Bug season can be a frustrating time of year to care for and enjoy horses. Not only are they irritating, but they also have the potential to spread disease. Like parasites developing resistance to dewormers, flies are showing resistance to specific repellents, making it harder to control them. As a result, you’ll need to implement a fly control strategy that uses both chemical and non–chemical techniques.
“The first step is to understand what pest you have,” said Erika T. Machtinger, PhD, assistant professor of entomology (the study of bugs) at Pennsylvania State University’s College of Agricultural Sciences, in University Park. “If you are trying to control one type of fly, but it is a different species, that is a problem. You may be unnecessarily putting controls in the environment that won’t work. It is a particular problem if you then treat with more and more insecticides.”
According to results from a 2019 study, house flies, in particular, have developed resistance to pyrethroids, which are frequently used in equine fly sprays.
“Resistance is very, very common, and there are concerns that this will continue to spread across the States, basically rendering all of our pyrethroid-based products worthless,” Machtinger said. “Unfortunately, the level of pyrethroids we can use on horses is very, very low because horses have a low tolerance for the compound and will have severe skin reactions.”
Prevention is essential to reducing pest presence because new active ingredients are slow to market. Once you know which pest(s) are problematic on your property, the next step is understanding integrated pest management (IPM), a broad-based approach combining chemical and nonchemical insect control strategies.
Non–chemical techniques, including eliminating pest breeding grounds such as manure, decaying organic material (weeds and uneaten hay), and standing water, are central to IPM. IPM also includes mechanical methods such as dragging pastures and installing fans and screens in stalls, or biological control methods such as parasitoid wasps, Machtinger added.
“Chemicals should be used judiciously on an as-needed basis,” she said. “People don’t often like to hear me say this, but resistance can develop to all forms of insecticides, even those considered natural. The biggest offender I have found is automatic spray systems. These can be a major source of resistance development because they spray but do not kill all the pests. Instead, they are knocking them down for a few minutes, and the horses, people, and pets in the environment are breathing in those chemicals, natural or not.”
Few repellents are better than others for house flies, she said. “Some of the pyrethroid-based repellents only lasted four hours whereas (a natural repellant comprising fatty acids and silicone oil) was the standout performer and had a 75% repellent success at around two days.”
Repellents can also be effective for horses when combined with protective boots, masks, and sheets. Permethrin-treated human clothing has been available for years but only recently for horses.
The U.S. military developed technology to protect troops in areas where they could get sick from tick or mosquito bites, and now horse can benefit from it too, Machtinger explained. “The permethrin product binds to the fabric of clothing and can act as a toxicant and a repellent to some arthropods but doesn’t rub off the human or animal using it. I think it could be a great additional protection, although there is little research on how it works and how long it works for horses in different situations.”
She emphasized that permethrin purchased as a fly repellent does not work like the technology designed for fabric treatment and should not be applied to horses or fly sheets, boots, or masks.
“Also, the self-treatment products for people are currently not labeled for horses, so they should not be used to treat horse clothing,” she said. “The best option is to purchase already-treated items or send your items to Insect Shield, which has a treatment labeled for horses.”
