Horn Fly Impact
Know Your Enemy
Know Your Enemy
Horn flies are the most pervasive and costly external parasites of cattle in North America. They're small, black flies, approximately four millimeters long. Infestation levels can increase rapidly - up to 4,000 flies per animal in untreated herds. At their peak, horn flies remain on cattle throughout the day and night. They can generally be found on the backs of cattle, often clustering on the animals' midlines and spreading down their sides.
Know The Signs
Know The Signs: Horn Fly Symptoms
The adult horn fly is a biting insect that takes 20 to 40 blood meals per day. The resulting stress interrupts the grazing patterns of cattle, causing them to go off feed and expend energy in an attempt to dislodge the flies. According to university research, calves from badly infested herds gain weight more slowly than uninfested calves and are lighter at weaning by 10 to 25 lbs. If severe infestations are left untreated, cows can go out of condition during the critical breeding period. Horn flies can also hinder future milk production of heifers and cause cattle diseases such as beef mastitis.
Treatment Strategies
Horn Fly Treatment Strategies
While there are both mechanical (walk-through fly traps or dragging of pastures) and biological (predatory arthropods such as beetles and mites) horn fly control strategies, chemical control strategies have been the most widely used. Techniques for application include backrubbers, dust bags, sprays, dips, ear tags (which can cause resistance), pour-on formulations, controlled-release boluses, and feed-throughs. The proven choice and industry leader, however, is Altosid® IGR, a feed-through insect growth regulator.

Horn Fly Population Guide

How Many is Too Many?

An effective control program keeps horn fly levels to fewer than 200 flies per animal. This at-a-glance guide will help you easily evaluate the effect that horn flies currently have on your herd. It’s easiest to make the most accurate estimate in the morning, when horn flies are on the upper sides of the cows.

So Effective. And So Easy to Use. 

To achieve effective, cost effective horn fly control with Altosid® IGR, just follow these simple instructions:


To maximize beef cattle in a cow calf operation, begin feeding Altosid® IGR to your cattle before horn flies appear in the early spring. Continue to feed Altosid® IGR during the summer months and then into fall. One general rule is to feed 30 days before average daily daytime temperatures reach 65 degrees and 30 days after the first frost in the fall. We call it the 30/30 Program, a proven strategy for achieving economical horn fly control.


If mixing Altosid® IGR with free-choice feeds, such as mineral supplements, put out a five- to seven-day supply and protect it from the rain.


To ensure cattle are consuming enough of the feed-through fly control, use one feeder for every 15 to 20 animals.


Keep the horn fly control supplement accessible to cattle by placing the feeders in areas where cattle congregate, such as near watering holes or loafing areas.


Check feed consumption for a few days. Increase or decrease the number of feeders, or move them if necessary, to allow for proper consumption. When used as directed, Altosid® IGR alone usually provides excellent season-long horn fly control. If fly populations exceed acceptable levels due to migration from untreated neighboring herds, use an approved adulticide, such as Prolate/Lintox-HD™ Insecticidal Spray & Backrubber for Livestock to decrease the fly population.

Protect Cattle from Horn Flies with the Original Feed-Through IGR

With more than 40 years of proven success, Altosid® IGR isn’t merely a leader among the class of feed-through insect growth regulators (IGRs). It created the class.

Get Certified

Looking to learn more about fly contol?

For additional information on how you can protect your cattle from horn flies using Altosid® IGR and an integrated pest management (IPM) strategy, become certified in Centralized Fly Control.