You can best control filth flies by managing moist organic debris sources:
Manure and bedding
Traps, stingless parasitic wasps and insecticides for filth flies are most effective when combined with debris management.
Managing water sources and providing your horse deep shade or housing can best protect them from aquatic biting flies.
Work with a veterinarian to have your horse vaccinated for mosquito transmitted viruses.
Flies are a natural part of keeping horses. Filth flies and aquatic biting flies are the main concerns in Minnesota. Understanding what these pests are and how they live and breed can help horse owners limit their fly pest problems.
Filth flies develop in moist organic debris such as:
Soiled animal bedding
Rotting feed debris
Stable fly adults have seven black spots on a gray abdomen. Their heads have bayonette-like mouth parts that pierce the skin for blood.
Biting stable flies cause horses and other livestock to swish their tails, twitch their flanks and stamp their feet. Only 5 percent of adult stable flies near a horse will be on the animal at any one time. The other 95 percent will perch on nearby fencing, buildings, and plants.
House flies don’t bite animals, but can spread fecal bacteria. House flies will feed at horses' eyes, body orifices and fresh manure. Like stable flies, only a small fraction of house flies are on a horse at any one time.
In Minnesota, filth flies reproduce continuously from May into October. Adult females lay 50 to 150 eggs every few days. Females place eggs in moist organic debris.
Small maggots hatch from eggs and feed on bacteria growing in the debris. Ideal conditions for maggots are in debris that is 40 to 80 percent moisture and 70 to 95 F.
Are 1/4 to 1/2 inch long
Have heads that taper to a point
Have round abdomens with two dark pores used for breathing
Common debris sources around horse barns:
Muck near leaky waterers
Old hay around feeders
Maggots mature into pupae and then into winged adults (flies). The entire life cycle can take as little as two weeks. Adults can live one to three weeks depending on the weather.
Control filth flies around your barn by managing debris. You should also scout weekly for possible maggot breeding sites. Long term, preventing debris will be more effective than chemical control.
Feed: Keep dry. Avoid ground feeding. Disk, spread or compost waste.
Manure: Clean up at least two times per week. Spread or compost.
Bedding: Replace weekly. Wood shavings and sawdust produce fewer flies than straw.
Waterers: Place in well-drained areas and away from where you feed horses. Keep in good repair.
Chemical and non-chemical control
Always carefully read and precisely follow label instructions when using chemical insecticides.
Insecticides are much less effective if you don’t manage debris.
You can use pyrethrum or resmethrin fogs and space sprays to kill adult flies indoors. These only provide temporary relief.
You can apply longer-lived pyrethroid and organophosphate residual premise sprays indoors and outdoors. These are most effective if you apply it to fly perching areas. Residual premise sprays may be effective for up to 3 weeks. Longevity depends on the cleanliness of the site your spray.
Stable and house flies 'perch' on solid surfaces where they won’t get disturbed, often above head height. You can identify perching sites by fly specks. Fly specks are small brown spots of fly waste. Identifying perching sites can help you determine where to apply residual insecticides for adult flies.
Poor debris management or off-site fly sources can limit the efficacy of fly traps. Sticky traps and ultraviolet electrocutor traps will catch and kill stable and house flies. Baited traps will attract and kill house flies, but not stable flies.
Stingless parasitic wasps
Stingless parasitic wasps are small, ant-like insects that kill filth fly pupae.
They occur naturally around animal premises.
They provide natural biological control of filth flies.
They are harmless to people and animals.
Female wasps lay eggs inside fly pupae and the wasp larvae kill the developing fly pupae.
You can purchase and release parasitic wasps to supplement natural populations. Success is inconsistent among studies. It likely depends on the amount of fly breeding media and number of fly pupae they must kill.
Fly repellants provide temporary relief from stable flies. Effectiveness can provide temporary relief from attacking stable flies. You should apply repellants to the legs, where stable flies are most likely to attack. You will need to reapply it if your horse walks through wet vegetation.
Bags of water
Hanging plastic bags of water around buildings has no evidence showing it repels house flies.
Recipes for stable and other biting fly repellents usually include:
- Bath oil
- Plant oils: may repel mosquitoes but not as effective as commercial products
- Herbal extracts
Some of the ingredients in these recipes may harm horses with sensitive skin.
Products registered with the Environmental Protection Agency (EPA) have undergone extensive safety testing. These products carry an EPA number on their label.
Hydrated lime or calcium hydroxide (commonly sold as barn lime) reduces moisture and can reduce ammonia odor in barn stalls. It can increase soil pH if you use it in large amounts.
The amount (usually minimal) of barn lime used in horse facilities likely isn’t good for fly control. Fly maggots tolerate a wide range of pH.
Using too much lime in pastures can stop some plant growth.
Aquatic biting flies
Black flies are 1/16 inch long and gnat-like insects. They commonly attack horses housed outdoors from May into July in Minnesota.
Black flies only bite during the day and few enter dark, shady areas. Horses can develop scabby lesions from repeated biting during outbreak times. These bites commonly occur in the following areas on the horse:
Inside the ears
You can salve the lesions with petroleum jelly to promote healing and prevent further biting.
Larvae develop in flowing creeks, streams and rivers. Adults can travel several miles from their larval source. Thus, these flies can attack horses on a site that may not have flowing water present.
Horse and deer flies
These 1/3- to 1-inch long, stout flies are active around swamps, where their larvae develop. Adults tend to stay near swamps but can travel a few miles for hosts. Horse and deer flies are only active in daylight.
Mosquitoes are 1/8 to 1/4 inch long. They can carry three viruses that can be deadly to horses.
Consult with your veterinarian to vaccinate against these viruses.
Most mosquito species are active from sundown into the night. A few can be active during the daytime as well.
Mosquito larvae grow in pockets of still water with decaying leaves and algae, including:
Naturally occurring tree holes
Artificial containers such as water troughs, old tires, etc.
Your horse can get relief from these flies (not mosquitoes) by being inside or in deep shade during daylight hours.
You can protect horses from mosquitoes by housing them indoors or behind screened doorways and windows.
Commercial repellents aren’t effective against black flies, horse flies or deer flies.
Manage water sources
Prevent on-site mosquito breeding by cleaning water tanks and garden containers.
Dispose of old tires.
Drill drain holes in tire swings.
Overturn or discard all buckets and containers that can hold rainwater for over a week.
Traps likely only kill a small fraction of flies present around horses. Trap use hasn’t been proven to improve horse comfort or protection from mosquito viruses.
Reviewed in 2018