What you need to know about fly control and your bottom line:

  • Biting flies, including stable flies and horn flies, increase animal stress and reduce time spent feeding, thus slowing animal weight gain and reducing milk yields.
  • Depending on the number and types of flies and the type of facility and management practices, milk production losses from flies can range from negligible to up to 30%.
  • Besides decreased feed efficiency due to fly biting activity, flies may also contribute to disease incidence in cattle, particularly mastitis in lactating cows and Pinkeye in young-stock.
  • To determine if investment in additional fly control is warranted, it is essential to first determine the types of flies present and the fly burden on your particular dairy.
  • Usually stable flies and house flies are the most common flies on confinement operations while horn flies and face flies are the most important flies on pasture based operations.
  • A cost-effective fly control program may incorporate a variety of methods but eliminating moist decaying organic matter should be the corner-stone of every dairy’s fly-control program.

Know Your Fly Enemy

Depending upon facility design and management, the fly species that are most economically important may differ from dairy to dairy. A good fly management program can keep fly populations controlled to below an adverse economic threshold level. When developing a suitable fly management program the first step is to determine what type of flies are on the dairy and how big their populations are. Identification of fly species that are prevalent is important because each species has different habits and may require a different management approach. The four most problematic flies that are commonly associated with modern dairies are the biting stable fly and the horn fly and the non-biting face fly and house fly.

Stable Fly  Adult stable flies of both sexes have long bayonet-type mouthparts used to tear through the skin taking large blood meals up to once per day. Stable flies feed preferentially on the lower body and legs of cattle and stable fly burdens are frequently monitored by direct counting on legs. Some studies have demonstrated control of stable flies may increase milk production by 10 to 20%. As few as 5 stable flies per cow is thought to have a measurable adverse economic impact. Stable Flies are strong fliers and have been reported to travel up to 80 miles from their breeding site.

Stable flies before and after a blood meal (University of California – Riverside)  

Stable Flies biting the legs of cattle. (University of California – Riverside)

Horn Fly  Like the horn fly the stable fly requires blood meals but feeds more frequently, some 10-12 times per day. Because of their frequent need to feed on blood, horn flies rarely leave the animal’s body. Horn flies congregate on the backs, shoulders and sides of cows, moving to the underside in very hot or rainy weather. Horn flies are often considered to be the most important pest of grazing cattle. Various authors suggest economic thresholds, ranging from 20 to 200 flies per dairy cow, which may result in up to a 20% reduction of milk production.

The bites of both stable flies and horn flies can be painful and high biting activity will negatively affect cattle.  Cattle often respond to the presence of numerous biting flies by exhibiting behaviors like tail flicks, head tossing, and stamping of feet.

Horn flies on the backs and sides of cattle. (Kansas State University)

House Fly  The most common and prolific fly on dairies, the house fly, is not a biting pest but can still be an important nuisance to cows and dairy employees. House flies can also serve as a vector of various diseases including bovine pinkeye. When house flies are particularly numerous, they become a nuisance to people residing close to the dairy; sometimes resulting in complaints and even legal action..  House Fly numbers and activity is frequently monitored by using baited traps or spot cards. While economic threshold numbers have not been established for House Fly populations, Texas Cooperative Extension has suggested target nuisance levels for different areas on the dairy.

House Flies on waterer. (Cornell University)  

Face Fly  Unlike stable flies and horn flies, house flies and face flies and do not bite, though you may see them feeding on blood at the edge of animal wounds.  Instead, the nonbiting face fly visits cattle for short periods where it feeds on sweat, tears, and nasal drainage.  These flies negatively impact cattle through the transmission of bovine pinkeye and eye-worms More than 50 face flies are considered significant and are linked to the spread of pinkeye.

Face flies (Organic Agriculture Center of Canada)

Most Common Flies on Conventional verses Pasture Dairies

The fly species likely to be prevalent at a dairy facility depends mostly on the type of facility:

Confinement Operations:   The stable fly and the house fly and are the most common pest species on conventional dairy operations. Stable flies feed primarily on the legs and may also be found on the belly. House flies are commonly found in animal housing areas, the milk house or parlor, and are known to readily disperse into the surrounding area . All of these fly species lay their eggs in wet cattle manure and the young flies (maggots) develop in the manure as long as the manure retains enough moisture.  Stable flies develop most often in older manure, and particularly like manure mixed with plant material (like straw bedding).  House flies will develop in both fresh and older manure, even when that manure is quite disturbed.

Pasture Operations:   Due to their preference for laying eggs on fresh, intact manure pats, the horn fly and the face fly are often the principal species on operations in which the cattle are turned out to pasture. Horn flies are typically found feeding on the shoulders, back and sides of cattle, though they will move to the belly when daytime temperatures are high. Face flies are generally clustered around the eyes and nose.

Dairy Economic Losses Due To Flies

Flies may reduce dairy productivity in a variety of ways: reduced employee productivity, reduced milk production, fly harassment of cattle with decreased feed efficiency, increased disease and even heat stress resulting from increased animal crowding. National losses due to stable flies alone are estimated a $360 million per year for dairy cattle and at nearly $1 billion for all livestock.  In addition to the direct damage the flies inflict upon cattle each year, their presence has repeatedly been cited as a nuisance, especially when dairies are in proximity to urban environments.  Lawsuits due to the nuisance caused by these flies have increased considerably during the last 10-20 years.

Decreased weight gain and milk production: Different studies (some dating back to the 1920’s) have variously demonstrated that depending on the type and infestation level, fly burdens can reduce milk production by up to 5% to 30%.  For young cattle, weight gains can be considerably reduced and feed efficiency is negatively affected resulting in higher feed costs. The reduction in weight gain and milk yield is primarily due to animals exhibiting fly avoidance behaviors (such as cattle bunching) that reduce their time feeding or resting.

Cattle crowding to avoid fly bites (Progressive Dairyman)

Mastitis: Studies have demonstrated that both the house fly and horn fly act as a vector for mastitis pathogens, particularly summer mastitis in heifers. Studies performed at Louisiana State University showed that dairies using fly control had significantly lower prevalence of mastitis in bred heifers, and studies at North Carolina State University found 53% of horn flies positive for a bacteria that causes mastitis.

Pinkeye: Face flies, and to some extent house flies, are instrumental in spreading the bacteria which causes bovine Pink Eye. Studies by North Carolina State University estimate that adult dairy cows infected with Pink Eye may see a drop as much as 25% in milk production. In terms of dairy economic losses however, Pinkeye is far more important as a disease of dairy young-stock. Drummond et al estimated that face fly infestations in the U.S. have been estimated to cause an economic loss of more than $53 million, primarily due to cattle infections with pinkeye.

Measurement of the Fly Population

Monitoring the fly burden (or perhaps more accurately the fly activity) at a dairy is the basis for all decisions about when and how to control the pest flies.  Fly activity should be regularly assessed so that changes in fly activity over time are easy to recognize.  For most monitoring programs, fly counts are made once each week.  If possible, pick a day each week that is sunny and with low winds, and conduct your fly monitoring in mid-morning so that individual flies are similarly active each week.  Because fly activity must be measured over time, it is important to use the same methods for monitoring fly activity so that fly counts can be compared across time.  Lastly, whatever method is used to monitor fly numbers, monitoring records should be retained for a minimum of 3-5 years for later reference. With experience, the dairy manager will begin to recognize when fly activity is nearing the economic threshold and flies must be more activity managed.

Measuring Fly Activity by Type of Fly

Horn Fly:  Because horn flies spend nearly their entire life on the animal, the best way to measure horn fly activity is to directly observe the number of flies resting on 5-10 randomly selected adult cows at your facility (counts on bulls and calves will be very different!).  It is best to count flies on cattle that are standing separately from other animals and counts are best made during mid-morning when temperatures are still cool and flies are clustered on the back of each animal.  Approach a single cow from one side and count the number of flies that can be seen on the back and side of the animal.  If you cannot approach closely enough to count flies with the naked eye before disturbing the animal, binoculars can be used from a greater distance. When fly numbers are high, you will likely need to estimate your fly counts by counting the number of groups of approximately 50 or 100 flies.  After counting flies on one side of the cow, quickly move to a position to view the other side of the cow and repeat the counting procedure.  Add together the counts from both sides.

An alternate fly monitoring technique is to count cow behaviors associated with high numbers of horn flies.  While observing a single cow, count the number of head tosses and tail flicks performed during 1 minute.  Observe 5-10 cows in this way.  More than a few of these behaviors within 1 minute is an indication of moderate-high fly numbers.  Counting animal behaviors may be less accurate for monitoring fly numbers than counting flies, but is considerably easier and may be a more accurate measure of animal stress which after all is what you really want to know!

Stable Fly:  Stable flies spend much of their life off the animal, but will move to animals to bite and feed on blood approximately once per day.  As for horn flies, the best way to measure stable fly activity is to directly observe stable fly activity during mid-morning on 15 cows standing apart from other cattle in the herd.  Because stable flies bite predominantly on the lower legs of cattle, approach a single cow from one side and count the number of flies that can be seen on the outside of the front leg facing you and the inside of the opposite front leg. Combine these numbers of flies into a single count to be recorded for each animal.  Count only flies oriented with their head pointed upward (toward the cow’s head) as this is the feeding posture for stable flies.  Other flies that may be on the cow’s leg will orient their body differently and should not be counted as stable flies.  When counted this way, an average of more than 5 stable flies per cow is expected to have economic costs.

Stable fly activity can also be determined by observing cow behavior, as the stable flies are very painful biters and cattle react to these bites quite strongly.  Typical cow behaviors caused by stable flies include leg stamps, tail flicks, and cattle bunching.  When cattle are noted to bunch or cluster within pens, and particularly if there is jostling of cattle to reach the center of the bunch, this is a clear indication that stable fly activity is too high.  Bunching cattle are not eating and drinking as they should.  When cattle are not bunching, stable fly activity can be determined by counting the number of leg stamps and tail flicks performed by a single cow.  Observe these behaviors for 15 cows, with a 2 minute observation period per cow.  More than a few leg stamps and more than 10 tail flicks in the 2 minute period is indicative of stable fly numbers resulting in economic costs.

Face Fly:  Although they do not feed on blood, face flies will visit cattle regularly to feed on eye and nose secretions.  Face flies can be observed on 10-15 cows at the same time that counts of horn flies or stable flies are performed on these animals.  Flies that appear to be feeding at the margin of the eyes or nose should be counted.  An average of more than 10 face flies per cow is associated with increased transmission of bovine pinkeye .  During feeding, face flies often scrape at the conjunctiva along the margin of cattle eyes, causing irritation to the eyes and excessive production of eye secretions.  When bringing cows in to the milking barn, the skin near the eyes should be checked for evidence of excessive eye secretions.  If excessive eye secretions are noted to have formed on many cows within a herd, face fly numbers are probably too high.

House Fly:  Unlike the other flies discussed, house flies cannot be monitored by observing cattle behaviors or counting flies on cows.  However, monitoring house fly numbers is very important as this species is known to transmit diseases to cattle (e.g., bovine pinkeye) and continue to increase in nuisance impacts to facility employees and neighbors.  House flies can be monitored using sticky traps or odor-baited traps that capture adult flies which encounter these traps.  A minimum of 5 traps should be deployed at locations where house flies have been observed to congregate.  Sticky traps, including sticky fly ribbons, will often catch many species of fly and the dairy manager must be able to identify the adult house fly from other species in order to make a proper count of house fly activity.  Odor-baited traps, including toxic fly baits, may also capture other fly species depending upon the trap and location where the trap is placed, and some odor-baited traps use a stinky liquid solution to attract and capture flies making identification of capture flies nearly impossible.

A better method to determine house fly activity relies on the behavior of flies to rest on facility walls or structural components.  Where flies tend to rest, they will leave numerous “spots” which are fecal and regurgitation spots.  This fly behavior can be taken advantage of by placing white 4” x 6” index cards at these locations allowing the flies to rest on these cards and leave their spots.  These “spot cards” are then replaced each week, with the number of spots on each card counted and recorded.  At least 10-15 cards per dairy provides a good estimation of changing fly activity.  After selecting spot card locations, these locations should continue to be used each week as the resting behavior of flies will be very different at other locations.  With some experience using this method, the dairy manager will be able to identify fly activity levels that exceed a treatment threshold.  Counting spots on these spot cards each week can be time consuming, but free down-loadable software recently developed by researchers at the University of California at Riverside is now available allowing users to scan spot cards using a flatbed scanner with fly spots counted automatically by the computer.

Download free “FlySpotter” Software to count fly spots on “Spot Cards”

House and stable flies breed in areas where moist organic matter is present. Common fly breeding sites on livestock operations include locations in and around (1) calf hutches, especially inside corners; (2) silo leak and spill areas; (3) animal stalls and pens, feed preparation, storage and manger areas, near water sources; (4) calf, hospital, and maternity areas; (5) water tanks; (6) feed troughs; (7) inside and outside manure handling areas.  Cornell University

Basics of Fly Control & IPM on the Dairy

After determining the fly species present and the fly burden on your dairy, your veterinarian or UC Cooperative Extension Advisor can help you determine which fly control methods will be most cost effective for your dairy. Since the 1940’s, fly management often relied primarily on insecticide use.  Reliance on this however promoted fly resistance to insecticides and inadvertently destroyed natural enemies of flies. Decades of research and experience have proven conclusively that, rather that depending on a single method of fly control such as pesticide spraying, a comprehensive Integrated Pest Management (IPM) program is far more efficient and cost-effective.  In studies on New York and Maryland dairies use of IPM practices cut fly activity in half using 80% fewer pesticide applications and reducing fly control costs from $7.66 to $6.17 per cow per year. See the bottom of the page for Cooperative Extension introductions to IPM on dairies. An excellent place to start are references from Texas A & M and University of Maine .

Below is a brief listing of some of the fly control methods available to dairy producers that may be included in their Integrated Pest Management program.

Sanitation (“Cultural Control”): Flies breed in manure and other moist decaying organic matter such as spilled feed or wet straw. The number of days that flies require to complete their development from egg to adult decreases as the average daily temperature increases.  Some fly species, like the house fly, can develop from egg to adult in as little as 7 days during summer . Because some 80% of a dairy’s fly population may exist in the pre-adult stages in wet organic matter, good sanitation is the most cost-effective way to control all types of flies. Without managing the source of the flies, it virtually impossible to control them using only pesticide sprays or natural predators. Eliminating moist decaying organic matter should be the corner-stone of every dairy’s fly control program. Appropriately graded pens, removal of old or spilled feed and repair of leaking water tanks are all examples of removing fly habitat. For many dairies the greatest reduction in flies may be realized by attention to the calf hutch, silage, and manure storage areas. Development in manure can be minimized either by allowing manure to dry before stacking or (if piled wet) ensuring that manure piles reach composting temperatures. An excellent list of sanitation control measures is available from Texas A&M University and the University of Maine.

For many dairies the greatest fly reduction will be achieved with attention to the manure storage, calf hutch and silage areas. Washington State University and California Agriculture

 Fly Baits & Traps: Target traps have proven effective at achieving some reduction in adult stable fly numbers in limited situations such as smaller dairies, zoo pens or dog kennels but larger dairies probably have too many flies to control efficiently using these traps as the sole means of fly control, and other fly species are not attracted to target traps. Fly baits may enhance house fly control but they do not attract stable flies or other problematic flies on the dairy. While traps/baits may capture great numbers of house flies they often do not substantially reduce the dense house fly populations that develop at livestock facilities.  Similarly electrocuting light traps attract and kill some flies, but these traps are only rarely useful around livestock operations, particularly as they are most attractive to insects at night when fly species of concern are not active.  Electrocution traps should never be used in milk barns or other areas where food products are handled or stored, as the electrocution shock can explode bits of fly body several feet from the trap.

Premise Sprays: While typically the least desirable method in a dairy’s Integrated Pest Management program, insecticides can be used to achieve some immediate reduction in total numbers of adult flies. Premise insecticide spays are generally categorized as either a “knockdown” or “residual” spray. Where and when sprays are applied will depend on their EPA label, the species of fly being targeted and any known resistance of flies to available insecticides. Federal law bans any sort of extra-label use of pesticides and it is essential that any pesticide be used exactly according to its EPA label. For instance although effective fly control is essential in dairy barns and milk-rooms, small amounts of pesticides can be detected in milk, and their presence is often illegal. A searchable database of pesticide products labeled for use in dairy cattle are available from  at the VetPestX website managed by the University of California at Riverside.

VetPestX website allows searches for pesticide products approved for use on dairies.

On-animal Pesticide Application: Pesticide and be applied directly to cattle through the use of sprays, pour-on products, back rubbers  and dust bag applicators. Horn flies are particularly susceptible to pesticide applications made to animals because horn flies must remain on animals for their entire adult life.  In fact, insecticide-treated cattle ear tags have provided good control of horn flies for the last 30 years.  However, other fly species spend relatively little time on their animal hosts and animal treatment may not provide significant management for these species.

Feed-Through Fly Control: Insect Growth Regulators (IGR) are fed to cattle and when excreted in the manure these chemicals prevent the pupa from developing into adults. Several IGR formulations including premixes, blocks and tubs are commercially available for dairy cattle.  Feed through products may not be appropriate for cattle on pasture as cattle may not consume sufficient quantities of the IGR, and the IGR may harm dung beetles that are needed to break down manure pats deposited onto pastures.

Organic Dairies: Organic dairies have particular challenges related to managing fly populations because they cannot use synthetic pesticide products as part of their total fly control programs. Currently, there is considerable research to evaluate the efficacy of organic-approved insect repellents that can be applied as sprays to cattle.  See below for links to resources specifically addressing fly control on organic dairies.

Predatory Insects: Biological control through releases of commercially available “natural enemies” that attack fly pupae is an appealing prospect. On California confinement dairies, most natural parasitism is done by wasps in the genus Spalangia.  Fly predators can be purchased commercially, with purchased predatory wasps typically a mix of wasp species that are known to occur through the United States.  Trials on commercial dairy operations to date have not demonstrated substantial reduction in fly activity when using commercially available fly predators, but in some situations (e.g., organic dairies) this may be a useful method of fly management to consider. If predatory insects are used on a conventional dairy it is essential to coordinate pesticide selection and application area to ensure that the commercially purchased insects are not inadvertently killed.

Additional On-Line Resources

University of California, Riverside Pest Management Website This comprehensive webpage developed through the University of California Cooperative Extension program a diverse compilation of resources including:

  • IMP information for particular pests,
  • Searchable database for registered pesticides in California,
  • Informational videos and documents,
  • Upcoming meetings and events,
  • Research funding opportunities,
  • Listing of other veterinary entomology websites
  • Contact information for California’s Cooperative Extension Entomologists

 Cornell University YouTube Video Series on Confined Dairy IPM This series of YouTube videos is an easy introduction to the concept of Integrated Pest Management (IPM) for confinement dairy facilities. Each video segment is approximately 11 to 15 minutes long.    

Part 1A Need and effectiveness of IMP, growing pesticide resistance.

Part 1B Use of baited jugs, sticky taps, spot cards, counting flies on animals sanitation beneficial insects.

Part 1C Best practices for commercial predatory insects & pesticide use.

Part 2 Types of flies on confinement dairies their biology & life cycle.

Part 3 Prime fly breeding sites, introduction to monitoring fly burden.

Part 4  Overview of types of IMP control, especially sanitation, traps & predators.

Part 5 Control with parasitic wasps, introduction to pesticide use and rotation.

Part 6 Managing pesticide resistance, does IPM pay for itself?

 Cooperative Extension References on Dairy Fly Control & Integrated Pest Management (IPM) A number of Universities have produced excellent introductions to IPM on dairies. See at the bottom of the page for Cooperative Extension resources:

University of CaliforniaTexas A&MCornell University

University of MaineUniversity of Nebraska

 Organic Dairies and Fly Management  Organic dairies have particular challenges related to managing fly populations without using chemical pesticides as part of their total fly control programs. Below are links to resources specifically addressing fly control on organic dairies.

YouTube Northeast Organic Fly Management Symposium (2:23 hour video)

Organic Agriculture Center of Canada

University of Arkansas

University of Minnesota

Northeast Organic Dairy Producers Alliance

Pennsylvania State University Webinar  Flies and Smells a Management Challenge  This webpage developed through the Pennsylvania State University includes a variety of resources including:

  • The archived webinar and accompanying slides
  • The extension publication Pest Management Recommendations for Dairy Cattle
  • The extension publication Fly Control in and around Cattle Barns and Pastures

National Cooperative Extension Website

Dairy Fly Control Articles on eXtension “eXtension” is the largest Cooperative Extension research-based learning network in the nation. A quick search for “dairy fly control” will yield dozens of papers, links and recorded webinars that can be useful.