North Dakota ranchers and farmers are reporting high populations of horn flies this summer.

Stable flies also have been more of a problem this year.

The horn fly problem appears to be particularly severe in south-central and southwestern North Dakota; however all regions of the state have been affected, according to Greg Lardy, North Dakota State University Extension Service beef cattle specialist, and Janet Knodel, NDSU Extension entomologist.

These blood-sucking flies feed up to 20 to 30 times per day and cause pain, aggravation and anemia from blood loss. As a result, livestock lose weight and generally are weakened. In cases of heavy infestations, horn flies can reduce weight gain by 0.5 pound per day and milk production by 10 percent to 20 percent.

Horn flies look like a grayish housefly, but are half the size and have piercing, sucking mouthparts. An easy way to identify horn flies is their behavior of clustering around cattle’s horns, shoulders and backs. On hot days or during rain storms, they often move to the animal’s belly.

Adult horn flies spend their entire life around cattle. Female flies lay their eggs in fresh cattle droppings. Maggots quickly hatch from the eggs and then transform into a pupae in or under manure pats. The life cycle is complete in two weeks and the flies produce several generations per year. Populations usually peak in late July and August. Horn flies overwinter as pupae under manure pads and produce an adult fly the following spring.

To monitor horn flies, count the number of flies on the heads, backs and shoulders of at least 15 cattle. A good set of binoculars will make the job easy. An average of more than 50 flies per side or 100 flies per animals is considered the “treatment threshold,” which is when producers should take control measures. Experts estimate that 200 flies per animal is the “economic injury level,” or when animals will have significant weight loss and aggravation.

Since horn flies stay on the animals all of the time, the best control strategy involves an integrated approach that uses several different methods. Control methods include ear tags, self-application devices (dust bags or oilers), pour-on or whole-body insecticide sprays, feed additives and a nonchemical walk-through trap.

Insecticide ear tags contain a synthetic pyrethroid or organophosphate. As the animal moves, the insecticide is released to the surface of the tag and contacts the cattle’s hair. Dust bags and back rubbers are available for cattle for self-treatment. Producers must provide enough self-treatment bags for all of the animals in a herd because bulls and older cows tend to dominate bags.

The drawbacks with ready-to-use pour-on, whole-body spray or duster insecticides are their application can be stressful for livestock and they have only a short residual for control.

Lardy and Knodel recommend that when fly populations are high, producers should combine strategies, such as ear tags early in the season and whole-body sprays later in the season.

Feed additives are insecticides that pass through the animal’s digestive system and destroy the developing fly maggots in the manure. They are effective in killing 80 percent to 90 percent of the developing fly larvae. These additives act as an insect growth regulator and prevent the fly maggot from maturing into an adult. One disadvantage of feed additives is fly migration from untreated herds, which can decrease their effectiveness for controlling adult flies on animals.

The nonchemical walk-through traps require cattle to pass through them to obtain water or access salt. As the cattle enter the trap, canvas brushes their backs, which disturbs the horn flies. The flies then are attracted to the light at the top of the trap and fly upward into an inverted-cone trap. Once captured, they are unable to escape. Research indicates that use of such a trap provides a 50 percent reduction in the number of horn flies in a herd.

Some producers report reasonable to good control through the use of integrated approaches. The use of feed supplements with insect growth regulators has been successful in some areas.

Research is under way on additional fly control, including fly traps with sex attractants, releases of sterile male flies and predatory wasps or dung beetles.

To avoid the development of insecticide resistance, Lardy and Knodel suggest producers:

• Do not place ear tags on cattle until horn flies are present
• Remove tags in the fall after a frost
• Rotate the insecticide classes in the tags annually

Stable flies have piercing, sucking mouthparts that penetrate the skin, causing irritation. These flies can be found on cattle and the front legs of horses. The fly bites are painful and animals respond by stamping their feet and switching their tails to dislodge the flies.

Knodel and Lardy say very few effective control mechanisms are available for stable flies in large pastures. In more confined settings, control measures include removing areas where the flies breed, such as manure piles; applying residual sprays to areas where flies roost; and applying insecticide sprays to the animals.

NDSU Agriculture Communication