Horn Flies: Violating Vector
Those tiny flies that stay put on the cattle, unless disturbed, could be causing more trouble than it appears. Horn flies, about 3/16” long, usually rest head down, often with wings in the delta position, on the back, sides or belly of cattle. A horn fly has piercing, sucking mouth parts and can take 20 to 40 blood meals per day. The horn fly will not bite man, but will feed on horses, sheep or dogs, without establishing a breeding population, until cattle are available.
Shawn Burgess, University of Arkansas, Stone County Agent, has been studying horn flies for three years. Burgess said severe infestations can cause yearlings to suffer an 18 percent loss in weight gain, an 8 percent reduction in calf weaning weights, and 20 percent less milk production for dairy cattle.
The horn fly, found throughout the continental United States and Canada, spends most of its adult life on cattle. They do not bother calves younger than six months of age. The female will leave the host to lay eggs (up to 500 during her three to seven week life span) in fresh manure. The manure loses its appeal within 5 to 10 minutes, which is why during the very hot, dry part of summer, the fly population decreases. Then the fall weather causes another peak in fly numbers. The complete life cycle from egg to adult can be completed in as little as 10 days. While the adult flies are killed by a freeze, the larvae burrow about 1.5 inches into the soil and over winter, ready to start the cycle in the spring.
Although horn flies have been implicated in the spread of summer mastitis, and suspected of spreading the pathogen causing pinkeye, Shawn Milliken, University of Arkansas, Searcy County Agriculture Agent, believes the economic loss is due to the “nuisance” factor. Cattle spend time bunching up, looking for shade trees, something to rub against and standing in water, rather than grazing. So, it is the reduced food intake that causes the decreased production and lack of weight gain.
Treatment is not considered economically profitable until horn fly populations reach 200 per beef cow or 50 per dairy cow. A fairly accurate way to estimate the number of flies on a cow is: imagine a cow with “A” just behind the shoulder, “B” lower and toward the middle, and “C” on the belly. A single patch of flies in any area equals 25 to 50 flies. A single patch of flies in any two areas equals 100 to 125 flies. A single patch that extends through A, B and C equals 200 to 350 flies. A patch that extensively covers all three areas equals 500+ flies.
Burgess noted that since the flies do not cover a great distance, they are “not a big disease factor.” In his studies they used liquid spray, treated ear tags, feed through minerals or tubs, back rubs, and dust bags. All these methods achieved control, which was defined as less than 200 flies per cow. He noted that the tubs, which kill the larvae, were the most convenient, but also the most costly.