Each summer, pastured cattle must deal with an annoying pest – the horn fly. These flies use their piercing-sucking mouthparts to take up to 30 blood meals from their host each day. This incessant feeding schedule and their large populations can severely impact growing calves and lactating cows.

Horn flies can also play a role in transmitting disease. In all, horn flies suck about $1 billion in weight gain/milk production losses and control costs each season.

The close relationship between cattle and horn flies can help with control. The flies leave the animal to lay eggs in fresh cow manure or to change hosts. Consequently, most application methods will expose flies to insecticide residues. These include forced-use dust bags or back rubbers, insecticidal ear tags, sprays and pour-on formulations.

One of the biggest challenges in horn fly control is rapid development of insecticide resistance and there are no clear-cut strategies that solve the problem.

Producers can do a several things to manage resistance:

· Do not treat for horn flies until numbers exceed 200 per animal. Cattle can tolerate up to this level before economic losses occur.

· If feasible, keep growing calves and lactating cows separated from mature stock. Fly reduction on growing and lactating animals is more likely to provide an economic return.

· Use periodic treatments with insecticides that have other modes of action (organophosphates, etc.) to break fly exposure to a single product group. Rotating products with different modes of action is a basic strategy that may reduce the potential for resistance.

· Remove ear tags in fall to reduce horn fly exposure to low concentrations of pyrethroids.

· Use a late-season application to reduce the number of horn flies that will enter the over-wintering stage on the farm.

If you don’t notice significant fly reduction within two weeks of applying tags, it’s a good indication resistant flies are present.

For more information about fly control, contact the Bell County Extension office.

Stacy White is the Bell County extension agent for agriculture and natural resources. Educational programs of the University of Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin. Source: Lee Townsend, UK extension entomologist