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Counting Horn Flies: No Easy Task, Whether In Person or In a Photo

horn fly

A single horn fly (Haematobia irritans) seems unimposing, but pest management tactics are necessary when the infestation on cattle reaches greater than 100 flies per dairy cow or 200 per beef animal. That’s why accurate methods for counting horn flies are crucial. (Photo credit: Craig Sheppard, University of Georgia,

By Ed Ricciuti

If your job is counting horn flies, a twitch of a cow’s hide can go a long way toward ruining your day. OK, perhaps that is a stretch, but it accurately describes a problem facing pest control managers who must manage a blood-sucking parasite that costs the cattle industry $1 billion a year in the United States alone.

Ed Ricciuti

Pest managers tally the number of horn flies (Haematobia irritans) on cattle because it makes economic sense to start control efforts only after a numerical threshold has been exceeded: 100 flies on a dairy cow or 200 on a beef animal. Economic loss is likely when numbers above the threshold are sustained.

Counting black specks half the size of a house fly is a formidable proposition when the flies are still. More often than not, though, counter and cow are enveloped in a constantly moving cloud of flies as the irritated animal tries to shoo them away. Trying to rid itself of flies, an infested cow continually swishes its tail, shakes its head and stomps its feet. Its skin is rippled by involuntary shudders, an involuntary aversion response to the flies, which each pierce the cow’s hide with a needle-like proboscis tipped by a raspy lower lip, grooved to channel in siphoned blood.

All that activity severely saps a cow’s energy and diverts it from grazing and other healthy behaviors that promote meat and milk production. Infested cows get little rest, feed less, and absorb fewer nutrients. Their normal rate of weight gain nosedives, as much as a half-pound daily. They also can lose a considerable amount of blood because, even though a single fly takes less takes two milligrams per meal, it returns to the table about two dozen times in a day, and it can be only one of hundreds at a sitting.

Traditionally, pest managers have calculated horn fly numbers by eyeballing them and clicking away on a hand-held tally counter or even using pencil and paper. More recently, still photography and even videos have entered the picture as well. Generally, observers extrapolate the whole-cow estimate from the number counted on part or parts of a cow’s body. The process is somewhat like trying to guess the number of jelly beans in a jar by counting those in the top layer then estimating the number in layers below. “Except,” says, researcher Brandon G. Smythe, “if all the jelly beans were black and could fly when you get too close to them.”

Smythe is program manager at the Veterinary Entomology Research Laboratory at the Center for Animal Health and Food Safety, New Mexico State University (NMSU). He is lead author on a paper, published today in the Journal of Medical Entomology, describing research aimed at refining and standardizing ways to count horn flies, making it easier for ranchers, technicians, and all concerned with their management.

Under controlled conditions, using four heifers a week for four weeks, the researchers compared visual count totals with counts of photographs of flies taken by an off-the-shelf digital camera and entered into a computer. The scientists infested the cows inside control rooms and counted flies on both their flanks outside, in paddocks, at 7 a.m., noon, and 5 p.m. Cows were situated facing south so light conditions were uniform. Horn flies move away from direct sunlight, which probably is why, says Smythe, morning counts were higher than at other times. At midday, especially, flies tend to mass underneath the cow and are difficult to see or photograph.

The research indicates that existing methods need plenty of fine tuning, says Smythe: “Our findings suggest inconsistencies in methods used to estimate horn fly populations.”

Among those findings, perhaps not surprising is that veteran observers produced counts that were more accurate and more often above the threshold than inexperienced rookies. Visual counts were more reliable, perhaps because the camera angle did not cover flies along the backbone and abdomen.

The researchers tried two ways with photographs to estimate the full-body number of flies: combining counts from photographs of both sides taken simultaneously and, the other, doubling the highest number photographed on one side of the animal.

The NMSU research did not produce an easy way to count flies but, importantly, suggests procedures for further evaluating methods. The ultimate goal is a technique that is uniformly accurate and one size fits all, less subjective than those in use. As things now stand, the study suggests observers need sufficient training to make solid counts.

The hope is to eventually make it so simple an untrained rancher or field hand—simulated in the research by the inexperienced observers—can snap photos of flies with cell phone and send it to a laboratory where technicians can use standardized analysis methods to get an accurate count and jump start control. Ultimately, says Smythe, it would mean that the process could be started by a complete novice, who “doesn’t spend the majority of his life counting horn flies like me.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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