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Horn Flies

Cow with Horn Flies on Back

Cow with Horn Flies on Back

Swarm of Horn Flies on Back of Cow

Cow with Horn Flies on Back

Biology

The adult horn fly, Haematobia irritans, is about half the size of a house fly or stable fly. Both sexes have piercing mouthparts which they use to penetrate animal skin to obtain blood meals. Horn flies are intermittent feeders that take 20 or more small blood meals each day. The flies normally congregate on the shoulders, backs, and sides of the animals. During very hot or rainy weather the flies move to the underside of the belly.

Unlike most other flies, horn flies remain on the animals almost constantly, leaving only for very brief periods to lay eggs on very fresh (less than 10-minuteold) droppings. Development from egg to adult is completed in 10 to 20 days. The flies overwinter as pupae in or under dung pats. Horn flies can be a serious pest of pastured cattle by causing reduced milk production, poor weight gain, blood loss, and animal annoyance and fatigue.

Monitoring

Horn flies are monitored by counting flies on the heads, shoulders, backs, and sides of at least 15 animals; counts in excess of 50 flies per side warrant insecticidal treatment

Management

Horn flies and face flies breed exclusively in very fresh droppings on pasture. As a result, cultural controls such as manure management practices in and around barn areas that are highly effective against house flies and stable flies will have no impact on horn fly and face fly populations.

Biological Control

Biological Control against these pests at present is limited to beneficial organisms that occur naturally in the field. Face flies are attacked by parasitic nematodes, and immature stages of both horn flies and face flies are attacked by predaceous mites, predaceous beetles, and parasitic wasps. Manure competitors such as dung beetles also limit fly populations by removing and burying cattle dung before immature flies can complete their development. Adult flies are attacked by predaceous yellow dung flies, and face flies are occasionally attacked by pathogenic fungi.

In spite of the diversity and importance of natural enemies of face flies and horn flies, methods are not known for exploiting these biological control agents in pest management programs. Parasite releases for house fly and stable fly control are not effective against these pasture pests.

Insecticidal Control

Insecticidal control options for horn flies and face flies include whole-animal sprays, self-applicating devices, feed-through insecticides and growth regulators, and controlled-release devices, such as ear tags and tapes. Table 2 lists examples of insecticides that are registered for control of pests of pastured cattle. Read product labels carefully for target pest information and for precautions to avoid contaminating milk and meat; not all products are effective against face flies, and some products cannot be used on lactating dairy cattle.

Whole-animal sprays provide rapid relief from fly pressure. Animal sprays are applied either as a dilute coarse spray, often applied under high pressure to soak the skin, or as a fine low-volume, more concentrated mist.

Self-applicating devices include back rubbers covered with an absorbent material treated with an insecticide-oil solution, or dust bags filled with an insecticidal dust. Back rubbers and dustbags should be placed in gateways, near water and feed sources, and in other areas where animals will make frequent contact with them.

Feed-throughs include insecticidal feed additives, treated mineral blocks, and bolus formulations. These treatments are generally less effective for face flies than for horn flies. In either case, feed additives have no effect on adult flies that are already present or that may immigrate from neighboring farms. Unless your farm is very isolated or you are participating in an area-wide management program, feed-throughs may not provide satisfactory fly suppression.

Controlled-release ear tags and tapes are generally very effective for horn fly control in the Northeast, and they often reduce face fly pressure as well. Because these products have not been used extensively in the Northeast, insecticide resistance is not a major concern at present. But in other parts of the country, high levels of resistance have developed in horn flies to pyrethroids such as permethrin, fenvalerate, resmethrin, and flucythrinate. You can prevent horn fly resistance from becoming a serious problem by following guidelines developed by a panel of experts in the field. These guidelines include the following:

  • Do not treat unless flies exceed threshold levels.
  • Use organophosphate insecticides, such as rabon or coumaphos, for early-season horn fly control, and reserve ear tags for late summer use.
  • Remove ear tags in the fall to reduce development of resistance to low levels of pyrethroids.

Although ear tags and boluses are controlled-release application methods, the amount of active ingredient they release decreases over time. Because of this, timing of ear tag and bolus placement is important. If at all possible, delay using these application methods until July so there will still be enough active ingredient left in mid-August, when horn fly populations reach their peak. Early tagging or bolusing of heifers at the time they are placed on spring pasture in April or May will greatly reduce the effectiveness of these treatments later in the summer when it is needed the most.