Horn flies and face flies are the two most common flies that bother cattle in pasture settings. From an economic standpoint, horn flies cause the most damage. Research indicates that a good horn fly control program can result in 12 to 20 pounds of additional weight gain for calves as well as reduced weight loss for nursing cows. The economic threshold for horn flies is generally considered as equal to or more than 100 flies/side or 200 per animal. Face flies do not cause the same type of economic damage and no economic threshold number is available, but they can disrupt grazing and contribute to the spread of pinkeye.
Common control methods include dust bags, back rubbers, insecticide-impregnated ear tags, livestock sprays and pour-on insecticides. A successful fly control program may incorporate one or more of these methods. Dust bags are only effective in situations where cattle must pass under them on a regular basis as they access food and water. The height of the bottom of the bag should force cattle to lift the bag with their head to pass under it. Lifting with the head provides treatment of face flies. As the bag drags along the back of the animal, horn and other biting flies are treated.
Probably the most common and familiar control method is use of insecticide ear tags. These tags provide good control of horn flies and possibly some reduction in face fly numbers when used correctly and when an effective insecticide is available. The primary issue with insecticide ear tags is horn fly resistance to the insecticides impregnated in the tags, especially the pyrethroid insecticides. Insecticides commonly used in ear tags include pyrethroids, organophosphates, abamectin and combination tags that contain both a pyrethroid and an organophosphate. Insecticide ear tags labeled for dairy cattle are limited to the pyrethroids.
The first key to good control includes knowing whether pyrethroid resistance is present and choosing a tag accordingly. Other keys are: placing tags in animals when fly populations begin to build and approach the economic threshold, follow label directions regarding the number of tags required for control (some require 2 tags/animal, others only 1), and remove the tags at the end of the fly season or before sending an animal to slaughter. Always read the label of tags before using. There may be limits for calves on some product labels that prohibit use for calves under 3 months of age or limit the use to only one tag. Remember, these are insecticide impregnated ear tags. Wear protective gloves when applying and removing these tags.
Back rubbers and oilers are similar to dust bags in that they work best when cattle have to come to them and use them to get access to minerals, water or a new pasture paddock. These devices combine an insecticide with diesel fuel, Number 2 heating oil, or a label recommended mineral oil as a carrier. In general, one gallon of oil solution per 15 to 20 feet of back rubber cable is sufficient for 30 to 40 head of cattle. Do NOT use waste oil or motor oil as the carrier. To increase the effectiveness of oilers against face flies hang strips of cloth at 4 to 6 inch intervals along the length of the device. Check and service back rubbers and oilers weekly. The insecticides labeled for use with dairy cattle is a more restrictive list as compared to labels for beef cattle.
Livestock sprays are available and labeled for both dairy cattle and beef cattle, but again the list of products labeled for dairy cattle is more restrictive. The challenge of using livestock sprays is getting good coverage of each animal and applying the insecticide so that no feed or water is contaminated. Pour-on insecticides are applied along the back line of the animal can also provide effective control of biting flies. Once again, there are more product options for beef cattle than dairy cattle. Some of the beef cattle labeled materials have significant slaughter intervals of greater than 40 days.
Several times in this article, I mentioned the differences in labeled materials for dairy versus beef cattle. Lee Townsend, Extension Entomologist at the University of Kentucky annually updates a publication entitled “Insect Control for Beef Cattle” and another publication entitled “Insect Control on Dairy Cattle” that contains current labeled insecticides. Those publications are available on-line at https://entomology.ca.uky.edu/categories/pests-cattle or contact me at the Wayne County Extension office at 330-264-8722 and I can send one to you.
One other “control” method not commonly mentioned is genetic resistance. There are some beef cattle producers rating cattle and selecting bulls, at least partially, based on a rating score for low fly numbers compared to peer bulls. One example of this that I am aware of is the Pharo Cattle Company in Colorado. Some food for thought.