicious male fly will dine on plant juices, nectar and other plant liquids, rarely upsetting the lives of our horses, until he searches out a female to play the reproductive game with.

The female will lay her eggs on vegetation that hangs over wetlands, streams, ponds or lakes. Nearly every species of horse fly is aquatic or semi-aquatic and their larvae require a moist area in which to mature and spend the winter. Conveniently, once the eggs hatch, it is a short drop into water for the lucky larvae.

Mature horse flies are attracted to shiny objects, movement, warmth and possibly carbon dioxide, which explain why human swimmers in the glittering water of backyard pools are often bothered by horse flies.

Because eradication of their breeding-ground wetlands is unlikely, and in most areas of the U.S is highly regulated, we will be forced to suffer the cohabitation of the dreaded horse fly during our summer daylight riding hours. If we can limit the amount of shiny bling that we and our horses wear, lower our body temperatures, don’t move and don’t breathe, we should be safe from the flies! Good luck to us all!