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Rein in horn flies

Horn flies will soon be bothering cattle. The best way to give the animals some relief from horn flies is to use several different control methods, say Janet Knodel, North Dakota State University Extension entomologist, and Greg Lardy, NDSU animal science department head.

Control methods include insecticide ear tags, self-application devices (dust bags or oilers), pour-on or whole-body insecticide sprays, feed additives and non-chemical walk-through traps.

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 to treat themselves. Provide enough bags for all of the animals in a herd because bulls and older cows tend to dominate bags.

Pour-on, whole-body sprays or duster insecticides can be applied, but handling the animals can stress them. The products only have a short residual. When fly populations are high, combine strategies, Knodel suggests. Use ear tags early in the season and whole-body sprays later in the season.

Feed additives containing an insecticide can kill 80% to 90% of the fly larvae. The insecticide passes through the animal’s digestive system and will spread through the manure where the fly maggots are found. These additives act as an insect growth regulator and prevent the fly maggot from maturing into an adult.

One disadvantage of feed additives is that flies can migrate from untreated herds. To avoid the development of insecticide resistance, don’t place ear tags on cattle until horn flies are present, remove tags in the fall after a frost and rotate the insecticide classes in the tags annually.

Research shows that nonchemical walk-through traps can help reduce horn fly numbers by as much as 50%. Traps can be set up to require cattle to pass through them to obtain water or to access salt.

As the cattle pass, a canvas brushes their backs, disturbing the flies. The flies are attracted to the light at the top of the trap and fly upward into an inverted cone. Once in the cone, they are unable to escape. Research is under way on additional fly control methods, including fly traps with sex attractants, and releases of sterile male flies and predatory wasps or dung beetles.

Source: NDSU Extension Communications

Tiny flies can cause big losses

In cases of heavy infestations, horn flies can reduce weight gain by 0.5 pound per day and milk production by 10% to 20%, says Greg Lardy, NDSU animal science department head.

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.

Tip: 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 animal is considered the “treatment threshold,” when control measures should begin, Lardy says. Approximately 200 flies per animal is the “economic injury level,” when animals will have significant weight loss and aggravation.


This article published in the July, 2011 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.