For us, they are a nuisance at our summer picnics and barbecues, but for your animals, they are more than just annoying. No matter where your cattle are this summer, pasture, range or corral, the one thing each location has in common are flies.

Understanding fly biology, monitoring for fly pressure and knowing when and how to treat can reduce disease problems and increase your bottom line.

Not all flies are the same

Flies can be grouped into two different types based on where you find them: pasture or premise.

Premise flies are those found mostly around farm buildings and corrals, mainly the common house fly and stable flies. House flies are a concern when it comes to disease transfer.

They have sponging-sucking mouthparts instead of biting mouthparts but can spread disease from animal to animal or from manure to feed.

Stable flies are fierce biters and are typically found on the animals’ legs and bellies. Stable flies resemble a house fly but are only about a quarter-inch in length.

As the name implies, pasture flies are those that affect cattle out on pasture. These would include horn flies, face flies and stable flies. Horn flies are blood-feeding flies found most often on an animal’s back, shoulders, side and sometimes bellies.

Horn flies are about half the size of a housefly and can be differentiated from stable flies by their slender appearance and location on the animal.

Face flies are non-biting flies that feed on secretions around an animal’s eyes, mouth and muzzle. Face flies are responsible for spreading the organism Moraxella bovis, the primary cause of pinkeye.

What’s the problem?

Flies, especially horn flies, are considered the number one pest for pastured cattle in the U.S. Losses from horn flies alone are estimated to be over $800 million annually.

The majority of this loss comes from reduced average daily gain in fed cattle and reduced weaning weight in calves. Several studies have shown weaned calf weights to be 10 to 27 pounds higher when effective controls of horn flies were implemented.

Research from the University of Nebraska showed a reduction in ADG of 0.44 pounds per head per day in an 84-day feeding trial compared with animals receiving insecticide applications for stable flies.

Horn flies have been shown to reduce milk production in pastured beef cows. Studies in dairy heifers have shown an increase in the prevalence of mastitis caused by staph. aureus when teat ends have scabs caused by horn flies.

Adults horn flies typically appear in late spring, peak during mid-summer to late summer and seem to disappear by late fall. Under ideal conditions, horn flies may complete their life cycle in less than 20 days.

Adults live for approximately 10 days, feeding 20 to 30 times a day and typically only leave an animal to lay eggs in fresh manure.

Eggs can hatch into larvae within 18 hours which live and feed in the manure for the next three to five days. Those larvae then pupate for three to five days, at which time the adults emerge to continue the cycle.

When to treat

Observation is the key to treating cattle for either horn or stable fly problems. Watch for the following signs that cattle are being bothered by an increased fly concentration.

• Decreased grazing time
• Increased traveling
• Increased tail switching and feet stomping
• Standing in groups
• Standing in water
• Increased lying time with legs tucked underneath body

Horn fly treatment is recommended when flies reach a concentration of 100 per animal. Documented economic losses begin at 200 flies per animal.

Since you probably have better things to do with your time than stand around and count flies, follow the rule of thumb that 200 horn flies will fit in the space of your hand. Economic losses from stable flies occur at a much lower level of five flies per leg.

Control methods

Producers who use an integrated pest management (IPM) approach to controlling fly populations have the best success. IPM uses biological, chemical and cultural methods to control flies.

Good sanitation is an example of a cultural control and should be the first step in any fly control program. This is important because you are reducing sites for flies to lay eggs.

In addition to manure, flies (especially stable flies) may lay eggs in spoiled or fermenting organic matter mixed with animal manure and moisture. Good sanitation steps to take for controlling flies include:

• Removing manure on regular basis from pens, harrow to break up manure piles in pastures
• Make sure corrals and barn have good drainage to prevent moist areas
• Clean around feeding sites to remove spilled feed, hay and straw

Other control measures include parasitic wasps (an example of IPM biological control), diatomaceous earth, bug lights and ultrasonic devices.

In university trials, only the use of parasitic wasps has been shown to have limited success at fly control when used in conjunction with good sanitation practices.

The other options have not been proven effective in university trials to reduce fly numbers.

When making decisions about using insecticides for fly control, it is important to realize that there are many effective products. Producers should work with their veterinarian to develop a program that fits their operation, is cost-effective and convenient.

The most common classes of insecticides used for fly control in cattle are organic phosphates, pyrethroids and spinosins. Methods for delivering the insecticide include ear tags, pour-on preparations, sprays, dusts and feed additives.

When using self-treatment devices (dust bags and oilers), location is very important.

Choose a place where animals have frequent contact to make sure animals are getting treated. Also use one dust bag per 45 animals to make sure animals have enough access to them.

Don’t forget about treatment for young animals. Because flies can decrease average daily gain, forgetting to treat those young animals may have more effect on your income than treating older animals.

Finally, you should rotate the class of insecticide, not just the brand or method of application being used to prevent selection of flies with a resistance to a certain insecticide.

An example would be to use an ear tag with pyrethroids, remove the tags when they lose their effectiveness and put out dust bags with organic phosphates.

So whenever you are out checking on your cows this summer, make sure to take note if your cattle are showing any signs of being bothered by the fly population – and take appropriate action. Your cows will thank you. 

Stephanie Etter is an extension educator for the University of Idaho.