Flies are a natural part of keeping horses. Filth flies, like stable and house flies, are one of the main concerns in Minnesota, along with aquatic biting flies. 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 including aging feces, soiled animal bedding and rotting feed debris. Biting stable flies cause horses and other livestock to swish their tails, twitch their flanks and stamp their feet. House flies don't bite animals, but can spread fecal bacteria. House flies will feed at horses' eyes, body orifices and fresh manure. 

Control filth flies around your barn by managing debris. Long term, preventing debris will be more effective than chemical control. Keep feed dry. Avoid ground feeding. Disk, spread or compost waste. Clean up manure at least two times per week. Spread or compost. Replace bedding weekly. Wood shavings and sawdust produce fewer flies than straw. Place waterers in well-drained areas and away from where you feed horses. Keep in good repair.

Always carefully read and precisely follow label instructions when using chemical insecticides. Keep in mind insecticides are much less effective if debris is not managed. Pyrethrum fogs and space sprays kill adult flies indoors, but only provide temporary relief. Owners can apply longer-lived pyrethroid and organophosphate residual premise sprays indoors and outdoors. These are most effective when applied to fly perching areas. Residual premise sprays may be effective for up to three weeks. Longevity depends on the cleanliness of the site sprayed. Stable and house flies 'perch' on solid surfaces where they won't get disturbed — often above head height. Owners can identify perching sites by fly specks. Fly specks are small brown spots of fly waste. Identifying perching sites can help 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 are small, ant-like insects that kill filth fly pupae. They occur naturally around animal premises, can provide natural biological control of filth flies, and are harmless to people and animals. Female wasps lay eggs inside fly pupae and the wasp larvae kill the developing fly pupae. Owners 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 the number of fly pupae they must kill.

Hanging plastic bags of water around buildings has no evidence showing it repels house flies. Additionally, 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, and using too much lime in pastures can stop some plant growth.

In a recent University of Minnesota study, researchers set out to determine which products best help horses avoid flies. Researchers found that a citronella spray reduced tail swishes and shoulder twitches, while leggings and leg bands reduced head-backs and hoof stomps. However, no one treatment reduced all of these behaviors in horses. 

This article was submitted by Roger Moon, PhD,  University of Minnesota.