It usually starts with a few flies around the calf hutches or some extra tail switching in the freestall barn. Then, almost overnight, cows are bunching, calves are irritated and employees are swatting flies left and right.
By the time all of this becomes noticeable, fly populations have often already been building for weeks. Fly control experts say the best chance to stay ahead of pressure is to start managing breeding areas before summer heat and rapid population growth take over.
Small Problems Turn into Big Populations Fast
Flies reproduce quickly once temperatures rise. According to Roger Moon, professor of entomology at the University of Minnesota, flies can complete a generation every 40 to 60 days during spring weather and as fast as every two weeks during the hottest parts of summer.
That means the bedding pile or leftover feed that seems harmless early in the season can become a major source of fly pressure later in summer. Calf areas are especially vulnerable. Wet bedding, spilled milk replacer, manure and leftover feed create ideal conditions for flies to breed.
“Fibrous plant material enriched with manure, urine and moisture are basically the perfect environment for maggots,” Moon says.
Some of the most common breeding spots on dairies include:
- Overwintered manure piles
- Soiled calf hutch bedding
- Bedded-pack barns that were not cleaned out over winter
- Feed buildup around bale feeders
- Wet feed refusals
- The crusted edge around manure lagoons
Moon recommends scouting these areas every one to two weeks during the spring and early summer using something as simple as a garden trowel to look for maggots before populations explode.
Different Flies Create Different Problems
Not every fly on the farm behaves the same way, which is why identifying the type of fly matters before building a management plan.
According to Claire LaCanne, Extension educator in ag production systems, dairies most commonly deal with stable flies, house flies, face flies and horn flies.
“You’ll want to determine what’s pestering your animals to figure out the various methods for managing that particular fly problem,” LaCanne says. “Identifying the type of fly or flies that you are dealing with on the farm along with understanding their lifecycle is key to developing an effective fly management plan.”
Stable flies and house flies are considered “premise flies” because they reproduce in confined areas like barns, calf bedding and manure piles.
Stable flies are the bigger concern from a cattle comfort standpoint because they bite and feed on blood. They are commonly found on the legs and trigger behaviors like bunching, tail switching and foot stomping.
“Stable fly presence can result in reduced production,” LaCanne says.
House flies, meanwhile, do not bite. Instead, they feed on secretions around the eyes and nose and are generally more of a nuisance, although they can contribute to disease spread around the farm.
Flies Cost More Than Annoyance
It is easy to think of flies as just another irritation that comes with summer, but the impact goes much deeper than that.
Heavy fly pressure has been linked to reduced milk production, lower weight gains and weaker immune response. Flies also contribute to the spread of diseases like salmonella, E. coli and pinkeye.
Then there is bunching.
Anyone who has walked into a pen during heavy fly pressure has seen it. Cows crowd together tightly with heads in and tails out, stomping and constantly shifting positions.
Moon says bunching is one of the clearest signs that fly pressure has gotten out of hand.
“You’ll see them milling for position, stomping and switching their tails,” he says. “Bunched stock grow slower, lactate less and have lower immunity because of stress.”
Researchers have also found that bunching creates another set of problems. Airflow between cows decreases, heat builds faster, resting time drops and cows spend less time eating.
What starts as cows trying to get away from flies can quickly lead to lower intake, less resting time and reduced performance across the pen.
Sometimes the Damage Shows up Later
One of the more frustrating parts of fly pressure is that some consequences do not show up until months later.
Stable flies repeatedly biting cattle legs can contribute to hoof problems over time because cows spend more hours standing and shifting weight instead of lying down comfortably.
Moon says animals dealing with prolonged fly irritation may eventually develop sole ulcers or abscesses, issues that often become noticeable in the fall long after peak fly season has passed.
Sanitation Still Matters Most
Even with all the new fly-control products available, most experts still come back to the same basic message: cleanliness matters.
“Cleanliness and sanitation is the most important step in a fly management plan,” LaCanne says.
Removing breeding material interrupts the fly life cycle before adult flies ever emerge.
That means staying ahead of manure buildup, keeping bedding dry and cleaning out problem areas before temperatures really warm up.
“To manage stable flies and house flies, start with sanitation,” LaCanne says. “Doing your best to remove possible breeding sites like rotting hay or grain, spilled feed or TMR, manure piles and other decaying matter is the most effective way to manage stable flies and house flies.”
She recommends scraping, hauling, spreading or composting soiled bedding every other week during the summer if possible.
Several additional management steps can also help reduce pressure:
- Move calf hutches and replace bedding after each calf
- Use sand, sawdust or wood shavings during summer months
- Mow grass and weeds around barns and lagoons
- Compost manure properly so temperatures reach at least 140 degrees Fahrenheit
- Place fly traps away from barns to draw flies away from cattle areas
Layering Strategies Works Best
Most farms that successfully control flies use multiple approaches together rather than depending on one product. LaCanne says scouting should become part of the routine.
“You should begin looking for signs of flies early in the season,” she says. “Dig or scrape around in areas with organic matter and search for larvae and pupae to figure out where your trouble areas are.”
Sticky traps can also help monitor population pressure and determine when additional controls may be needed.
When fly pressure builds despite sanitation efforts, additional tools can help.
Premise sprays may help suppress stable flies and house flies in enclosed areas, though LaCanne stresses they should be paired with sanitation rather than relied on alone.
Fly baits are most effective against house flies, while pasture fly traps can help reduce horn fly pressure on grazing cattle.
Biological controls are also gaining attention on dairies.
Some farms release parasitoid wasps, often sold as fly predators or fly parasites, to target fly pupae before adults emerge.
“Parasitoid wasps can provide effective management when used with other methods, especially diligent sanitation,” LaCanne says.
However, she cautions that insecticide use can interfere with beneficial insects, making it important to think carefully about where sprays are applied.
Start Before the Flies Force You to
One of the biggest mistakes farms make is waiting until fly pressure becomes obvious before taking action.By the time cows are bunching and calves are restless, fly populations are already well established.
According to Moon and LaCanne, the farms that manage flies best are not necessarily the ones reaching for more sprays in July. They are the ones that dealt with breeding areas early, before populations had a chance to build.
