Fly control is always a hot topic with dairy producers because there are not a lot of viable options to alleviate fly pressure. Nuisance flies are typically most active from May to October in northern regions and are active year-round in warmer climates.
These flies feed on cattle whether housed indoors or on pasture. Cows are irritated by fly feeding behavior and can become very stressed under excessive fly populations. Cows also become restless and spend less time lying down when under heavy fly pressure.
Prolonged exposure to fly irritation can lead to a decrease in production. Although flies cannot realistically be eliminated from a farm, producers benefit from fly management with more comfortable animals and people. Proper management can keep fly populations in check to minimize negative effects.
Three important blood-sucking pest flies on dairy cattle in the Upper Midwest are the stable fly, horn fly and face fly.
Stable flies develop as maggots in a wide array of decomposing organic matter, including soiled animal bedding and soiled feed debris that accumulates wherever cattle are confined.
Dairy farm surveys indicate calf hutch bedding is a prominent source of stable flies around dairies, and choice of bedding material can minimize stable fly production; for example, pine shavings and sawdust contain fewer flies than straw.
More recently, it has also become apparent that feed debris and manure that accumulate during winter are also important sources of stable flies, especially where overwintered debris piles remain intact into the following summer.
Adult stable flies closely resemble house flies but are easily distinguished by piercing mouthparts that protrude from under the head. These flies are often found on the legs of cows. Bites from stable flies are painful. Frequent bites stress cattle, leading to decreased weight gain and milk production.
Control of adult stable flies is difficult. Only a small percentage of the total population are found on cows at any given time, and most chemical sprays are rubbed or washed off of cows before achieving satisfactory results. Reduction or elimination of breeding sites is one of the most common methods to manage populations. Any effort to manage breeding sites must often be coordinated with neighboring farms to prevent dispersal.
Horn flies are small, biting flies that are primarily pests of cattle. The horn fly develops in fresh cattle dung pats and nowhere else. These flies spend almost all their time on a cow, often along the back or sides, where they feed several times per day.
Cows can temporarily dislodge horn flies with head throws or tail flicks, but flies will quickly settle on the same or a nearby cow once the cow stills. Several factors may influence horn fly attraction to a particular cow, including color, breed of cow, time of day and innate heritable resistance. Great effort is spent on controlling this fly. Chemical, mechanical and biological control methods have been developed as ways to manage horn fly populations.
Face flies resemble house flies and feed on bodily secretions, usually around the eyes and mouth of cows. Face flies spend relatively little time on their host. These flies are most active during the day and are typically a problem to pastured cattle, as they seldom enter barns or animal shelters. Although feeding habits of face flies are annoying to cattle, there is little evidence of negatively affected growth or milk production.
Fly presence encourages cows to move more frequently to newer areas and to alter feeding bouts. Cows annoyed by flies exhaust energy once directed toward production in an attempt to dislodge flies. Intensity of attacks varies with time of day and weather conditions.
Flies are particularly active when winds are low and temperatures are high. Under intense attack, cows often abandon eating and bunch close together. Bunching is a herd response to fly activity where cows attempt to limit surface area exposed to attack. Oftentimes, cows will gather in a tight circle with heads in the center. However, cows in such close proximity have increased risk of heat stress and weight loss.
A new solution for fly control
At the University of Minnesota West Central Research and Outreach Center dairy, we have been evaluating a unique method (Spalding Cow-Vac) for controlling pasture flies without the use of chemicals. The system is compatible with grazing dairying because a trap can be positioned at the entrance to a milking parlor, where cows come and go twice per day.
During the summer of 2015, we evaluated the efficacy of this tool in on-farm dairy production systems to control horn flies, stable flies and face flies. The study partnered with eight grazing dairy farms in Minnesota, and herds ranged from 30 to 350 cows in size.
The results of fly counts and milk production for the presence or absence of the Cow-Vac on farms are in the accompanying table (Table 1). Horn fly numbers on cows were reduced by 44 percent on-farm in the presence of one of these devices compared to those without it.
Stable fly and face fly numbers were similar on-farm, whether it was present or absent on farms.
Milk production was similar for farms with and without this specific fly control method. In summary, these results indicate it was effective in reducing horn fly numbers on cows and reduced horn fly growth rates during the pasture season in dairy production systems.
Sanitation should be the primary control option on any dairy. Because synthetic pesticides are not allowed on organic dairies, proper sanitation is of the utmost importance. Manure and feed provide the ideal habitat for house and stable fly production. Manure and old feed should be removed daily, or at least twice a week, from calf pens, holding areas, feed areas and milking areas.
To ensure success on your dairy, producers need to properly identify key pests, understand their biology and habitat, monitor their populations and then reduce the fly population through mechanical or biological management techniques. Ultimately, there are many tactics you can try out on your own farm.
Take notes and evaluate how well things worked, what didn’t work and where you can find additional answers to improve the well-being of cattle and reduce pests. 
PHOTO: The University of Minnesota West Central Research and Outreach Center dairy has been evaluating a unique method for controlling pasture flies without the use of chemicals. The system is compatible with grazing dairying because a trap can be positioned at the entrance to a milking parlor, where cows come and go twice per day. Photo provided by Brad Heins.
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Brad Heins
- Associate Professor Organic Dairy Management
- University of Minnesota West Central Research and Outreach Center, Morris
- Email Brad Heins
