Control Flies, Prevent Disease

The following commentary does not necessarily reflect the views of AgWeb or Farm Journal. The opinions expressed below are the author's own.

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 this week, when an unusually large hatch of black flies, in this case southern buffalo gnats, apparently caused the deaths of a bull and a cow on an Arkansas farm.

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.

Talley says when horn flies are present, control efforts can bring an average of 1.5 pounds of extra gain per week. Generally, 200 to 300 flies per animal constitutes an economic threshold, where treatment will pay.

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.

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.

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.

Several species of ticks, particularly the dog tick, serve as biological vectors for the anaplasmosis 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.

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.

Insect growth regulators (IGRs) and feed-through larvicides offer additional options. These generally are formulated with mineral blocks or mixes. They 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.

The cost of insecticide ear tags averages 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.

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