Suppressing Stable Fly and Horn Fly Populations
By Jeffery K. Tomberlin, Ph.D., A.B.F.E., assistant professor and
Extension specialist, Texas A&M University, and Kimberly H. Lohmeyer, Ph.D., research entomologist, USDA, ARS, Knipling-Bushland U.S. Livestock Insects Research Laboratory

A population of more than 50 stable flies per cow can reduce weight gain by 25 percent. Horn fly populations more than 200 flies per animal can cause 15 to 50 pounds in weight reduction from birth to weaning. Proper identification of the costly pests leads to successful management.

The stable fly and the horn fly look similar, but stable flies are slightly larger and have longer bayonet-like mouthparts that protrude from the front of the head. Stable flies primarily attack the legs of cattle. Horn flies feed mostly on the back, shoulders and sides.
Stable flies are present only when feeding. Horn flies rest on the cow between feedings. Also, a horn fly typically orients its head towards the ground while feeding.

Optimal conditions for flies
Stable fly eggs are typically laid in wet straw, or other decomposing vegetation, mixed with urine and feces produced by confined animals. Stable flies develop from egg to adult in three to four weeks.

Female stable flies live approximately three to four weeks and lay about 500 to 600 eggs during their lives. They are typically active during the spring.

Horn flies lay their eggs in fresh cow manure pats and develop from egg to adult within 10 to 20 days. Adult flies live approximately three weeks, feeding 20 to 30 times a day.

In Central Texas, horn flies are first observed in early spring. Populations tend to peak in early summer, then decline when the weather becomes hot and dry. In the fall, populations usually surge again when temperatures drop and rainfall increases. Populations typically decrease after September or October, depending on the temperature.

Multi-pronged approach
Use an integrated pest management approach to suppress pest populations.

Remove potential fly-breeding sites by maintaining high sanitation standards. Remove and spread decomposing vegetation or soiled bedding material from a confined animal facility as often as possible.

On rangelands, hay bale feeding sites are ideal stable fly-breeding areas and require the same sanitation practices used in confined animal facilities - removal and spreading the vegetation when it becomes mixed with feces and urine.

Natural predators, parasites or pathogens are biological controls used to suppress a pest species. Few biological control tactics have been found for controlling stable and horn flies, but some species of wasps, parasitic to the flies but not a pest to people, lay their eggs in immature stable flies. The wasp offspring feed on the stable fly maggot and eventually kill it.

Wasp parasitoids are available commercially and can be used to boost natural populations. However, you will not observe immediate results nor will the biological control be 100 percent effective. This control should be used with other methods, such as sanitation.

If stable or horn fly problems persist, consider an insecticide. Many compounds are available for suppressing adult and larval stable fly populations.

Insecticides
For stable flies, animals can be treated as needed with ready-to-use sprays containing pyrethrins. Residual wall sprays, such as Demon¨, Cislin10¨, or Dyfly Plus¨, can be applied to surfaces where the insects rest.

Backrubs, which use compounds such as coumaphos (Co-Ral¨) or permethrin (Atroban¨), are also suitable methods for reducing stable fly populations. However, backrubs only treat the animal’s back leaving areas such as the animal’s legs unprotected.

Spray products, such as Disvap III¨, are labeled for use on dairy cattle.

Ear tags are effective in suppressing horn flies. Avoid tagging cattle until there are more than 200 horn flies per animal. This will minimize the development of early-season resistance to the insecticide in the tag. If you do not tag until March or later, the tags will still be effective later in the year, when horn fly numbers are highest.

Read ear tag labels carefully to determine when to remove them from the animals. Do not use tags beyond their recommended useful life Ð this exposes the flies to lower doses of insecticide and may increase the chances of fly populations developing resistance.

Rotate classes of insecticides (not brand names) every year. Most ear tags contain either a pyrethroid or an organophosphate insecticide. If you use the same class of insecticide two years in a row, horn flies can quickly become resistant.

Do not use ear tags that contain both pyrethroids and organ-ophosphates. These combination tags do not slow resistance development and may increase this problem. After tagging cattle, add a supplemental treatment, such as pour-on, dust bag, or back rub, at mid-season (July or August).

Free-range cattle can be treated with small sprayers and dusters powered electrically from the back of a truck. Sprays can also be ap-plied during seasonal roundups, though their effectiveness is short-lived.

Both free-range and confined cattle can be given feed additives that contain insecticides which pass through the digestive system of the animal and kill fly maggots in the manure. Regulating the intake of feed additives can be difficult because some animals take in the proper amount while others receive too little.

Do not treat infestations of less than 200 horn flies per cow or 20 stable flies per cow. Treatments when fly numbers are below these levels do not provide an economic return and may speed the development of resistance. Read the treatment label to make sure it is suitable for use on cattle. Always wear protective clothing when applying insecticides.