Get a Jump-Start on Fly Season

Understanding insect life cycles and the conditions that encourage or discourage populations to proliferate guides management decisions that can help you beat the bugs, reduce the risk of disease transmission, and keep your horse comfortable during peak pest season. | iStock

Practicing smart insect control tactics year-round can help you keep populations under control

Biting insects can cause unsightly welts and itchy bumps. They can also transmit pathogens that cause diseases such as West Nile virus and pigeon fever. Unfortunately, however, these pests are simply part of barn life. While it’s impossible to eliminate every irksome insect, you can take a variety of steps to minimize their numbers.

“Insects are ectotherms, which means they develop faster the more the weather warms. Flies generally have very fast life cycles,” says Erika T. Machtinger, PhD, an assistant professor of entomology at Pennsylvania State University, in State College. “This means that if fly populations are not controlled early, they can get out of hand quickly and can be near impossible to control.”

For many fly species found around the barn, it only takes seven to 10 warm days for their eggs to transition through three larval stages to become pupae and hatch into adult flies, explains Sonja L. Swiger, PhD, a professor and veterinary/medical extension entomologist at Texas A&M AgriLife Extension, in College Station. Colder temperatures slow the process to 10 to 14 days, which is still enough time for a population explosion, especially given that one adult female stable fly can lay up to 600 eggs, she adds.

Insect species thrive in different conditions. For example, mosquitoes and horse- and deerflies develop in water. “Filth flies” such as blowflies or houseflies thrive in rotting materials like garbage and decomposing organic matter such as hay, manure, uncovered or dropped feed, and grass clippings.

“Filth flies are decomposers, or at least the larvae are,” explains Swiger. “The larvae are one of the few organisms that can and will inhabit manure and consume it.”

Flies are very good at finding places to breed and lay eggs, and some can fly 2 to 8 miles, so if another farm down the road isn’t as clean as yours—think garbage storage or disposal—you might get flies coming in from that area, she added.

Sanitation Practices

Reducing or removing habitats where insects thrive is key to controlling their populations. The first step is to develop a manure management plan. Removing manure promptly keeps overall fly numbers lower by reducing breeding grounds.

For filth fly management, Machtinger recommends:

Keeping manure piles or pits as far from the barn as possible.
Spreading manure in a thin layer (one-half inch or less) to dry manure out faster than the flies can develop.
Covering manure piles to deter flies and reduce fly development.
Keeping manure piles or pits as active compost by turning and aerating them.

“Composting properly is a great tool,” Swiger says. “It is necessary to produce the right amount of heat in the compost pile and to turn it; otherwise, larvae can still develop. It can also be thinly spread on fields as a fertilizer. Spray-on larvicides or parasitoid wasps are other useful options when there is nowhere to spread the manure.”

Sanitation practices also include a water management plan. Stagnant ponds and poorly drained streambeds can attract fly populations; it’s essential to aerate ponds and open areas to encourage creek or stream flow. Other steps involve improving drainage around the barn or in the paddocks and filling in deep ruts to prevent water from pooling.

“When substrates get oversaturated or are continuously saturated, the matter continues to break down and can sustain larval development,” Swiger says. “So, a good water management plan is important.”

The next step is to inspect the area for items such as tires, tarps, and empty buckets that collect even a small amount of rain. Mosquitoes and other water-loving larvae only need a skim of water to reproduce, so you might find yourself flipping buckets or tubs after every rain to eliminate their habitat.

It can be more challenging to reduce fly populations in smaller outdoor enclosures than larger ones—a higher density of horses means more manure and, potentially, mud in a smaller space. Muck out paddocks regularly as you would a stall. But managing manure in very large pastures or on farms with other livestock can also be difficult. Dragging large pastures takes time and other resources, and species such as cattle can attract a lot of flies; cleaning up decaying hay, reducing mud and accumulated manure piles, and spacing pastures as much as possible can help limit population numbers.

Creating “buffer zones” around pasture perimeters can also be helpful. Some insect species thrive in tall weeds. Mowing a path a few feet wide encourages bugs to move their breeding grounds (if only a little) farther from your horses.

Fly Sprays and Repellents

Sprays and repellents are staples in any stable. Products labeled safe for horses can be applied directly to the animal’s coat, while premise sprays are designed for treating areas around the barn. Before using either, read the label and follow the instructions, which explain how much and how often to use them.

Direct application You can choose from hundreds of fly sprays and repellent lotions. The manufacturer and package design might look different, but the ingredients are nearly all the same, says Machtinger.

“There are very few active ingredients that can be used on horses for fly control,” she says. “Nearly all of them have a pyrethroid (a synthetic insecticide), and most fly populations across the U.S. are resistant to pyrethroids.”

Natural fly repellents are also gaining popularity. Manufacturers use a variety of oils and ingredients that naturally deter biting flies. These can range from peppermint and tea tree oil to clove oil, citronella, and geranium oil, among others.

The big questions are, “How long will it last?” and “How effective is it?” After receiving hundreds of calls from horse owners with these two questions, Machtinger’s team tested numerous products to find out. The results? The effects of most pyrethroid-based repellents only last about four hours.

“There were a couple of plant- or fatty-acid-based repellents that were much more effective and lasted quite a bit longer,” she says. “None lasted more than one day with high repellency. So, you need to use effective repellents daily for them to make a difference.”

More is not better, Machtinger warns. Buying concentrated livestock permethrin (a type of pyrethroid) to make a “higher dose” is ineffective and potentially dangerous. She adds that horses are highly sensitive to permethrin-based products and can experience severe skin irritation.

Premise treatments Repellents and sprays labeled for premise treatment can help reduce fly populations, but they cannot be applied to an animal unless it is listed on the label, Swiger emphasizes.

“These products will all come as liquid concentrates and need to be mixed with water in a hand pump sprayer or something bigger before being applied, and animals should not come into contact with these products,” she says. “The products labeled for on-horse use or other livestock will come in many different concentrations, spray bottles, aerosols, liquid concentrates, wipes, and such.”

Machtinger discourages the use of automatic fly spray systems. While these systems can treat large spaces with precise timing and amounts, their excessive use accelerates the development of pesticide resistance in flies and can cause respiratory aggravation in humans and other animals, she says.

Fly Traps and Gear

Reducing insect numbers successfully requires a combination of strategies that addresses the multiple life stages, which vary depending on the species. In the barn, Swiger recommends sticky tapes hung out of wind and walkways and pheromone traps for houseflies and blowflies. Pheromone traps will attract lots of flies to the immediate area, so place them outside the barn or away from animals and people.

“They only work as good as the human,” she says. “If the tape is full of flies or dirt, it is no longer working. If the pheromone traps are full or dried up, they cannot collect more flies.”

Similarly, decomposing flies in these pheromone traps can be a breeding and development site for more flies, so empty them regularly.

Outside, larger bag traps and ball traps attract different fly species using attractant scents and shapes, in the case of horsefly ball traps, to lure bugs away from horses. Another option, parasitic flies, are biteless and stingless insects that feed on biting and stinging bug larvae while never becoming pests themselves.

You can further protect your horses by using fly sheets, masks, and boots. To deter biting midges that can cause sweet itch, you can purchase and outfit your animals in smaller-mesh sweet itch fly sheets. These tend to have less ventilation—use them with caution so horses don’t overheat, says Machtinger.

“If there are biting horse- and deerflies, I recommend using a zebra-striped sheet,” she says. “Research has shown that horses dressed or painted in zebra stripes have fewer bites than those that aren’t. This has to do with how the fly sees, and the different colors confuse the fly.”

Machtinger adds that all sheets, boots, and fly masks can be commercially treated with permethrin to help kill flies and other arthropods, such as ticks. Some fly boots come pretreated, or you can send gear out to Insectshield.com, the only company she is aware of that treats horse gear.

Beware of Other Bugs

Mosquitoes, stable flies, houseflies, and horseflies are the most common biting insects on horse properties. Regardless of where you live, you’ll be dealing with these bugs. However, if you live in Texas or regions with similar terrain and weather conditions, fire ants can be an issue in pastures.

“In Texas, we recommend baiting land/pastures twice a year, first on Mother’s Day and then the first week of November, every year,” Swiger says. “Fire ant bait is the only option for use around livestock, but other products can be used in yards around the barn or house.”

Also watch for blister beetles. This beetle species produces acid in their abdomens that, if consumed by a horse, is toxic and can cause colic or, worse, death. They appear in summer on flowering plants and can be found in alfalfa.

“In Texas, we harvest alfalfa before blooming, but we can still have blooms and beetles,” Swiger says. “Blister beetles can be seen on roadsides in wildflowers and other weeds, so it is important to eliminate flowering plants that horses could consume.”

Know Your Opposition

Learn what potential flies you have on your property. Each fly species requires different control methods, so if you target houseflies when you have stable flies or deerflies when you have horn flies, you will not be happy with the results.

“Sanitation is the base of all fly control activities, but there are other control methods such as mechanical and physical control, biological control, and chemical control that can be combined for the best results,” Machtinger says. “Reach out to your state land grant institution, usually the state university Cooperative Extension, and ask them for help if you are unsure what to do. That is what they are there for.”