Calculating the Economic Impact of Biting Stable Flies to California Dairies


Dairy cows bunching
Dairy cows bunching to avoid stable flies. Photo by Wagdy ElAshmawy, UC Davis. .


Flies are ubiquitous, disease-carrying pests that annoy everyone, everywhere – and that’s just the species that don’t bite.

Biting flies, like stable and horn flies, have all the negative attributes of other flies, plus a serrated dagger-like mouthpart to pierce skin and suck blood. They are problem pests in animal agriculture, and new multi-disciplinary research on California dairies has calculated the economic loss caused by stable flies and a defensive behavior cows takes to avoid them, called bunching.

“Producers would notice in mid-spring to early summer that their dairy cows would aggregate in very tight groups,” explained Sharif Aly, a professor of epidemiology at the UC Davis Veterinary Medicine Teaching and Research Center in Tulare. “It was a puzzle as to what was really causing it, but observations pointed at stable flies.”

Sharif and several collaborators, ranging from entomologists and economists to veterinarians, began a series of studies looking at the causes and costs of cow bunching and stable flies on dairies and what control measures producers can take. The Western IPM Center is one of the funders of the ongoing research.

What they found is that stable flies are a serious seasonal problem for dairy producers that costs the average 2,000-milking-cow dairy at least $10,000 a year in lost milk production in addition to the cost of control measures. And although calculated on an annual basis, the actual losses occur during the active stable fly season from early May to mid-July. (The flies are temperature sensitive and die off when average daily highs get above about 90 degrees.)

“These cows need to rest 12 to 14 hours a day,” explained Aly. “When you see cows bunching, they can be away from shade, away from feed or water for hours. They bunch in a tight circle and try to get to the center, away from the flies. It’s stressful because the bites are painful. They need to rest and chew their cud, ruminate, and instead they’re on their feet.”

As part of the research, veterinary clinician Wagdy ElAshmawy at the Tulare center did detailed surveys of 20 dairies in the southern San Joaquin Valley, documenting production practices, environmental factors, fly counts and bunching behavior. He found some clear patterns:

  • Higher stable fly counts resulted in more bunching
  • Stable flies and bunching are worse in pens near the perimeter of dairies and worse generally on dairies surrounded on three sides by field crops
  • Fly counts were higher on dairies that incorporated agriculture by-products or wet distillers grain in their feed
  • Bunching was reduced on dairies that regularly cleaned manure away from fence lines and pen edges

Looking at daily milk production (and factoring in lots of variables), economist Fernanda Ferreira, also located at the Veterinary Medicine Teaching and Research Center, calculated that cows that bunch produce on average about one pound less milk a day than cows that don’t. Higher fly counts also resulted in lower milk production, even if the cows didn’t bunch.

The value of that lost milk production added up to $9,200 to $10,200 during the stable fly season for an average dairy and equaled about $3.5 million in lost production for the five-county region the researchers studied.

Controlling Stable Flies

UC Riverside veterinary entomologist Alec Gerry was also part of the research team, which recently posted four 15- to 25-minute videos online highlighting their research and findings. Gerry’s portion of the project focused on fly behavior and control, which includes preventing the development of adult flies and reducing fly populations if adults do emerge in large numbers.

Most of the prevention recommendations involve reducing the amount of decaying manure and damp organic matter the flies lay eggs in. Recommendations include:

  • Replacing bedding regularly in freestall-style dairies
  • Grading pens flat to reduce low spots
  • Eliminating water leaks
  • Cleaning manure away from fence lines and pen edges in the early spring
  • Scraping pens to break up and remove manure
  • Drying collected manure to less than 45% moisture content and composting it to create heat and destroy eggs and stable fly larvae

Some prevention techniques used in other animal production systems are less effective in dairy settings, Gerry said. Insect growth regulators that control other fly species are less effective against stable flies, and the release of parasitic wasps that target fly larva hasn’t yet been proven effective in dairy settings.

Jordan Tonooka, a UC Davis doctoral student researching animal behavior and welfare, led a trial that compared two interventions to control adult fly populations, automatic sprayers that treat cows with an insecticide or repellant as they leave the milk parlor, and insecticide-treated fabric panels hung above pens. Tested in three pens on one dairy in a preliminary study , the data analysis by post-doctoral scholar Essam Abdelfattah found that the fabric panels didn’t do any good and the sprayer had mixed results – showing higher on-cow fly counts in the morning but lower on-cow counts in the evening compared to a control group.

“There’s more research to be done,” Aly said. “Our goal is to develop a sustainable integrated pest management program that incorporates management changes, and to look at the economic impacts of that program. Effective stable fly control would have economic and animal-welfare benefits.”


Learn more: See four videos about the research and findings at