One of the more common insects affecting livestock is the horsefly. Much larger than a typical house fly or stable fly, horseflies can grow to be just over an inch and have a terrible bite.
I noticed them first in unusual numbers while driving slowly through a local national forest as I was studying the area for our upcoming hunting season. My truck was surrounded by enough of these flying insects that, for a moment, I thought I may have stirred up a hornet’s nest. Making sure to keep my windows up, I stopped to identify what was buzzing and hitting my truck. It was horse flies.
Since then, I’ve received calls from area residents about these ornery flies, where they come from, and how to control them.
Entomologists have identified 52 species of horseflies. They range in size from 3/8 to just over 1 inch long and vary in coloration by species. Some are all black while many have colored patterns on their abdomens and wings.
Few flies can be confused with horse flies as there are very few other tough flies which are capable of biting man and animals. They commonly feed on cattle and horses. Other mammals at times are certainly part of their diet, and given the chance, they certainly can pester humans.
Horse flies are significant livestock pests with their painful and persistent biting behavior. Heavy attacks can lead to reductions in weight gains of beef cattle, reduced milk yield, reduced fed utilization efficiencies and hide damage from the puncture wounds. Horses under attack will be irritable and distracted by trying to avoid being bitten.
Their biting activities can be annoying in outdoor areas, as the bites can be very painful and remain swollen for several days. Yet while this fly may be ‘quite annoying’ to people, their incessant attacks on livestock can cause them to lose weight. Additionally, their biting and feeding is capable of mechanically transmitting animal diseases, including viruses, bacteria, protozoans, and nematodes of animals.
Like the mosquito, only adult female horse fly species are attracted to man and animals in search of a blood meal. The female mouthparts are modified for piercing and sucking blood. Male horse flies are nectar feeders and often hover at certain times of the day, presumably to attract females and maintain a territory.
Once the females have had enough ‘blood meals’, they will lay eggs later in the season. Females often lay eggs in specific locations, such as on vegetation overhanging water. Eggs are laid in masses that darken to brown or black before larvae hatch out and drop to the ground or into water.
Their larvae are generally whitish, spindle-shaped, and develop through six to thirteen stages (instars) over one or more years before pupating. Larvae have chewing or tearing mouthparts; adult Larvae live in species specific habitats, although most are aquatic, semi-aquatic or terrestrial. They are generally predaceous and cannibalistic, feeding on other insect larvae and earthworms
Come spring, horse fly larvae will pupate and begin emerging as adults in late spring and summer. Unlike many common horse flies typically have one generation per year.
Controlling them is difficult. One author suggested that adult horse and deer flies can be ‘collected by swinging an insect net around one’s head’. This activity could be accomplished while walking through an infested area, such as along a wooded path, near swampy areas, on the beach or other bodies of water at the right time of the year.
Special traps developed by researchers, called malaise traps, are baited with a source of carbon dioxide (such as dry ice), and visually attracting objects (simply a black plastic sheet or sphere) are very effective for obtaining large numbers of the flies. This set up may be quite effective at a farm/ranch or large outdoor venue.
Using topical pesticides is ineffective due to horse fly behavior, but short-term control is possible. Aerial applications (fogging) and treating for the larvae are ineffective. The best option is to provide shelter for the animals and people, or pasturing livestock away from infested areas.
