House flies (Musca domestica) are drawn in huge numbers to livestock facilities, and they tend to swarm around the heads of cattle. A new study examines whether this swarming is enough of a nuisance to affect how much cattle will eat at the trough. (Photo by Tomaz Nascimento de Melo via iNaturalist, CC BY-NC-ND 4.0)

By Ed Ricciuti

Ed Ricciuti

Add to the many noxious qualities of house flies (Musca domestica) that they just might hike the cost of a good steak. At least that’s what is suggested by research published in March in the Journal of Medical Entomology.

The study, by researchers from the U.S. Department of Agriculture and New Mexico State University, examined how eight Angus heifers reacted when exposed to an introduced infestation of house flies while consuming grain dispensed from an experimental feeding trough, called a bunk. The heifers were obviously flustered, tossing their tails repeatedly while twitches rippled their skin, acting much more agitated and finicky about food than uninfested heifers used as controls. Yet, in the end both ate the same amount.

Results of the experiment, says NMSU’s Brandon G. Smythe, Ph.D., senior author on the study, start to put together “the thousand-piece puzzle we might need to assemble before we can truly say whether or not house flies influence cattle performance in feedlots.”

Most beef on the market comes from cattle confined in feedlots before slaughter to quickly fatten them, boosting weight and producing more flavorful and tender beef. To yield high-quality beef, cattle need to have the proper ratio of fat to muscle. If cattle are too lean, their meat will lack flavor and tenderness. Feedlots facilitate the process by concentrated, efficient feeding of cattle.

The grain given to the cattle was supplemental to their basic diet of alfalfa cubes. Supplemental grain commonly is used to spur growth rates.

A heifer that eats normally can add about three pounds per day in a feedlot but, if feeding declines, all bets are off. That’s why clouds of pesky house flies tormenting cattle could “account for a previously unrecognized source of feeding disruption that causes economic loss” for cattle producers, according to the paper. “We know from other fly species that defensive behaviors can be indicative of infestations that often lead to things like decreased weight gain,” says Smythe.

House flies (Musca domestica) are drawn in huge numbers to livestock facilities, and they tend to swarm around the heads of cattle. A new study examines whether this swarming is enough of a nuisance to affect how much cattle will eat at the trough. (Image originally published in Figueroa-Zamudio et al. 2026, Journal of Medical Entomology)

Higher costs of raising cattle usually ends up hitting the consumer hard, especially when it comes to high-end products such as top-grade steaks. Thus, it behooves cattle producers to corral as many undue feedlot costs as necessary.

“The feeding mechanism [in the study] as well as the timing of the feeding and the feed stuff offered are not representative of real-world scenarios,” says Smythe. “While our study doesn’t necessarily represent a feedlot, we designed it to focus on capturing the potential effects of house flies on cattle during feeding activities.”

Given that large amounts of manure and other waste concentrates when masses of cattle are confined in the relatively small spaces of feed lots, they normally teem with various species of flies, including house flies.

Several flies common in feedlots are implicated in causing diseases of livestock. House fly control to combat diseases alone costs poultry, pig, and dairy farmers about $1 billion a year, even though the full impact of the various species of flies on actual beef production is not known. It is known, however, that cattle bothered by houseflies can be reluctant to feed.

The aim of the study was to seek evidence as to whether house flies do, indeed, impact grain intake by cattle, focusing on the head area. House flies, especially females, swarm around the heads of cattle, seeking protein from secretions of the eyes and face.

Heifers were acclimated to inserting their heads into the experimental feeding boxes, with and without fly infestations. The heifers subjected to flies showed significantly more signs of what is called “feeding hesitancy,” unenthusiastic feeding or even reluctance to eat, which can be caused by illness and, notably, stress.

“To our knowledge, this study is the first documented report of experimentally inducing defensive behaviors in cattle, including feeding hesitancy at the bunk level due to house fly activity” says Smythe. This study provided a very narrow but unique approach to determine if house flies have the ability to irritate cattle during feeding activities.

While it is clear that the animals responded to the presence of the house flies, inferences on the performance impacts associated with this responses remain to be seen. “How the presence of house flies influence cattle across an entire feedlot stay is a mountain we haven’t climbed yet,” Smythe says.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His most recent book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.