Fly control in concentrated animal feeding operations such as dairies is a challenge, especially as the weather heats up and there is abundant moisture and organic matter available.
Flies not only are a pest, but they also decrease production efficiency. Flies cause livestock to expend extra energy fending them off instead of resting, feeding and milking. Other issues directly associated with fly pest problems on dairies include increased medication costs, veterinary costs, increased potential for disease spreading and possible increased public complaints. For example, it is estimated that stable flies (biting, blood-feeding fly) can lower milk production by 15 to 30 percent. Additionally, contagious mastitis is also spread by high fly populations. Flies also can hinder worker productivity along with spreading disease to humans.
Fly identification
Identifying the type of fly or flies that you are dealing with on the dairy along with understanding their lifecycle is key to developing an effective fly management plan. Common flies found on dairies include house flies, stable flies and blow flies. These flies are commonly classified as “filth flies” as they like to breed in spoiled or spilled feed, bedding, decaying organic matter and manure. They also prefer moist conditions and decaying organic matter to lay their eggs for reproduction. Of the flies listed above, only the stable fly is considered a biting fly and requires a “blood meal.”
Common pests often found in pastures are typically horn flies, face flies, horse flies, deer flies and mosquitoes. All of the pests for pastures listed, except face flies, are biting insects that require a “blood meal.” To learn more about these pest populations in livestock, you may want to view Cornell University's website.
Control options
Control of filth flies is best achieved with sanitation and a combination of knockdown and residual sprays. It is important to read the label of knockdown or residual sprays, as many are not cleared for use on lactating dairy cows or in the milking parlor. The best time to apply insecticides for fly control of house and stable flies is during the hottest part of the day. At that time the insects retreat to cooler areas, such as vegetation around pens, under bunks or shaded areas. Knockdown sprays are most effective when the air temperature is between 65°F and 90°F. Residual sprays should only be applied to shaded fly resting areas because ultraviolet light breaks them down when exposed to the sun. Rain may wash some residual sprays off or decrease their effectiveness, so they may need to be reapplied.
Cleaning areas
Areas to concentrate cleaning efforts in fly control management include: cattle pens, calf housing and loafing barns, drainage areas, manure storage areas, freestalls, in and around waterers, feeding aprons, feed mixing areas or any other place where there is decaying organic matter. These areas should be cleaned and have organic matter moved away and spread on fields if possible every 10 days to break the fly’s reproductive cycle. Manure stored in silo-type storage units may crust on the top, but cracks allow flies to deposit eggs in wet material below the crust. Agitating or adding water may be necessary as fly larvae drown in water. Periodic rains will help drown fly larvae in large lagoons.
Milking parlor
Keeping flies out of the milking parlor can prove to be challenging. Keeping screens repaired and sanitation in check, along with proper ventilation (which will allow the milking area to dry between milkings), will help minimize potential fly populations. It is important to read the label of any pesticide product before use to make sure it is labeled for use in the milking area and for proper application methods.
Grazing considerations
If you graze your cattle and do not keep them in dry lots or freestall barns, they may be attacked by other fly species; horn flies, face flies and horse flies are the most common. Horn flies are bloodsucking insects that can bite an animal up to 20 to 40 times per day to feed. This deters grazing and feeding, resulting in decreased intake and milk production. Horse flies are typically not as prevalent in pastures unless the pasture is close to a woodland or wet marshy area. The female horse fly will deliver a violent bite with its mouthparts, often causing the animal to bleed after the bite, allowing it to feed on the blood from the wound, which can also attract the female face fly. Horse flies will lay their eggs on vegetation near marshes and can have a life cycle from 70 days to two years, depending on the species.
The face fly primarily feeds on animal secretions around the eyes and nose, and are the common means of spreading the pinkeye bacteria, Moraxella bovis. It is also important to note that both the horn fly and face fly lay their eggs in manure, so manure management in pastures by dragging will decrease fly breeding grounds. Fly control for cattle going to pasture is best achieved if the animals are forced to pass under low-hanging dust bags or oilers so their face and back get treated. Once again, it is important to check the label of the products used to make sure they are approved for their use in both lactating and non-lactating dairy animals.
Calf hutches
One area of fly control that is often overlooked on dairies is fly control in calf hutches, which may produce enough house and stable flies to be considered an economic problem. These flies act as vectors for viruses and bacteria/agents for diarrhea between calves, not to mention the dangers of maggots during dehorning and around the umbilical cord. Weekly cleaning or moving of hutches to cleaner “sun-dried” areas during the warm summer months will help reduce the breeding ground for these flies.
The bottom line
Fly control on dairies is an important pest management consideration that impacts the bottom line by affecting overall animal productivity and health. Sanitation is key in providing effective fly control around the dairy and in pastures. 
References omitted but are available upon request. Click here to email an editor.
This article originally appeared on the South Dakota State University Extension website.
Tracey Erickson is a South Dakota State University Extension dairy field specialist.
PHOTO: Photo courtesy of South Dakota State University Extension.
