Despite their name, stable flies can now be found beyond the barn and in the field. Historically an issue for dairies and feedlots, these pests have emerged as a problem in pasture systems to the detriment of animal health and performance.

In a recent BeefWatch newsletter from University of Nebraska Extension, Dave Boxler explains that the uptick in stable fly pressure in pastures coincided with the shift from feeding cattle small square bales to using round bale ring feeders where higher volumes of wasted hay create the ideal environment for stable flies to reproduce.

“The females deposit eggs in spoiled or fermenting organic matter, sometimes mixed with animal manure, moisture, and dirt,” the extension educator writes. “In pasture settings, winter hay feeding sites where hay rings are used can often be a source for stable flies through the summer if moisture is present.”

Both male and female stable flies require blood and prefer to feed on animal legs and bellies. This can cause tail flicking, skin twitching, leg stamping, and bunching. Boxler states that after feeding, stable flies will resort to and congregate around shaded areas under windbreaks, barns, and water tanks to digest their meal.

Reduced daily gains

Even though there is limited research that documents the effects of stable flies in pastured livestock, one study from the University of Nebraska recorded a reduced rate of average daily gain by 0.44 pounds in untreated grazing cattle compared to cattle that were sprayed with an insecticide. Researchers have determined an economic injury level of five flies per leg to warrant treatment; however, this is commonly exceeded in pastures, and executing insecticide applications can be difficult.

“Stable flies are on the host for relatively short periods of time and tend to congregate on the lower parts of the legs and belly area. Targeting these areas with many of the common treatment methods is challenging and often unsuccessful in reducing fly numbers,” Boxler says. He adds that stable flies usually lay eggs away from host animals, making it hard to locate and eliminate them at the source.

Low-pressure sprayers and mist blowers are two options to apply organophosphates and pyrethroid insecticides, which are labeled for direct application. Boxler notes that despite being the most effective method, this type of chemical control can still be relatively weak and brief. He adds that insecticides can also be sprayed along windbreaks and barns where stable flies rest in the shade but cautions to avoid applying product on or near water tanks.

Fly traps are another option to mitigate stable flies, but they are typically more effective in confined settings like dairy barns and feedlots. Boxler says trap number and placement in pastures is still under evaluation.

The best way to reduce stable fly presence is to minimize hay waste and clean up spillage around hay rings before flies have the chance to repopulate. Boxler says spreading hay waste in a thin layer can help limit larval development. Spraying larvicide around hay-feeding sites is another option to keep stable flies at bay in pastures.