Ecology of lice on sheep. VII. Population dynamics of Damalinia ovis (Schrank)

Abstract

The numbers of D. ovis decline in the spring, remain low during the summer, and increase during the winter. A density of two lice per square inch is necessary in the autumn if numbers are to increase during the winter to 200 per square inch, the density found on heavily infested sheep in the early spring. The time needed for this increase has been calculated using the deterministic matrix model ofpopulation increase described by Leslie (1945), and 4-5 months are required when there is only a small mortality. Thus, when the favourable winter period lasts only 3 months, the increase in louse numbers may be insufficient to sustain the losses which occur in late spring, summer, and early autumn. The factors responsible for these losses determine the density of the louse population at the commencement of winter. Should the winter period last c. 6 months there is usually a direct relationship between the number of lice on a sheep in the late autumn and in the following spring. However, should winter last 9 months it is suggested that other factors become increasingly significant, and determine the eventual size of the louse population.