Controlling harmful, bothersome flies on horses a complex scientific problem

Flies are a major menace to horses and a difficult problem for owners to control. (Erika T. Machtinger photo)
Flies are a major menace to horses and a difficult problem for owners to control. (Erika T. Machtinger photo)

“Flies on horses are a major problem sometimes harmful to health and a complex quandary for owners to control.”

There are a wide variety of methods to help control flies on horses, according to Erika T. Machtinger, entomology professor at Penn State University.

“A combination of approaches is generally required for satisfactory results,” Machtinger pointed out.

Pesticides, typically referred to as fly spray, are generally the first product horse owners turn to for fly control.

“These products are poisonous,” Machtinger emphasized. “If it kills flies or keeps flies off of horses, it is a harmful and must be handled with caution.”

Before use, pesticide labels should be read and understood completely. “Handle pesticides carefully and store them in their original containers,” Machtinger advised. “They should be out of reach of children, pets, and other animals.

“It is recommended to wear sanitary gloves whenever applying insecticide and always wash hands carefully after use,” Machtinger demanded.

Forages, streams, and ponds can be contaminated by pesticides if not used properly. “Always dispose of empty containers right away, in a safe manner and place,” the entomologist urged.

There isn’t one silver bullet to fly management, Machtinger reemphasized. More risks are associated with fly populations than just nuisance problems.

“Flies can transmit pathogens that cause disease or other conditions in horses,” Machtinger said. “They can have a negative impact on horse condition and physiology.”

Farms can be perfectly managed and still have pest problems. “Basic water management can reduce many natural sources of water where flies populate,” Machtinger said. “Proper farm and facility drainage should be established, and leaky waterers, hoses, or other water sources should be repaired quickly.”

Proper ventilation, like the addition of outdoor-rated fans, can aid in quick drying of bedding and stored manure. “Many fly eggs hatch in less than12 hours, so proper manure removal and storage can limit pest problems,” Machtinger said.

Along with frequent cleaning, bedding choices can influence fly development.

“Sawdust tends to support fewer developing flies than other bedding choices like shavings and straw,” Machtinger said. “Adding a drying agent can keep stall floors unsuitable for mass fly production, although flies often find small, protected places.”

Other sanitation practices can reduce the likelihood of fly development. “Keeping feed storage areas clean, removing spilled feed quickly, and covering feed bins will eliminate many sugar resources,” Machtinger said.

“Fly screens on windows in barn areas can reduce fly presence,” Machtinger continued. “Fly sheets, masks, and boots provide protection for horses. Fans in the stable area can interrupt flight and prevent flies from landing.”

Two problems exist with relying on chemical insecticides for fly control. “Since fly resistance to active ingredients has been increasing, it is becoming difficult to find products that work,” Machtinger said. “Second, while insecticides reduce adult flies in contact with the compound, they don’t address the source of fly development.”

Several types of chemical control exist. “Residual insecticides and premise sprays are applied to walls, ceilings, and other places where flies rest,” Machtinger said.

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Fly baits can be effective when competing food sources are limited. “Since baits are toxic, it will be necessary to prevent other animals and children from being exposed to them,” Machtinger reiterated.

Larvicides are specific insecticides that can be applied to fly development sites where large numbers of flies are predicted.

“On-animal fly sprays or roll-ons or wipes are available with a variety of active ingredients, percentages, and trade names,” Machtinger said. “These may or may not be effective depending on local resistance and application.”

Feed-through regulators are administered as supplements to the horse, pass through the digestive system, and are excreted in manure. “Every horse at a property must be fed these feed-throughs to prevent flies from developing,” Machtinger said.

Selection and percent of active ingredient in fly control products are very scientific.

Pyrethrins and pyrethroids are most common for fly control on horses. “They’re compounds that have natural insecticide properties with knockdown benefits,” Machtinger, “However, they may not kill the insect pests, just repel them. These generally will break down quickly.”

Piperonyl butoxide is another compound often found with pyrethrins. “This acts as a synergist to make the products more effective,” Machtinger said.

Cypermethrin, permethrin, and resmethrin are synthetic forms of pyrethrin called pyrethroids. “They are more stable than the pyrethrins and have longer-lasting effectiveness,” Machtinger said.

“More natural products containing fatty acids are as effective as synthetic compounds in repelling flies,” Machtinger said. “They have a longer duration of effectiveness.”

For applications to be most effective, horses should be clean, free of dust, dry and all insecticide label directions followed.

“The products should be applied to target areas and then brushed lightly in the direction of the hair,” Machtinger recommended. “Pay particular attention to the legs, belly, neck, face, and shoulders.”

Insecticide resistance is a frequent and legitimate concern when using pesticides. “Resistance allows some pests to continue to survive and reproduce in the presence of a toxin,” Machtinger explained.

Cross-resistance is when developed resistance to one toxin leads to resistance of a similar toxin. “Resistance can develop when pests are repeatedly exposed to a toxin,” Machtinger said.

After initial exposure, most pests may die but a few naturally resistant individuals survive and produce offspring that are resistant. “The fly life cycle is short, and offspring are produced so populations can quickly develop resistance,” Machtinger verified.

“To reduce resistance, it is important to not expose multiple generations of pests to the same insecticide,” she said. “This is important whether you are using fly sprays, wipes, feed-throughs, baits, or other methods of application.

“Rotate that choice with another insecticide which may have a different mode of action,” Machtinger suggested.

“Fly control on horses is an extraordinarily complex problem for horse owners. It requires continual changes and then often is still not completely effective,” Machtinger most honestly concluded.