If you want to control flies on your farm in the summer, the time to act is spring, according to Roger Moon, Professor of Entomology at the University of Minnesota.
“It’s best to start engaging your fly-control strategies in April or sooner,” Moon told the audience of a recent webinar hosted by the I-29 Dairy Coalition. “Source reduction is the most effective long-term approach to keeping fly populations down.”
Moon advised scouting for flies every 1-2 weeks starting in spring, using a garden trowel to check for maggots in possible fly-breeding areas. He said maggots thrive in fibrous plant material that is enriched with manure, urine and/or water. Common sites on dairies are:
- Manure piles that have been over-wintered
- Soiled bedding in calf hutches
- Bedded-pack barns that have not been cleaned out through the winter
- Caked hay around large-bale feeders
- Other wasted feed from winter; and
- The top crust and/or ring around the edge of manure lagoons.
Spring cleaning all these areas will help remove the breeding haven for multiple generations of flies throughout the summer. Moon said flies in Minnesota will complete a full generation about every 40-60 days in the spring and fall, and as rapidly as every 2 weeks in the hottest summer months.
If flies gain a foothold on the dairy, both animals and their caretakers will pay the price as the summer wears on. “An obvious indicator of heavy fly infestation is groups of animals that are bunching,” Moon explained. “You’ll see them with their heads in, tails out, milling for position, stomping, and switching their tails. Bunched stock grow slower, lactate less, and have lower immunity due to stress.”
A less obvious, but serious, consequence of fly infestation is hoof damage. Stable flies bite and cause painful irritation as they draw blood from animals’ legs and flanks. If cows and heifers spend several weeks fighting off these pests, they may eventually develop sole ulcers and abscesses, which negatively impact milk production and reproduction. Often these symptoms don’t show up until fall.
Another common fly species on dairies is the house fly. Moon said that while they do not have the mouth apparatus to bite, they do spread pathogenic bacteria, and are a general nuisance.
Dairies with stock on pasture also will see horn flies. These smaller flies bite like stable flies, and swarm in clouds on animals’ withers. They breed only in cattle dung pats, so only occur where stock are pastured.
Polyvinyl fly tags can effectively control horn flies. Moon cautioned the active ingredients used in tags or pour-ons should be rotated each year to slow development of resistance. Unfortunately, tags and pour-ons will have no impact on stable flies and house flies.
Moon said other measures that should be taken around the farm to keep flies in check are:
- Change calf hutch bedding by moving hutches after every calf and disposing of the last location’s bedding.
- Switch calf and heifer bedding to sand, saw dust or wood shavings in the summer.
- Mow grass around the farm site and keep weeds down.
- Utilize residual premise sprays, but only as a fall back, not a backbone.
- Compost manure; heat to at least 140°F to ensure fly larvae are killed.
- Place fly traps strategically throughout the farm to capture house flies.
- Release predatory wasps.
Still, Moon suggested removing breeding material is the foundation of fly control; all the other measures are secondary. “Aim to have bedded-pack barns cleaned out, and other breeding areas on the farm cleaned up, by May 15,” he advised. “You and your animals will reap the rewards from your efforts for the rest of the summer.”
The Veterinary Entomology Website is a valuable, nationwide resource for identifying flies; selecting mitigation products; and locating professionals who can assist with fly control.
