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by Lee Townsend, Extension Entomologist, and John Webb, Research Specialist

University of Kentucky College of Agriculture

Development of insecticide resistance in a pest, such as the horn fly, is a selection process similar to that used by cattlemen to improve herds. Animals with undesirable traits are carefully culled, the best stock are kept. By the repeated use of insecticides, producers can cull horn flies that are susceptible or easily controlled. Surviving or resistant flies are left to breed and produce resistant offspring. The result is a control failure. That is, the product that once gave good control no longer kills flies.

In Kentucky, unsatisfactory horn fly control, due to resistance, can be expected if ear tags containing the synthetic pyrethroid permethrin are used. No control problems have been seen in herds using tags containing other synthetic pyrethroids or organophosphate insecticides.

Researchers from Kentucky, several other states, and the USDA, have developed the following list of suggestions to follow in horn fly control programs.

Horn fly treatments are not automatically needed. Most researchers agree that an average of less than 200 horn flies per animal will not cause a measurable loss. Flies may never reach this level in some herds, so an insecticide application is not needed.

Begin treatment when there are an average of 200 or more 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.

Target lactating cows and growing calves. The greatest potential for loss to horn flies appears to be with lactating cows and growing calves.

Rotate synthetic pyrethroids with organophosphates so that a different class of insecticide is used each year. ENT-11, Insecticide Recommendations for Beef Cattle, provides information on products and application alternatives.

Remove insecticide ear tags in the fall before the first frost. This reduces the amount of time that flies are exposed to a product and allows the number of susceptible flies to increase. Change application methods regularly. Use dust bags, backrubbers, or sprays rather than relying continuously on ear tags. Continued use of an insecticide in a slow release form (ear tag) may eventually lead to resistance.

Rotation of organophosphate tags with synthetic pyrethroid tags is a strategy to prolong the effective use of synthetic pyrethroids and to decrease the chances for resistance to organophosphates.

Revised: 05/94

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.


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