Controlling Flies on Pastured Cattle

Grazing season is here for much of the country, so make sure you're cattle are ready to combat pesky insects like flies this summer.
By: Dave Boxler, UNL Extension Educator

As we move closer to summer grazing season, livestock producers should start to consider their fly control management systems. There are three primary fly species that economically impact grazing cattle; horn fly, face fly, and stable fly.

Horn Fly
The horn fly is considered the most important blood-feeding pest of pastured cattle and can have significant economic impact on cattle operations. Horn flies are 3/16 – long and are normally seen on the backs, side, and poll areas of cattle. During the warm part of the day, horn flies can be observed on the belly of cattle. Both male and female horn flies will spend most of their lives on cattle feeding 20-30 times a day. After mating, the female will leave the animal only long enough to deposit eggs in fresh cow manure. Eggs hatch within one day, and larvae feed and mature in the manure, pupating in the soil beneath the manure pat. Newly emerged horn flies can travel several miles searching for a host (Figure 1). The entire life cycle can be completed in 10 – 20 days depending on the weather.

Economic losses associated with horn flies are estimated at more than $800 million annually in the United States. Horn fly feeding causes irritation, blood loss, decreased grazing efficiency, reduced weight gains, and a decline in milk production. Furthermore, horn flies have been implicated in the spread of mastitis. Many studies have been conducted in the U.S. and Canada to assess the economic effects of horn flies on cow and calf weaning weights. Nebraska studies have demonstrated calf weaning weights were 10 -20 pounds higher when horn flies were controlled on mother cows. The economic injury level (EIL) for horn flies is 200 flies per animal (Figure 2). Yearling cattle can also be impacted by the horn fly; other studies have indicated yearling weight can be reduced by as much as 18 percent.

There are many chemical methods available to reduce horn fly numbers; backrubbers, dust bags, insecticidal ear tags, pour-ons, oral larvicides, and sprays. Insecticide ear tags are a convenient method of horn fly control. However, many horn fly populations in Nebraska are resistant to pyrethroid insecticides. One of the resistant management recommendations for horn flies is to rotate insecticides. Backrubbers and dust bags are an effective way to reduce horn fly numbers if cattle are forced to use them. Sprays and pour-on will provide 7-21 days of control and will need to be repeated throughout the fly season for effective control. Oral larvicides prevent fly larvae from developing into adults. An important factor when using an oral larvicide is insuring daily consumption. A complicating issue when using an oral larvicide is that of horn fly immigration from neighboring untreated herds which will mask the effectiveness of an oral larvicide.

Designing an effective horn fly control program will depend on efficacy, cost, convenience, and head health management practices.

Face Fly
Face fly adults closely resemble house flies except they are slightly larger and darker than the house fly. Other differentiating characteristics include: 1) the abdomen of the male face fly is orange and the female has an orange stripe; the abdomen of the house fly is white or light gray and 2) the compound eyes of male flies nearly touch, but are separated in the house fly. The persistence and habit of congregating about the eyes and nose of animals helps distinguish the face fly from the house fly in the field. House flies may congregate on the faces of cattle in confined feedlots and dairy pens. Face flies are pasture flies and are not found in feedlots. The face fly is a non-biting fly that feeds on animal secretions, nectar, and dung liquids. Adult female face flies typically cluster around an animal’s eyes, mouth, and muzzle, causing extreme annoyance (Figure 3). They are also facultative blood feeders, gathering around wounds caused by mechanical damage or other injury. Face flies are present in the field throughout the summer with peak populations occurring in late July or August. Face flies are most numerous along waterways, areas of with abundant rainfall, canyons where the canyon floors have trees and shaded vegetation, and on irrigated pastures.

Feeding of the female face fly around the eyes causes eye tissue damage, which creates susceptible tissue for eye pathogens. In addition to be very annoying, face flies vector Moraxella bovis, the causal agent of bovine pinkeye or infectious bovine keratoconjuntivitis. Pinkeye is a highly contagious inflammation of the cornea and conjunctiva of cattle. If coupled with the infectious bovine rhinotrachetis (IBR) virus, M. bovis can cause a much more severe inflammatory condition. Controlling face fly numbers is a key to reducing most pinkeye problems. Attaining adequate face fly control can be difficult because of where the flies feed and the significant time they spend away from the animal. The best methods of reducing face fly numbers is using a treatment where the animals are forced to contact an insecticide on a daily basis such as a dust bag, oiler, spray or an insecticide impregnated ear tag. Ear tags should be applied at the label recommended rate. Both cows and calves must be treated if control is to be achieved. Pinkeye vaccines are available and should be considered if face flies and pinkeye are an annual problem. Commercial and autogenous pinkeye vaccines are available; please check with your local veterinarian about the use of these products in specific geographical area.

Stable Flies
Stable flies have been pests of Nebraska feedlots and dairies in for many years. More recently the stable fly has expanded its range to include feeding on pastured cattle (Figure 4). Stable flies are blooding feeding flies, mainly feeding on the legs of cattle. Stable flies stay on the animal long enough to obtain a blood meal and then seek a shaded place to rest and digest the blood meal. Stable flies may take blood meals several times a day. Stable fly bites are painful and when flies are abundant cattle stomp their legs, bunch at pasture corners, or stand in water to avoid being bitten. The effect of stable flies on weight gain performance of pastured cattle is similar to that of livestock in confined operations. Research conducted at the University of Nebraska, West Central Research and Extension Center utilizing yearlings, recorded a reduction in average daily gain of 0.44 lbs. per head per day in 84-day trials with animals that did not receive an insecticide treatment compared to animals that received a treatment. The economic threshold of 5 flies per leg is easily exceeded in Nebraska pasture conditions.

The stable fly is about the size of a house fly, but is dark gray and has irregular spots on its abdomen. The mouth part protrudes bayonet-like in front of the head. The life cycle of the fly from egg to adult can take 14-24 days in Nebraska depending on weather conditions. While the source of early season flies it not well understood some probably develop from overwintering larvae. Other early season flies may be migrants from southern locations but evidence is lacking. Nevertheless, we do know that stable flies can move at least 10 miles or more.

The female stable fly deposits eggs in spoiled or fermenting organic matter mixed with animal manure, moisture, and soil. The most common developing sites are in feedlots or dairy lots, usually around feed bunks, along the edges of feeding aprons, under fences and along stacks of hay, alfalfa, and straw. Grass clippings and poorly managed compost piles also maybe stable fly developing sites. Winter hay feeding sites where hay rings are used can often be a source for stable fly development through the summer if the proper amount of moisture is present.

The only adult management option available for the control of stable flies on range cattle is the use of animal sprays. Sprays can be applied using a low pressure sprayer or can be applied with a mist blower sprayer. Weekly applications of these products will be required to achieve reduction in fly numbers. Sanitation or clean-up of wasted feed at winter feeding sites may reduce localized fly development. If sanitation is not possible these sites may be treated with a larvicide (Neporex®). But, the application of either procedure may not totally reduce the economic impact of stable fly feeding.

For Current Nebraska control recommendations go here. 

When applying any insecticide control products please read and follow the label instructions.