ENTFACT-501: Managing Pyrethroid-Resistant Horn Flies  |  Download PDF

by Lee Townsend, Extension Entomologist, and John Webb, Research Specialist 
University of Kentucky College of Agriculture 

Development of insecticide resistance in horn fly populations is the result of a selection process similar to that used to improve herds.  Cattle producers can cull horn flies that are susceptible to a certain group of insecticides by using products with the same mode of action year after year.  Surviving or resistant flies are left to breed and produce resistant offspring.  As a result, products that once gave good control may no longer kill flies or may lose their effectiveness earlier in the season.  Insecticide resistance has become a problem in some areas of Kentucky, particularly with the use of insecticide ear tags containing active ingredients with the same mode of action – attacking the same site in the insect.  

Specific steps can be taken to manage resistance, including: 

1)  Target treatment to lactating cows and growing calves because they have the greatest potential for loss to horn flies and the greatest chance for a return from the cost of treatment. 

2) Rotate among insecticides with different modes of action.  ENT-11, Insecticide Recommendations for Beef Cattle, provides information on products and application alternatives. 

3)  Wait to treat until there is an average of 200 or more horn flies per animal. This may not occur until early to mid-June. Treating too early, especially with ear tags, may mean poor control in late summer when the flies are most abundant. 

4)  Use alternative insecticides and application methods late in the season to reduce the percentage of overwintering flies with resistance. 

5)  Remove insecticide ear tag as soon as horn fly numbers begin to decline in the fall.  This reduces the amount of time that flies are exposed to a product and allows the number of susceptible flies to increase late in the season. 

6)  Change application methods regularly. Use dust bags, back rubbers, pour-ons, or sprays rather than relying continuously on ear tags. Continued use of insecticides from the same class in a slow release form (ear tag) may lead to resistance. 

Revised: 01/11 

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication. 

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE! 

Images: University of Kentucky Entomology