Now Is The Winter Of Our Discontent: Well, At Least Where Cattle And Lice Are Concerned

Livestock Update, February 2001

Rod Youngman, Extension Entomologist, Virginia Tech

Winter is the time of year when several species of lice typically reach their peak activity on cattle in Virginia. Lice can occur on beef and dairy animals year round, but unlike most insects, lice infestations usually decline during the spring and summer months because warm sunny days quickly heat the animal's hide to temperatures lethal to lice. When outside conditions cool off during late fall and winter, lice populations begin to increase because temperatures on the hide surface become favorable for their development. It should be noted that low numbers of lice are able to survive the summer months by residing in the inner ear tips of many animals.

Lice Species: There are four species of lice which occur on cattle in Virginia. The longnosed cattle louse, shortnosed cattle louse, little blue cattle louse, and cattle biting louse. The first three species are referred to as "sucking lice" because they have piercing-sucking mouthparts and feed on blood. The cattle biting louse has chewing mouthparts and feeds on the outer layer of skin. All four species are capable of causing intense irritation and discomfort to the animal.

Life Cycle: Once present, lice spend their entire life cycle on the animal including mating and laying eggs. Although lice are relatively small insects, with adults reaching a maximum size of about 1/8th inch, they can easily be seen on the animal without aid of a magnifying lens. Lice eggs, or nits, are glued onto hair shafts and hatch in 10-14 days, with another 2-3 weeks required for lice to reach the adult stage. Knowledge of the louse life cycle is important to ensure proper insecticidal management of a lice infestation.

Damage Symptoms: Often the first indication that an animal is infested with lice is hair loss resulting from excessive rubbing activity against fence posts or other suitable objects. Because lice-infested animals spend so much time rubbing themselves in an attempt to reduce the irritation, milk production and weight gains can be adversely affected. Milk production in lice-infested dairy animals can be reduced up to 25%. Heavy lice infestations in beef cattle can dramatically decrease the rate of weight gain over a season. In general, lice-infested animals appear unthrifty and can be more difficult to work with.

Monitoring: Regular monitoring can help cattle producers identify potential lice infestations before they result in economic losses to their herds. According to New York Cooperative Extension recommendations for dairy cattle, lice inspections should be made every two weeks throughout the fall, winter, and spring months. When sampling a herd, select 10% (or up to 15 animals) each of mature cows, heifers, and calves. Use a flashlight to carefully inspect several square inches of skin surface. Be sure to systematically check several body regions including the tail head, hips, back, neck, and head. The tail head and neck tend to have higher lice populations than the other regions because the animals are not able to groom these regions effectively. When selecting animals to sample, it is important to choose both healthy and lousy-appearing animals. Once sampling is complete, calculate the average number of lice per square inch. Infestations of less than 3 lice/in2 are considered light, infestations of 3-10 lice/in2 are moderate, and infestations of more than 10 lice/in2 are heavy. Sampling should be continued at two week intervals for herds identified with light or moderate infestations. For animals or herds identified with heavy lice infestations, at least two insecticide applications, 14 days apart, are recommended. The first insecticide application kills all of the lice stages except the egg. Lice eggs have a thick shell which enables them to survive an insecticide application. When the second insecticide is applied 14 days later, enough time has passed for most of the lice eggs present at the time of the first application to have hatched, but not enough time for the resulting immature lice to reach the reproducing adult stage. Consequently, two properly-timed insecticide applications can greatly reduce, if not eradicate, a lice infestation on cattle.

Insecticide Selection: If monitoring has revealed the presence of a heavy lice infestation and insecticidal treatment is warranted, producers are encouraged to consult their county extension agent and refer to the Virginia Cooperative Extension 2000 Pest Management Guide for Field Crops (no. 456-016) for choice of suitable control materials. It is very important to review the label of any insecticide being considered for lice on cattle because many insecticides recommended for lice also have grubicidal activity. Cattle infested with cattle grubs (i.e., insect parasites which develop internally in cattle) should not be treated in Virginia with any grubicidal material from November 1 to February 1 to avoid the possibility of serious host-parasite reactions. Some products which have grubicidal activity include: Ivomec Eprinex pour-on (eprinomectin), Warbex (famphur 13.2%), Co-Ral (coumaphos), Spotton (fenthion 20%), Tiguvon (fenthion 3%), and Neguvon (trichlorfon). Other products which can be used at any time of year to control lice on cattle because of their lack of grubicidal activity are Atroban (permethrin 11%), Brute (permethrin 10%), Permectrin II (permethrin 10%), Ectiban EC (permethrin), and Saber Pour-On (lambda-cyhalothrin 1.0%). Remember, it is always good practice to read the label thoroughly and follow all instructions before using any pesticide product.

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