The ecology of the sheep tick, Ixodes ricinus L. The infestations of hill sheep

Abstract

In general, female ticks infest the hairy areas of sheep: head and legs; the (comparatively) bare areas: axillary and inguinal; and certain woolled areas: chiefly the short-woolled margins of the hairy and bare areas; they do not attach in the long wool of withers, back, rump and flanks.More female ticks attach on the head than on the axillary region, and more on the axillary than on the inguinal region, obviously on the principle: first come best served.Different tick stages tend to attach in different zones of the same regions; two theories are put forward to account for this.On a hill pasture where the main tick activity occurs in spring only, the average totals of female ticks carried per annum by ewe, hogg and lamb were in the proportions 8:6: 1. Host size is thought to be the chief governing factor.Under existing British hill pastoral conditions, the upper limits to female tick infestation appear to be of the following order: average spring peak total count per ewe reaches 100–150, with extreme individual limit about 300; and about five times as many ticks are carried per ewe over the entire spring season.On heavily infested hill pasture, stocked at one sheep to 1–5 acres, typical flocks feed from 100 to 600 female ticks per acre per annum. The densest sheep stocking results in the highest tick density.For every one female tick, about eight nymphs were fed by sheep in one season.The better the condition of a sheep, the less bare of wool and apparently (to touch and sight only) the more greasy, and the lower the tick infestation.In any age group of sheep on one grazing, there is considerable range in weekly infestation weights but individuals tend to keep their relative positions consistently in this range. Sheep differing markedly in their infestation weights show a remarkable lack of difference in the majority of physical and physiological characteristics of body form and body surface. (Fleece humidity was omitted from study.) Notably, there is no indication of significant differences in fleece greasiness. Apparently only two characteristics go with the lighter infestations in Blackfaced sheep. These are coarser fleece hair and less clearance between fleece tips and ground, having doubtful and minor effects respectively.The most important factor in individual differences of infestation within an age group is very probably individual activity, i.e. the more ground covered in unit time, the more ticks picked up.