The Horse Owners Survival Guide to Fly Season

Equine Care

Whether it’s the common house fly, deer fly, horse fly or blow fly, we can all agree that they are one of the biggest frustrations of horse ownership. Flying insects are a nuisance for sure, but more than that, they are bearers of disease. In North America alone, there are 16,000 types of flies. Some of them eat by vomiting on their food to break it down, some of them eat or lay eggs in dead animals, some of them are bloodsuckers, most love an open garbage can… and they carry whatever pathogens they pick up along the way on their hairy legs and in their mouths. So when they land on your arm (or your fork), they pass these pathogens on to you.
Here are a number of common types of flies in North America and some of the diseases they carry.


Dirty Common housefly viewed from up high, Musca domestica, isolated on white

The housefly is recognizable by its grayish color and one of the most prolific of pests. Found all over the world, it can carry salmonella, staphylococcus and E. Coli, and cause diseases such as typhoid fever, cholera, hepatitis, polio, tuberculosis and dysentery. These flies don’t bite, but they ingest food by vomiting on it to start breaking it down. Then they sponge it up with their mouths. Their diet includes organic scraps in the garbage, exposed food in the home, carcasses and manure. Manure piles are a popular breeding ground for houseflies, as are dirty trash cans.


Blow flies are a cool metallic blue or green color and a bit larger than houseflies. They are located throughout North America, but they are especially prevalent in the Southwest. Blow flies feed on decaying meat and organic matter. Their habitat is often around dumpsters. The blow fly prefers fresh meat and is often the first visitor to a new carcass, where it lays its eggs, which quickly hatch into maggots (larvae). For this reason, maggots can be used by forensic experts to establish time of death. Blow flies also lay eggs in wounds and have been used medicinally to clean wounds. They can transfer bacteria that cause dysentery, typhus and cholera.


Horse flies are often large and agile in flight, and the females bite animals, including humans, to obtain blood. They prefer to fly in sunlight, avoiding dark and shady areas, and are inactive at night. Adult horse-flies feed on nectar and plant exudates; the males have weak mouthparts and only the females bite animals to obtain enough protein from blood to produce eggs. The mouthparts of females are formed into a stout stabbing organ with two pairs of sharp cutting blades, and a sponge like part used to lap up the blood that flows from the wound. The larvae are predaceous and grow in semi aquatic habitats.
Female horse-flies can transfer blood-borne diseases from one animal to another through their feeding habit. In areas where diseases occur, they have been known to carry equine infectious anemia virus.

The life cycle of these pesky creatures varies depending upon the species. For example, the life cycle of a house fly begins in the egg stage. A female house fly is capable of laying up to 150 eggs in a batch. Over a period of a few days, she will produce five or six batches of eggs. Female house flies favor damp, dark surfaces such as compost, manure and other decomposing organic material for egg laying. House fly eggs resemble individual grains of rice.
Within a day, house fly eggs hatch into larvae, also known as maggots. Maggots are legless, white insects that feed from the egg-laying site for three to five days. During this time, maggots molt several times. They then choose a dark place to pupate.
Fly pupae are similar in function to butterfly cocoons: their hard, brown shells protect the inactive, developing flies. Over the course of three to six days, the pupae develop legs and wings, ultimately emerging as full-grown house flies. Within two to three days, female house flies are capable of reproduction and the process begins again!

We certainly will not eradicate all of the flying insects that plague us and our horses, however with a few simple steps, we can reduce their numbers and improve our time at the barn.

1. Control Moisture:
Insects are drawn to wet areas, where they breed as well as drink. Keep stalls dry; eliminate standing water in low-pasture areas such as receptacles and old tires or feeders; create good drainage around your barn; repair plumbing leaks and cover rain barrels.

2. Manage Manure:
Probably the single most important way of controlling flies is proper manure disposal. Stable flies, the most annoying of all the biting flies that bother horses, breed in manure. Houseflies also prefer manure for reproduction.
The best way to keep these fly numbers down is to frequently remove manure from your horse’s stall or paddock. Barn aisles, turnout areas and riding arenas should also be cleaned. Clean at least once a day; more often if you can.

3. Air Power:
For stalled horses, strategically place large fans in your barn or aisle way to air blast the flies away. For pastured horses, provide access to open, breezy expanses.
Minimize the Buffet:
Dispose of garbage in enclosed containers with airtight lids. Cover grain bins and other food sources securely.

4. Biological Controls:
An eco-friendly way to combat flies is to use biological methods. Tiny parasitic wasps are widely available, and work by destroying the fly larvae before it can hatch. Signing up for a monthly delivery of these predatory insects during fly season can help keep the fly population down at your barn.
Harmless to humans and so small you can barely see them, the wasps are shipped still in the egg stage. Once a few of the wasps hatch in the plastic bag after delivery, you sprinkle the contents in and around your horse’s living area.

5. Traps:
Available in a variety of different types, fly traps work by attracting flies, either through color or scent. Once the fly lands on or enters the trap, it can’t escape and eventually dies.
The simplest type of fly trap uses a sticky surface to capture flies. The flies are drawn to the color—usually orange or yellow—and land on it. The glue-covered surface sticks to their feet, keeping them from flying away. Other traps use bait to lure the fly inside. Some traps have water inside, and the flies drown because they can’t escape. Others simply trap them inside the container, where they die due to lack of food and water.
Sticky fly traps can be hung anywhere flies congregate, but should be safely out of reach of curious horses. Baited traps should be placed far from stalls since they will initially attract more flies to the area before the insects become trapped.

Simple DIY traps are effective and easy to make.

6. Feed Through Supplements:
Supplements designed to prevent flies from breeding in manure or to discourage flies from biting are another option for your horse. Insect growth regulators (IGR) are chemicals that disrupt the fly’s ability to reproduce by affecting the pupa stage in the life cycle. When the active ingredient in the IGR is present in the horse’s manure, the fly larvae can’t develop to adulthood.
A feed-through product containing an IGR is added daily to the horse’s food, and then passes through the horse and is deposited with the manure. The IGR has no effect on the horse, but wreaks havoc on the fly population.
Natural supplement formulas containing garlic, apple cider, yeast and other ingredients with natural fly-repelling properties are reported to discourage flies from biting. These products must be fed daily to be effective.

7. Topical/Sprays:
Both insecticidal sprays and repellents discourage flies from landing on your horse.
Oil-based sprays have more staying power than water-based products, and both are best used in conjunction with other types of fly control. They should not be used in the vicinity of parasitoid wasps, however, since the beneficial insects are also susceptible to insecticides.
When applying fly sprays, be sure to use an adequate amount as directed on the label. If you don’t coat the horse’s hair with the spray, flies can still land and bite. For applying around the eyes, ears and muzzle, spray the product onto a washcloth and wipe it onto the horse’s face. For a more natural chemical-free option try Equiderma Neem & Aloe Spray.