Cattle bunched up to fight flies

Cattle often stop grazing and bunch up in corners to fight flies.

Horn flies, face flies and stable flies are the most common and most treated pests on North Dakota livestock operations.

“Left untreated, these pests can cause significant losses in production,” says Janna Block, Extension livestock systems specialist based at NDSU’s Hettinger Research Extension Center. “Irritation caused by flies leads to behavior changes and can reduce the amount of time spent grazing, which can result in weight loss and decreased milk production. Additionally, flies are a factor in the transmission of diseases such as keratoconjunctivitis (pinkeye) and summer mastitis.”

While integrated pest management is commonplace for controlling crop pests, similar concepts can apply to controlling livestock pests. They include using the right type of control at the right time for the right duration to control pests effectively.

Timing is important

“Many North Dakota livestock producers apply pest control prior to pasture turnout, which may be optimal for control of some pests but not others,” says Miranda Meehan, NDSU Extension livestock environmental stewardship specialist. “The first step to determining when to apply control is to properly scout pastures and cattle to determine fly type and fly populations.”

Horn flies are gray and are about half the size of a housefly. Horn flies bite and spend most of their time clustered around the head, shoulders and back of cattle.

These blood-sucking flies feed up to 20 to 30 times per day. This constant biting causes cattle pain and stress, and can reduce the cattle’s weight gains by as much as 20 pounds.

When fly counts reach 200 flies per animal, the economic threshold has been reached and animals will have significant weight loss. The economic threshold is the pest density at which producers should take action to manage the pest.

Face flies look like large, dark house flies. They are nonbiting flies that feed on animal secretions, plant nectar and manure liquids. Face flies may transmit pathogens responsible for infecting the eye and causing pinkeye in cattle.

The life cycle of a face fly is approximately 21 days. Populations tend to peak in late summer. Because of the extreme irritation these flies cause, small populations can do a lot of damage. An average of 12 to 14 flies can reduce grazing time by an hour a day, and when populations reach more than 20, animals will bunch up and/or move into shaded areas.

Horn and face flies typically are not present at pasture turnout and do not reach economic thresholds for applying control until midsummer. However, populations in some areas may be peaking earlier than usual due to mild winter weather conditions and/or early moisture.

Stable flies are similar in size to house flies but have circular markings that distinguish them from horn flies. In addition, these flies bite on the abdomen and legs, feed on blood and are very disruptive to cattle grazing. They breed on organic matter.

“The only method to control adult populations is weekly application of insecticide sprays,” advises Gerald Stokka, NDSU Extension veterinarian and livestock stewardship specialist. “The best way to reduce stable fly populations is through habitat control measures such as cleaning feed grounds and keeping barns clean and dry.”

Variety of controls availableA variety of tools can be used for fly control, including insecticide ear tags and strips, sprays and pour-ons, insect growth regulators (IGR), and dust bags or back rubbers.

Ear tags contain insecticides that are released slowly into the animal’s hair by movement, so ear tags should not be applied until fly populations are nearing the economic thresholds (typically from mid-June to July). Read insecticide labels carefully because recommendations can vary for the number of tags to apply (one or two), age of cattle that can be tagged and chemical class of active ingredient (pyrethroid, organophosphate or a combination). Research shows that horn flies typically target adult animals and that tagging calves is unnecessary.

Stokka recommends rotating the class of insecticide each year and removing tags when they no longer provide effective fly control to help prevent flies from becoming resistant to the insecticides.

“Controlling face flies can be challenging because flies spend a lot of time away from the animal,” Block notes. “Dust bags and insecticide ear tags are recommended treatments for these pests. However, unlike when treating for horn flies, both cows and calves should be treated for face flies.”

To achieve proper fly control, pour-ons and sprays must be applied every two to three weeks throughout the fly season. Applying these products before pasture turnout likely will not be an effective fly control method.

“Additionally, all avermectin pour-ons and injectables may provide up to several weeks of fly control but are more important for controlling internal parasites and should be administered at the proper timing and dosage to meet that objective,” Stokka says.

Feed additive insecticides can be included in mineral formulations for cattle. The additives pass through the animals’ digestive system and destroy the developing horn fly maggots in the manure. These additives are effective in killing 80% to 90% of the developing fly larvae in animals that have consumed the product.

Feed additives should be offered at least 30 days prior to fly emergence in late June or early July. Proper intake of these products is extremely important for optimum control. Continuous use of these products may speed up resistance in the fly populations.

Dust bags and back rubbers contain insecticides that treat livestock through direct contact. These are typically inexpensive; however, they will have the best control when placed where cattle are forced to use them consistently. Ideally, they should be placed near water or mineral supplements, which can be challenging, depending on pasture size and other characteristics.

Nonchemical methods of control include fly traps, parasitic wasps and habitat control practices such as using rotational grazing to help distribute manure. As with the other control methods, producers need to watch for economic thresholds and determine what control measure will work best in their operation.

When applying any type of pest control, be sure to read the label carefully prior to application. Monitoring populations to see if the product is achieving the desired level of control also is important. If a product is not effective, the fly population may have developed a resistance to that type of insecticide and another method and/or product should be used.

“Pest control can be costly,” Meehan says. “Producers can reduce costs by following principles of integrated pest management and applying the appropriate products at the appropriate time for the appropriate control of pest populations.”

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