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

US - 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.
calendar icon 3 August 2007
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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.

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