Insect Growth Regulators & Larvicides – Options for Fly Control

By: Heather Smith Thomas

It helps to have several tools to combat the flies that negatively impact cattle. One strategy that can be effective under certain conditions is the use of insect growth regulator (IGR) hormones. These natural hormones guide development of immature stages of flies and other insects. If normal concentrations in the insects are upset, flies don’t progress to adult stage and die as maggots.

Dr. Lee Townsend, Extension Entomologist, University of Kentucky, says several companies make a feed-through product containing a synthetic version of IGR. “Levels of this juvenile hormone tell the fly maggot whether it should stay a maggot longer or become a pupae and then an adult. Feed-through products containing IGR pass through the animal’s digestive tract and end up in manure in small amounts—just enough to disrupt fly development,” he explains.

In order to be effective, there must be a certain amount added to manure. “If you are using a mineral supplement for administering this hormone, every animal must eat at least the specified amount, every day. If some animals are under-consuming the supplement, it won’t be as effective for controlling the flies,” says Townsend.

Success also depends on the fly situation. “If you have a small acreage with nearby neighbors who do not control their flies effectively, horn flies and face flies can come in from the neighbor’s place even if you are getting good control of maggots with a feed-through product on your own cattle. In this situation you need a supplemental fly control strategy such as an oiler or dust bag for cattle to use.” A multi-pronged attack usually works best, rather than trying to rely on one control method.

The feed-through insect growth regulators work best for horn flies. In a feedlot, you will still have problems with stable flies, and house flies, because they don’t breed in straight manure. “They lay their eggs in spilled feed, moist feed, old rotting hay or straw. If horn flies are the main problem, the feed-through growth regulator can work well. The face fly is a larger maggot and the dose may be too low to be effective. It may give some protection against face flies, but works best for horn fly control.” In a pasture situation where horn flies (rather than stable flies) are the biggest problem, with no other herds nearby, an IGR product may be helpful.

“Methoprene is the most widely used IGR for horn fly control and is available through a number of different brands. You can buy it as a premix in loose mineral or blocks—already mixed in and ready to go—or concentrates (supplements) to blend into feed rations,” says Townsend.

Current USDA research is looking at the possibility of using this same chemical around a feedlot to treat breeding sites (such as rotting organic matter) rather than dosing each animal. “I haven’t seen any products yet that are labelled for this use. It is still in the development phase. Maybe in the future it might be an option, to treat areas where stable flies and house flies are breeding,” Townsend says.

“Insect growth regulators tend to last several weeks, so you wouldn’t have to retreat these sites very often.” If you applied these products to breeding sites just before the peak of fly season, you could cut back the population immensely.

“These methoprene granules are already being used in mosquito control–dropped in standing water to halt mosquito development. Thus it’s just a matter of showing that it works in a livestock situation and seeing if a company wants to label it for that particular use,” says Townsend.

Since this is a hormone specific to insects, it would not adversely affect other creatures in the environment. “There would be no concern about it affecting fish or mammals. The dose makes a difference, as well. Small, thin-skinned maggots are more susceptible to absorbing that material than some of the other insects such as predators that are also in the manure, feeding on maggots. Thus it is fairly selective,” he explains.

Another type of product, often called a larvicide, interferes with chitin, which is one of the main components of the exoskeleton of the insect. “This product interferes with formation of the external skeleton. After the insect molts it is very soft until that external skeleton hardens. If it can’t harden, the insect will dry up and cannot survive.”. Larvicides helps control house flies, stable flies, face flies and horn flies. There are several brands, also used as feed-through products in cattle.

As with any pest control product, IGRs and larvicides work best as part of an integrated pest management program.