The ecology of Lice on sheep. 4. The establishment of maintenance of populations of Linognathus ovillus (Neumann) the ecology of Lice on sheep.

Abstract

Linognathus ovillus has been found on all regions of the sheep except the lower aspects of the limbs, but populations are maintained throughout the year only on the parts of the body covered with hair, particularly the face. The inability of L. ovillus to multiply below a constant temperature of 30°C, or to survive prolonged exposure to cold temperatures, probably prevents maintenance of populations on the lower parts of the legs. Only adult lice were transferred from sheep to sheep, and fewer were required to establish a population on the hairy face than on the parts of the body covered with wool. The greasiness and length of the wool caused females to lay eggs less densely, required a greater density of males and females for fertilization, and resulted in a greater mortality of nymphal and adult lice. Lice disperse continually from the face into the surrounding wool, and the consequent density of lice in this wool frequently becomes sufficient for the population to maintain itself, thus resulting in a swarm of lice in that area. Shearing removes few lice from the parts of the body covered with hair, but many from those parts covered with wool. Thus, whereas the density of lice on the face is little affected by shearing, that on the body is reduced considerably. When a sheep is exposed to atmospheric temperatures of c. 28°C the temperature next to the skin rises to over 38.5°C. Few eggs develop and hatch at this temperature. These two factors probably account for the disappearance of L. ovillus from the body after shearing in the spring, and during the following summer. The occurrence of lethal temperatures within the fleece during the summer may also be a contributory factor. Observation of an infested flock over a period of a year showed that the sheep which were most heavily infested in the autumn were also the most heavily infested in the subsequent spring. This indicates that the size of a louse population on a sheep in the spring may frequently be directly dependent on its size at the commencement of the previous winter.