Diary cows grazing
Updated: June 11, 2021
By Sarah Potts

Stepping up your Fly Control Program

Summer is here and with that comes, what sometimes seems, a never-ending battle with flies.  Flies can pose a significant problem for all animals on the farm.  Following a (usually much welcomed) bout of wet summer weather, fly populations sometimes seem to explode.  There are three major types of flies with economic significance to the U.S. dairy industry: horn flies, face flies, and stable flies.  Control measures vary somewhat depending on which type of fly is causing issues.  Below are four steps you can take to help reduce the fly load for your animals.

Identify the pest

  • Horn Flies
    The horn fly is a biting fly that causes significant irritation, particularly to animals on pasture. The horn fly can cause avoidance behaviors, such as bunching, that reduce feed intake and increase energy expenditure, which can negatively affect production.  The horn fly is typically found on the back or topline of the animal during the cooler morning hours and on the underside or belly during the warmer afternoon hours.  
  • Face Flies
    The face fly is a non-biting fly that also impacts pastured animals. This fly feeds on bodily secretions found around the eyes, nose, and mouth, causing significant irritation.  The face fly also serves as a vector for bovine keratoconjunctivitis (pinkeye) and infectious bovine rhinotracheitis (IBR).  Pinkeye is a highly contagious bacterial infection of the eye and IBR is a severe viral respiratory infection.  Both conditions cause reductions in productivity.  Controlling face fly numbers, in addition to vaccination and quarantine protocols, can help prevent widespread pinkeye and IBR infections in the herd.  
  • Stable Flies
    The stable fly, like the horn fly, is also a biting fly that causes irritation to the animal. These flies tend to accumulate on the front legs of cattle and provoke avoidance behaviors, such as foot stomping and tail switching, and can negatively impact feed intake.


Economically important flies for livestock include horn flies, face flies, stable flies, and house flies.

Fly populations can be managed using an integrated approach.
  • Weekly cleaning and sanitation is key to remove fly breeding grounds.
  • Supplementary control measures:
    • Baits, tapes, and traps for house flies and stable flies.
    • Self-application, controlled-release, or topical insecticides for horn flies and face flies.


  • House Flies
    Although it does not bite like stable flies and horn flies, house flies are a major annoyance to animals. These flies also pose a disease risk because they can serve as vectors for mastitis, pinkeye, E. coli, salmonella, and others.

Determine the severity of the problem

Simple observation of your animals can help you determine if there is a significant problem with flies. The “economic injury level” is defined as the fly load at which production begins to be negatively affected. 

To determine if there is a horn fly problem, focus on examining the fly load on the backs of animals during the mid-morning hours before the flies retreat to the underside in the heat of the day. Count the flies on at least 10 to 15 different animals. The economic injury level for horn flies is 100 or more flies per animal.  

Face flies are much more obvious to detect, as they congregate around an animal’s face. As with stable flies, count the number of face flies around the eyes, ears, nose, and mouth for at least 10 to 15 different animals to determine if there is a problem. The economic injury level for face flies is 10-15 flies per face. 

As previously mentioned, stable flies generally prefer the legs of animals so if you see flies accumulating around the legs (often the front legs), especially in the cooler morning hours, you likely are seeing stable flies. The economic injury level for stable flies is 5 flies per leg.

Choose an integrated treatment approach

While the initial reaction may be to reach for the fly spray, it is important to recognize that fly sprays and other insecticide treatments will not provide adequate long-term solutions for controlling flies. To be effective, a good fly control program must utilize an integrated approach.

The cornerstone of an effective fly-control program is sanitation. Cleaning up organic material, which provides an ideal environment for flies to reproduce, interrupts the fly life-cycle and helps keep numbers down. Most flies, especially stable flies and house flies, prefer to lay their eggs in decaying organic matter, such as manure, feed, and bedding. Though cleaning is usually not on the top of everyone’s summertime to-do list, try to make a point to remove this material at least weekly during fly season.  Focus on places where animals congregate, such as the feed bunk and waterers. For cows and heifers on pasture, remove manure and old feed from around any sacrifice or feeding areas.  For calf barns, clean up any spilled milk, sweet-feed, and milk powder. Store opened bags of milk replacer in airtight containers to keep the flies out.  In addition to organic matter removal, try to keep these areas as dry as possible by ensuring proper drainage and fixing any water leaks. 

Other tools used to help control flies include sticky tapes, traps, parasitic wasps, oral larvicides, and topical insecticides. Sticky tapes and baits work well for house flies, which usually do not spend a large amount of time physically on animals. For maximum effect, sticky tapes should be strategically placed at fly-entry points, such as doorways and windows and changed every few days. Parasitic wasps can also be used to help control stable and house fly populations. These insects prey on the developing fly pupae, which reduces the number of fly larvae that reach maturity. However, conditions must be right in order to promote proliferation of these natural fly predators in order for them to provide effective control. Oral larvicides are insecticides that are consumed daily in the feed and pass into the manure. Once in the manure, the larvicide it will kill developing fly larvae for a period of time.

Topical insecticide treatments are most effective against face flies and horn flies. Regardless of the insecticide selected, be sure to carefully read the label before use to ensure proper administration.  Not all products are approved for use in lactating cows. Topical insecticides can be applied through self-application devices (rubs, dust bags), controlled release methods (fly tags), and direct application (pour-on, sprays). Self-application devices are most effective when incorporated into a forced-use system, where animals must come into contact with the insecticide in order to obtain water, feed, or shade. Fly tags can be useful for animals on pasture, but it is important to use them properly to help prevent the development of resistance. Fly tags should only be used when indicated by the economic injury level (10-15 face flies per face or over 100 horn flies per animal) and should be removed at the end of the fly season. Pour-on and animal spray insecticides can also be useful. However these only work to temporarily reduce fly populations and must be reapplied every few weeks throughout the season.  

Evaluate and reassess regularly

As with any intervention program, it is always a good idea to assess progress to ensure that what you’re doing is working. Check your animals often (at least every other week) to ensure you have adequate control (i.e., fly numbers are below their economic injury levels). If you see fly populations starting to creep up, it’s time to revisit your fly control program and see where adjustments can be made. Perhaps some fine-tuning is all that is needed.  Or, perhaps, it’s time to try a new approach to see if it will work for your farm.

This article appears on June 8, 2021, Volume 2, Issue 2 of the Maryland Milk Moo's newsletter.

Maryland Milk Moo's, June 8, 2021, Vol.2, Issue 2

Maryland Milk Moos is a quarterly newsletter published by the University of Maryland Extension that focuses on dairy topics related to Nutrition and Production, Herd Management, and Forage Production. To subscribe to this newsletter, click the button below to enter your contact information.