Dave Boxler, University of Nebraska Extension

Treating cattle for lice when it is convenient—usually during preconditioning and preg-checking—isn’t necessarily the most effective approach.

While late summer and early fall endectocide (drugs that kill both internal and external parasitic insects) treatment may work on most internal parasites and horn flies, lice may escape.

Nebraska cattle are usually infested with more than one species of lice, and calves, yearlings and older poor-conditioned cattle usually have the heaviest lice infestations. Heavy louse populations can lower milk production, stunt growth, cause hair loss, an unthrifty appearance and anemia.

University of Nebraska and other studies indicate heavy lice populations (more than 10 lice/inch2) may reduce calf weight gains by as much as 0.21 lb./day. These studies also indicate calves fed at a higher nutrition level had lower lice populations and were affected less severely by lice than calves fed a maintenance ration.

Timing of cattle lice control

Cattle lice are cold weather insects, thriving during wintery conditions. During summer months cattle lice undergo a period of dormancy called estivation, when their reproduction is reduced significantly. Temperatures above 78° F in September, October and November will suppress louse development, because the cattle skin temperature will exceed 100° F.

If endectocide treatments are applied too early during a warm extended fall, like in 2021 and 2022, lice will develop slowly and can largely escape the endectocide treatment.

Later, when cold weather does set in, lice outbreaks can occur if the endectocide has broken down. Livestock producers who use a fall treatment strategy should monitor their cattle for signs of lice, especially during December, January, and February, and consider re-treating. However, using the wrong products at the wrong time can do more harm than good.

Cattle lice treatment options

Cattle lice treatment products fall into several categories: animal sprays, non-systemic (contact) pour-on, and endectocides (systemic pour-on, absorbed internally and systemic injectable). Some non-systemic pour-ons require just one application and some require two applications spaced 14 days apart. Systemic injectables work better on the three species of sucking lice than on the little red chewing louse. A systemic pour-on can effectively kill both chewing and sucking lice.

Using systemic control products between Nov. 1 and Feb. 1 is not advised as they may cause a host-parasite reaction from killing developing cattle grubs while they are in the esophagus or spinal canal of the animal. A systemic product used during fall weaning will not be a problem, and if a systemic product was used in the fall, a follow-up from November to February shouldn’t cause those issues, if it’s necessary.

Producers who did not use a systemic control product during fall weaning should consider using only non-systemic control products during this November to February time frame. If replacement animals are brought into a herd during the winter months, they should be examined for lice. If present, the animals should be isolated and treated before introduction into the existing herd.

Insecticide treatments, regardless of application method, should be rotated through insecticide Mode of Action groups (MoA) as outlined by the Insecticide Resistance Action Committee, to reduce the likelihood of developing resistance. The IRAC website offers an app to download or a web-based database that outlines which insecticides are in which groups. Since there are 36 different groups, it’s best to check the classifications prior to investing in an insecticide strategy.

Continual use of products from a single numbered group against a pest species can lead to reduced control (resistance to all products in the group). The recommended management practice to manage resistance is to alternate insecticide classes, and that applies to animal sprays, dusts, non-systemic pour-ons, and endectocides (injectable and pour-on formulations).

To reduce control failures due to insecticide resistance, do not apply pesticides within the same group number repeatedly, and always follow label directions. Insecticides and endectocides are an investment of time, money and cattle stress, and there are real costs to insect infestations. Putting a few extra minutes into correctly applying these tools can make all the difference between an effective and ineffective insect control program.

When applying any insecticide control product, please read and follow label directions.