Summer Fly Control in Feedlots

By : Warren Rusche, SDSU Extension Beef Feedlot Management Associate, Courtesy of extension.sdstate.edu

Key Takeaways

A brown and black cow standing in a feedlot with flies on its face.
  • Flies not only are an annoyance, they can reduce performance and worsen heat stress.
  • Successful control strategies start with sanitation.
  • Sprays, biological controls, or feed additives may also help reduce fly pressure in feedlots.

Just as longer days mark the beginning of summer, so does the arrival of increased number of flies in feedlots.

Stable flies and house flies are the most common pests of cattle in feedlots. Of the two species, stable flies cause the greatest economic loss. As few as five stable flies per leg can reduce ADG by 3 to 20% during a summer. Most of that lost performance results from the indirect effects of flies, such as bunching and increased heat stress, along with the energy losses from flighting flies. Flies also act as disease vectors and can lead to poorer relations between feedlots and their neighbors.

Sanitation is Key

Controlling feedlot flies starts with sanitation. Without this key step, any of the other mitigation strategies will likely be disappointments. Stable flies breed in mixtures of spilled feed and manure, especially around feeding aprons, under feed bunks, feed storage areas, and under fences. Any wet or stagnant area where with combinations of organic matter, manure and soil can be a potential trouble spot. Removing feed and manure from these areas promptly deprives the flies of potential breeding areas, which in turns helps prevent explosive increases in fly populations.

Stable flies are most active between 10 A.M. and 4 P.M., after that they seek shaded areas to roost. Eliminating weedy areas around the edges of pens, near feed storage areas, or along the edges of manure containment facilities reduces the number of “safe spaces” for flies which should help keep populations in check.

The life cycle of a stable fly lasts about 2 to 3 weeks, so scraping pens every 14 days reduces the opportunities for the flies to reproduce. Hoof action also disrupts the life cycle, so keeping pens fully stocked will help to reduce fly numbers, if mud issues can be avoided.

What About Chemical Control?

Chemical controls can be a part of a fly control plan in conjunction with sanitation efforts. Stable flies prefer to feed on the front legs of cattle, meaning that getting good spray coverage for this pest is difficult. Spraying the animals directly would only provide short-term relief but could still be useful as a quick “knock-down” to reduce pressure while implementing other practices. Premise sprays may be more effective, especially in areas where the flies rest such as vegetation or on shady sides of structures.

Biological Control Strategies

Biological control using parasitic wasps is another option to consider. Parasitic wasps lay eggs in the pupae of flies. These wasps can reduce the number of adult flies in a feedlot and delay the peak in population if the parasite population is high enough during the fly breeding season. This strategy is not a “quick fix”, but rather a part of a longer-term strategy to reduce fly pressure.

Finding a reputable supplier who can provide the right species and manage the release is a key factor to successfully implement a biological control strategy s for optimal results. Periodic releases throughout the fly season are more effective than one massive release.

Sanitation is still critically important with biological control programs. If the fly breeding areas are too large, the flies will simply overwhelm the ability of the parasitic wasps resulting in little to no effect on cattle performance or overall fly populations. Reducing the number of breeding areas will concentrate the flies and help make the parasite control more effective. It is important to remember not to use chemical control in all the breeding areas when using parasitic wasps. The parasite requires live pupae to reproduce.

How About Feed Additives?

Several feed additives have been developed for use with pasture cattle, particularly for face and horn flies. If a feeder is considering using either an insect growth regulator (IGR) or larvicide product, make certain that they are labeled as being effective against stable flies and house flies.

Adding garlic to mineral mixtures at a rate of 2% has been promoted as a potential fly repellent. The amount of research data on this strategy is limited. One published study from Canada reported reduced fly numbers in grazing cattle when garlic was added to the mineral. While this is only one study and it was not conducted in a feedlot setting, this could still be worth considering as a component of a multi-faceted control strategy, particularly if the costs were reasonable.

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