In three experiments, the chief hosts, sheep, were withheld from a grazing until 0–3 weeks after the normal spring peak of female tick activity. This did not give rise to any appreciable prolongation of female tick activity into the normal summer offseason. More than half the female ticks actually available during the entire spring season were unfed when activity came to an end. This was due to the withholding of hosts. Hence that aspect of the ‘two-brood’ theory which claims that activity ends in spring, because all the available ticks are fed, is untenable as the complete explanation of bimodal annual activity.

Increase in sheep-stocking density, over the normal for a grazing, lowers the average infestation per sheep for that season at least.

The experimental data suggests the following:

(i) Delay in stocking a grazing with sheep results in a slightly higher-than-normal population of active female ticks which is fairly quickly absorbed after reintroduction of sheep.

(ii) A varying proportion of the tick population becomes active each week throughout the tick season, increasing up to the peak and decreasing thereafter.

(iii) The average duration of individual activity in a natural population of female ticks is probably not more than 1 month, perhaps less.

Practical applications of the findings to hill sheep farming are indicated.