Fly Service

Offer feed-through products before fly season begins so treated manure is waiting as flies emerge and begin breeding.
Offer feed-through products before fly season begins so treated manure is waiting as flies emerge and begin breeding.
(John Maday)

If fly season has not yet begun in your part of the country, it soon will. While we’ll probably never “win” the war on flies, early action, and a season-long control strategy can prevent stress, lost performance and disease associated with flies and other external parasites of cattle.

The issue made headlines recently, when an unusually large spring hatch of black flies, in this case southern buffalo gnats, apparently caused the deaths of a bull and a cow on an Arkansas farm.

Larry Hawkins, DVM, a technical service veterinarian with Bayer, says four types – horn flies, stable flies, house flies and face flies,  cause the most economic damage in cattle production, with horn flies leading the pack.

In addition to animal-welfare and performance impacts, flies, ticks and other external parasites transmit disease such as pinkeye and anaplasmosis. Prior to last year’s changes to the veterinary feed directive (VFD) rules, producers sometimes used medicated minerals or other free-choice feeds containing chlortetracycline (CTC) for prevention of pinkeye, an extra-label application. By law, medicated feeds can only be used in accordance with the label, and with the VFD rules in place, veterinarians must confirm the intended use complies with product labels before writing a VFD order.

Given those restrictions, and with antibiotic use falling under increasing consumer scrutiny and regulatory action, producers can benefit by working with their veterinarians to focus more on parasite control and less on medicated feeds for preventing those diseases.

Oklahoma State University Livestock Entomologist Justin Talley, PhD, says external parasites cause enormous economic losses to the cattle industry in the United States. Horn flies lead the way, causing an estimated $1.36 billion in annual losses in U.S. livestock herds. Stable flies cause an additional $672 million in losses, followed by horse flies at $296 Million, face flies at $191 million and ticks at $162 million.

Horn Flies

Hawkins says each horn fly can take 20 to 30 blood meals every day, reducing yearling weights by as much as 18%. Good control of horn flies can boost weaning weights by 10 to 15 pounds ad yearling weights by 15 to 50 pounds, compared with similar cattle without horn fly control. Horn flies have also been implicated in the spread of summer mastitis. Talley adds that generally, 200 to 300 flies per animal constitutes an economic threshold, where treatment will pay.

Stable Flies

Stable flies tend to accumulate on the lower part and legs of the animal, with five to 10 flies per leg a typical economic threshold. Populations typically peak in the spring and again in the fall.

Stable flies, with their painful bites to the legs and bellies of cattle, can reduce calf weight gains by a half-pound per day. Fortunately, the season for stable flies generally is short, but during a typical 20-day season, they can reduce calf gains by 10 pounds. They also can reduce milk production by 30 to 40% and annual dairy output by 139 kg per cow. Stable flies breed in decaying vegetation, so cleanup around old hay rings or feed bunks provides a critical control measure. Stable flies can travel five to eight miles for a meal though, so producers often need to use controls in addition to sanitation. Stable flies generally cause most problems during the spring and fall, with their populations declining during the heat of the summer.

Face Flies

Face flies are not a biting fly, but cause stress and play a role in transmission of Moraxella bovis, the pathogen that causes pinkeye in cattle. Pinkeye reduces calf value by an average of $10 to $12 per hundredweight, and just 12 to 14 flies on the face of an animal can disrupt grazing behavior enough to reduce weight gains.

Face flies are intermittent feeders, often feeding on mucus around the eyes and mouth of an animal. They make short visits to the host animal, so an insecticide needs to act quickly. Pyrethroid ear tags generally offer good protection, Hawkins says.

House Flies

House flies often cause the most damage in dairies and feedlots, and can transmit the bacteria that causes mastitis in dairy heifers, which can then spread from animal to animal. When dairy heifers develop mastitis after their first calving, Hawkins says the disease often traces back to house flies infecting the heifer’s teat during the development stage. 

Horse and Deer Flies

Horse flies and deer flies can serve as mechanical vectors for Anaplasma marginale, the bacteria causing anaplasmosis in cattle. Veterinarian Gregg Hanzlicek, at the Kansas State Veterinary Diagnostic Laboratory, says flies carry, but don’t amplify the anaplasmosis pathogen, meaning the bacteria flies pick up when feeding is the maximum they will be able to pass on to the next animal when they take their blood meal. Control is important though, as flies can travel significant distances and introduce anaplasmosis into beef herds.


Ticks are not flies, or even insects, but they enter into this discussion as biological vectors for A. marginale. Several species of ticks, particularly the dog tick, carry and spread the pathogen. When ticks feed on a positive animal, the bacteria establish inside the tick and reproduce, and the concentration can reach very high levels.  When the tick feeds on an animal it will pass those bacteria through the saliva. Some ear tags, sprays and pour-on insecticides are labeled for use against specific species of ticks, and others provide activity against ticks beyond what is listed on the label.

Fly-Control Methods

Because stable flies attack the legs and lower bodies of cattle, backpack sprayers or sprayers mounted to ATVs can enable individual treatment during fly outbreaks. Hawkins also suggests spraying facilities and fences where stable flies congregate while not feeding.

Horn and face flies primarily harass pasture cattle, so pesticide ear tags provide the convenience of long-term control. They generally protect cattle through the fly season, but in areas where the season begins early and lasts long, Hawkins says producers might need to supplement pour-on or spray insecticides late in the season.

Fly control efforts should begin early in the season, before populations explode. Talley says to avoid encouraging pesticide resistance, don’t use pyrethroid tags more often than one year in three, and do not use organophosphate tags more than two years in a row.

Horn flies and face flies, and to some extent stable flies and house flies, breed in manure, making feed-through insect growth regulators (IGRs) and larvicides effective control options. These generally are formulated with mineral blocks or mixes. The active ingredients pass through the animal and kill or inhibit fly larvae in the manure. In the southern United States, use these products in the early spring, with timing moving later further north, to match the time flies begin laying eggs prior to the seasonal spike in population growth.

University of Tennessee beef Extension specialist Jason Smith, PhD, says that while feed-through fly control can be a valuable component of a comprehensive fly control program, misuse often results in lost opportunities and limited return on investment. Thus, it is important to understand the science behind these products and how they should be used in order to maximize their efficacy.

Talley says that while IGRs and larvicides are delivered to the manure in a similar manner, IGR products are generally species-specific. Some commonly available IGR products are only effective on horn flies.

In contrast, larvicides , because of their different mode of action, can target horn flies, face flies and stable flies. Larvicides might, however, affect insects other than flies that reproduce in manure, Smith says.

Because these feed-through products affect flies in manure, Smith stresses a need to begin feeding these products at least one month prior to flies emerge to limit reproduction. Similarly, product efficacy is dependent upon feeding these products throughout the duration of the fly season. As a result, it is necessary to extend feeding through the second major killing frost, Smith says.

These products work in a dose-dependent manner, Smith notes, so it is imperative that cattle consistently consume the labelled amount of feed required to deliver the necessary level of IGR or larvicide. If using a free-choice mineral supplement, this requires tracking mineral consumption, and relocating mineral feeders as necessary to achieve consistent intake.

Smith also says feed-through products work best used in combination with other methods of control, such as fly tags, topical insecticides and proper manure management.

As for cost, Talley says insecticide ear tags average around $3.20 to $4.45 per cow per season. Pour-on products range from $2.50 to $9.50, depending on the number of treatments needed. Generally, Talley says, ear tags combined with an IGR provide cost-effective control.

Service Opportunity

Hawkins says some veterinarians have taken to offering fly-control programs for their clients. Helping producers design, implement and evaluate their fly control provides an opportunity for valuable service while filling the summertime gap between calving season and pregnancy checking in cow-calf operations.

Effective fly control requires planning and flexibility to account for seasonal and regional variation. You can help your clients identify strategies for protecting animal health, welfare and performance while preventing pesticide resistance in flies and antibiotic resistance in pathogens.


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