Photo by Stephen Ausmus.
Photo by Stephen Ausmus.

Filth flies are a pain, especially stable flies that will travel several miles just to bite livestock, pets and people. They often attack the ankles and lower legs, inflicting sharp, stabbing bites.

For cattle producers, these attacks can be costly, resulting in lower milk production in dairy cows, less weight gain in beef cattle, and reduced feed efficiency.

Agricultural Research Service (ARS) scientists are developing improved methods to locate stable fly habitats, finding easier and more efficient ways to control them, and using innovative techniques to stop flies from reproducing.

Staking Out Sites

In the past, stable flies have mainly been associated with stables and barnyards, but over the past three decades, they've become a significant pest in pastures as well and are now the most damaging arthropod pest of U.S. cattle.

Large bales of hay, placed in fields and used as supplemental cattle feed, are partly to blame. Scientists at the ARS Agroecosystem Management Research Unit (AMRU) in Lincoln, Neb., determined that hay-feeding sites are the primary sources of peak fly populations during summer months in the state.

The accumulation of wasted hay, manure and urine at these feeding sites creates an ideal habitat in the pasture for stable fly larval development, says AMRU entomologist David Taylor. For 100 years, the primary method used to control stable flies has been to clean up sites where they dwell. However, infested hay-feeding sites are often in remote locations.

"Producers need to know if the time it takes to clean up a site is worth their time and expense," Taylor says. "We developed a model to assess the economic impact of stable flies and to provide a cost-benefit analysis to producers."

The model was based on four classes of production: dairy; cow/calf; pastured and range stocker; and animals on feed. The analysis showed that stable flies cost the U.S. cattle industry more than $2.4 billion each year.

Preventing Flies from Growing

Insecticides used to control flies at hay-feeding sites usually wear off in two or three days, according to Taylor. An insect growth regulator that prevents stable flies from developing can be more effective.

Taylor and his team used a commercial product called cyromazine to control immature stable flies. The product, which has been used to manage other species of flies, inhibits flies from developing into adults.

In the study, one application of granular cyromazine sprinkled on a hay-feeding site reduced the number of adult stable flies emerging by 97 percent. Treatments took only 10 minutes, cost $10 per site, and remained effective for two to five months.

"We wanted to develop a method where the producer could apply a single treatment and be done," Taylor says. "They can quickly treat sites while doing other chores or checking on cattle."

Luring Flies to Drive Them Away

Another technique for controlling stable flies involves identifying natural compounds that drive them away and developing attractants to lure them into traps.

AMRU entomologist Jerry Zhu calls this his "push and pull" strategy. The "push" forces stable and other filth flies away from host animals, and the "pull" entices flies into traps—baited with low-toxic insecticides, sticky substances or a combination—to kill them.

Zhu is experimenting with plant-based chemicals like catnip to drive filth flies away from livestock.

"Catnip oil and its active compounds—nepetalactones—are powerful repellents against stable flies," Zhu says. "To date, it's probably the best repellent identified for flies that bite. It's also good at reducing larval development."

Several sprayable catnip oil formulations that decrease stable fly field populations have been developed by Zhu and his colleagues. They have also worked with Microtek Laboratories, Inc. of Dayton, Ohio, to test a new granular catnip product that prevents flies from laying eggs.

Killing Flies with a Virus

At the ARS Center for Medical, Agricultural and Veterinary Entomology (CMAVE) in Gainesville, Fla., scientists have found that a potential biological control agent—salivary gland hypertrophy virus (SGHV)—that's been experimentally successful in controlling house flies also kills stable flies.

Collaborating with scientists at the University of Florida and Denmark's Aarhus University, entomologist Chris Geden in the Mosquito and Fly Research Unit at CMAVE studied the distribution and host range of the virus, and the effectiveness of different SGHV application methods.

SGHV is one of three viruses known to occur in pests they infect, Geden says. The virus replicates in the salivary gland and multiplies in female flies. The salivary glands increase in size, while ovaries remain small. Female flies infected with the virus never produce eggs again, and male flies do not mate.

Healthy house flies are believed to contract the virus after feeding on contaminated food particles left by infected flies. The virus, which was successful at stunting the growth of house flies, also severely affected stable flies in an experiment.

"When we injected stable flies, not only did they become infected, but they died very quickly. Of those that didn't die, many did not have developed ovaries," Geden says. "Also, infected stable flies produced 50 percent to 75 percent less feces, suggesting that they weren't feeding on blood or biting as often as healthy flies."

Stable flies that had the virus did not lay eggs, did not bite as much and had a significantly shorter lifespan than uninfected pests, Geden says.