Published: July 2021 | Updated: June 2023
If there’s one thing horse owners know, it’s that flies are no fun. In fact, they can be a downright nuisance throughout the summer months. Unfortunately, getting rid of flies altogether simply isn’t feasible. However, there are a number of ways you can reduce the fly population on your farm premises.
Why Flies Are a Problem For Your Horse
Not only can biting flies seriously irritate horses and make them miserable in the pasture, they can also interrupt training or riding time. Worse, flies pose a health hazard to our horses by transmitting disease. In fact, the following conditions are all caused by different types of flies:
Summer sores develop when houseflies, face flies, and stable flies transfer parasitic worm larvae to areas around the horse’s eyes, nostrils, mouth, genitals, or wounds. The larvae can’t complete their life cycle in these locations, but they can lead to a hypersensitivity reaction, resulting in chronic non-healing wounds.
Eyeworms can infect horses when face flies transmit Thelazia lacrymalis, a worm that resides in eye glands and ducts. This results in irritation and can also lead to secondary infection.
Pigeon fever occurs when houseflies, stable flies, and horn flies spread the bacterium Corynebacterium pseudotuberculosis. Infection commonly leads to external abscesses on the chest, though some horses develop more serious forms including internal abscesses and ulcerative lymphangitis (infection of the limb).
Equine infectious anemia (what a Coggins test is looking for) is transmitted from horse to horse by horse flies and deer flies. This virus is often fatal.
Hypersensitivity is caused by biting flies and gnats. Horses may develop hives, intense itching, hair loss, skin thickening, abrasions, and skin ulceration that can put them at risk for secondary skin infections.
Onchocerciasis is transmitted through Culicoides species which act as the intermediate host for the nematode parasite Onchocerca cervicalis. The immature forms of this worm are found in horses’ skin (often at the midline of the abdomen). Adults, however, reside in the nuchal ligament of the neck. This parasite can cause skin irritation that results in scales, crusts, ulceration, hair loss, and loss of skin pigmentation.
Aside from transmitting disease, flies can also have a negative impact on horses in other ways, such as:
- Causing pain and irritation, and leading to pest avoidance responses including tail swishing and head and neck movements;
- Causing changes in grazing behavior such as reduced grazing time and subsequently, lower forage intake;
- Reducing energy available for growth, reproduction, and body condition maintenance; and
- Increasing stress responses such as blood cortisol concentrations, heart and respiration rates, and rectal temperatures.
Why Flies Are Attracted to Your Horse Barn
There are two main reasons why flies are attracted to horse barns and surrounding areas: manure and organic matter such as wasted hay or feed on the ground. Flies act as scavengers, consuming rotting organic matter, and many fly species also lay their eggs in manure or waste hay as well.
Flies also tend to congregate around wet areas such as puddles and wet stall bedding, so the wetter your premises, the more flying pests you will likely attract. If there are cattle or other livestock housed nearby, this too, can increase the amount of flies.
What You Can Do to Maximize Your Horse Fly Control System
We’ve all heard it before, but an ounce of prevention is worth a pound of cure, and this is certainly true when it comes to fly control. However, prevention will take some work. The key? Proper sanitation practices in barns and pastures, primarily revolving around manure and moisture management.
Regular removal of manure and wet bedding in stalls is of utmost importance, but removal of manure from pens and pastures is also helpful. When multiple horses are housed on a small farm, this will be even more crucial for fly control.
Regarding moisture management, proper drainage in and around barns should be a priority. Stall bedding type can also affect fly development, and sawdust appears to be the most favorable choice when it comes to fly prevention and management. Likewise, adding a drying agent to stalls can also help to discourage flies.
However, aside from both of these strategies, a multi-pronged approach can often be helpful.
Other methods of fly control that horse owners can implement include:
1) Fly predators
Fly predators consist of several different insect species, including Muscidifurax raptorellus, Spalangia cameroni or Spalangia endius, and Muscidifurax zaraptor. These tiny insects feed on the larvae of the common house fly, horn fly, biting stable fly, and the lesser house fly, making them a great ally to have for pest fly control.
Several companies sell fly predators and with each monthly shipment, larvae should be scattered around existing manure piles where they will hatch and go to work. When using fly predators, however, you will need to start in early spring and continue to put them out throughout the summer months.
2) Fly traps and tapes
Fly traps and tapes can also be placed around the barn as a way to manually kill flies. Though they may not kill a huge number of flies, these products are fairly inexpensive and can help to play a small part in fly control.
3) Premise sprays
Premise sprays can be an effective and larger scale method of controlling pest fly populations. These products are typically applied to barn walls, ceilings, or other places where flies may congregate. However, it is important that animals and water sources are protected when using premise sprays. It should also be noted that flies can build up resistance to insecticide chemicals over time, especially with indiscriminate use. Therefore, rotating the type of active ingredient and using premise sprays judiciously is advised.
Fans help to deter flies inside barns by aiding in the drying of stall bedding. The moving air they produce can also create a more inhospitable environment for flies while helping to keep horses more comfortable during hot summer months.
Protection With Horse Fly Control Products
Aside from treating your pasture and barn, some horses may still be affected by flies more than others. Older horses, those with reduced immune systems, and those that have previously developed skin conditions such as Culicoides sensitivity may need added protection through one or several of the following methods:
1) Feed through fly control for horses
Two main categories for feed-through fly control products exist: those containing organophosphates and those that are organophosphate-free.
Organophosphate products contain the active ingredient tetrachlorvinphos and are designed to pass through the digestive system and into manure with minimal absorption. When house and stable flies lay their eggs in the manure, the eggs will hatch but won’t grow into adult flies because the tetrachlorvinphos affects the larvae’s nervous system. Organophosphate products have proven to be safe and 99% effective against house and stable flies if used daily.
Organophosphate-free products must currently be purchased through a veterinarian. These products work similarly to organophosphate products, but the active ingredients, known as insect growth regulators (IGRs), are different.
Two commonly used IGRs include diflubenzuron and cyromazine, both which work to inhibit the formation of chitin, the primary structural component of the exoskeleton on fly larvae. FDA safety studies have shown that these products are also safe for horses and are only toxic to insects.
In order for feed-through products to be effective, they should be started at the beginning of fly season and fed to all horses on the premises daily. It should also be noted that feed-through fly control products don’t kill adult flies.
2) Horse fly spray
Fly sprays are the most common means of fly control on many horse farms. These products range in ingredients and price range, and there are also “natural” sprays available (typically made with essential oils) for horse owners who are more holistically-minded.
Fly sprays are either water or oil based and both come with advantages and disadvantages. The water based sprays are less messy, but they may not have the “staying power” that oil based sprays have. The oil based sprays may last longer, but they can attract dirt and make your horse grimy in some instances.
Fly sprays can be effective, but most only work short term. Therefore, other methods of fly control are typically needed as well.
3) Spot-on treatments
Another topical fly control option is spot-on treatments which work similarly to spot-on tick treatments for dogs. These products are applied to specific parts of the horse’s body and generally provide some degree of fly protection for up to two weeks.
4) Fly masks, sheets and boots
Fly masks, sheets and boots (which cover the lower legs) come in a variety of sizes and styles and can be quite helpful in giving your horse added protection from biting flies. These products are made with thin, breathable mesh which will prevent your horse from overheating. If using any of these products, however, it’s important to make sure they fit correctly and don’t cause rubbing or irritation.
Flies can undoubtedly be a problem for horses, but by employing several of the above strategies in a comprehensive fly control program, you can help to keep your horse more comfortable this summer and also reduce their risk of inflammation and disease caused by these flying pests.