In contrast to the fly pests, lice are relatively small and inconspicuous. Four species of lice attack dairy cattle in the Northeast. By far the most common is the cattle chewing louse, Bovicola bovis. This species is about 1/8 inch long when fully grown, has a yellow-brown appearance, and is most commonly found on the animal’s neck, back, hips, and tailhead. B. bovis are not blood feeders, but they use their mouthparts to rasp away at animal skin and hair.
In addition to chewing lice, three species of sucking lice feed on the blood of dairy cattle: the long-nosed cattle louse (Linognathus vituli), the short-nosed cattle louse (Haematopinus eurysternus), and the little blue louse (Solenopotes capillatus). Sucking lice have mouthparts specialized for penetrating animal skin. They spend most of their time with their heads firmly attached to the skin. Sucking lice often take on a darker appearance than chewing lice as they become engorged with blood.
Female lice lay their eggs by attaching them to hairs with a strong glue to prevent them from falling off. The eggs, known as nits, hatch in 10 to 14 days, and the young lice (nymphs) complete their development within several weeks. Lice, in contrast to some other livestock pests, are permanent parasites that spend their entire lives on the host animal. All four types of lice cause extreme annoyance to the host animals. Milk production declines in heavily infested cattle, and the animals’ preoccupation with rubbing leads to hair loss, reduced feed conversion efficiency, and general unthriftiness. Infested animals become irritable and difficult to work with, especially during milking. People working around lousy animals are exposed to greater risk of injury and are also annoyed by stray lice they pick up from infested animals during handling.
Although louse problems are generally perceived as being most severe during the fall and winter months, animals of different age groups show distinct differences in the seasonality of infestation. Lice are most common on mature cows in December through March, with peak populations found in March. In contrast, calves housed inside barns show high levels of infestation throughout the year, with peak populations in June. This difference may be due to the fact that cows are placed on pastures in the spring, where exposure to direct sunlight heats the skin to levels lethal for most lice. Calves kept in the cool environment of the barn are not able to take advantage of sunlight’s natural curative properties.
Other animal housing conditions also affect louse populations. Cows in stanchion barns are twice as likely to be infested as cows in free stalls, owing to the greater opportunities of unrestrained animals to groom themselves. Calves housed in communal pens inside barns are 10 times as likely to be infested as calves in individual hutches. The effectiveness of hutches results from a combination of the animals’ isolation from one another and the opportunity for calves in hutches to spend time in direct sunshine.
Because lice often are inconspicuous, many producers do not detect them until their cattle begin to show hair loss from the animals’ grooming activities. But by the time the infestation has progressed to this stage, populations of lice are already well above economic injury levels, and treatment becomes very difficult owing to the large numbers of lice involved. Effective management of cattle lice below economic injury levels requires sampling of apparently healthy as well as noticeably lousy animals for the presence and relative numbers of lice. Such surveillance should be conducted every 2 to 3 weeks throughout the fall, winter, and spring months.
Lice can be monitored easily with a flashlight and a little practice. Sampling involves carefully inspecting sections of skin on a representative sample of animals in the herd, either 10 percent or 15 animals in each group: mature cows, heifers, and calves. The best regions to inspect are the head, neck, shoulders, back, hips, and tail. If sampling indicates that B. bovis is the dominant species present, assessment of the neck and tailhead alone is sufficient to detect most infestations. Treatment is recommended when counts average over 10 lice per square inch.
Producers can save on the cost of insecticide treatments for lice by adopting cultural control practices. First, replacement animals brought into the herd should be isolated and carefully inspected for lice before they are allowed to mingle with the rest of the herd. Second, careful and regular monitoring for lice can detect problems before an infestation gets out of control. Third, housing calves in hutches will reduce infestations on these valuable replacement animals by 90 percent without any insecticide applications.
Many insecticides and application procedures are effective for managing lice. Insecticides registered for control of lice are listed in Table 4. As with any insecticide application, it is essential to consult the label to ensure the insecticide is registered for use on dairy cattle, and if so, whether it may be used on lactating animals. Before selecting an insecticide, consider how it can be applied to meet individual needs and production practices. There are several categories of application methods: self-application devices, whole-animal sprays, pour-ons, and dusts.
Self-application devices such as dust bags must be placed in areas where animals will contact them frequently and treat themselves with repeated, small doses. Whole-animal sprays have the advantage of ensuring good coverage over the entire animal’s body. But severe louse problems on mature animals are most common in winter, and it generally is wise to avoid soaking animals in periods of cold weather. Applications with foggers and mist blowers can overcome these problems. With these types of applications, a small quantity of concentrated pesticide is propelled as an aerosol made up of very small spray particles. The concentrated aerosol can then be applied evenly over the animal’s body, greatly reducing the amount of liquid used.
Another method of application is the use of pouron insecticides, in which a small quantity of pesticide is poured down the backline of the animal. The most popular application method for lice is dusting by hand. Dusts are easy to apply, require no mixing, and can be used year-round.
Insecticides must be used properly to achieve satisfactory control of lice. Many louse-control products require two treatments, 10 to 14 days apart. The second treatment is essential to kill newly hatched lice that were present as eggs at the time of the first treatment and were therefore not killed. Failure to make the second treatment in a timely manner will create problems requiring many more subsequent treatments.