The Bionomics of Ixodes ricinus L., The “Sheep Tick” of Scotland

Extract

Life history of Ixodes ricinus.

1. Under laboratory conditions, at 60–70° F. the pre-oviposition period was usually found to last 15–22 days.

2. Eggs began to hatch after about 4 weeks at 60–70° F. and after 37 weeks at 50–60° F. in the laboratory.

3. Unfed larvae placed on a hedgehog mostly dropped off gorged in 88–120 hours (shortest feeding period 67½ hours). Larvae gorged on sheep in 65 hours.

4. From larvae gorged in August and kept in the laboratory, nymphs emerged after 6–7 weeks. From larvae gorged in September and October, nymphs emerged after 28–37 weeks.

5. Unfed nymphs gorged in 4–7 days, mostly in 5 days.

6. From nymphs gorged on sheep in spring and summer, and kept in the laboratory at 60–70° F., adults emerged after 10–19 weeks.

7. Females gorged on sheep in 7–11 days, mostly in 8–9 days.

8. Sex ratio: about 5: 9 (basis: 362 males and 671 females emerged from nymphs in the laboratory).

9. Longevity unfed: males and females survived 21 months; one male survived for 31 months in the laboratory. Larvae survived 2 years; some were able to feed after fasting 15 months, and nymphs after fasting 13 months.

Bionomics.

10. Hosts: one or more stages of I. ricinus have been found on the following hosts. Mammals: horse, dog, fox, hedgehog, hare, rabbit, stoat. Birds: hoodie-crow, rook, jackdaw, starling, golden plover, barn owl, kestrel hawk, and merlin hawk.

11. When Blackface and Cheviot sheep were kept on the same pastures in spring, the former became more heavily infested with ticks than the latter. It is said by shepherds that ewes weakened by lambing are more liable to infestation than strong barren ewes. Comparative counts of the infestations of a barren ewe and two ewes with lamb appear to confirm the shepherds' view. Non-acclimatised (experimental) sheep, when exposed on a tick infested farm, rapidly. lost condition at first, and subsequently began to recover. When these sheep, whose health and general condition were low, were compared with native sheep their average infestation with ticks was found to be higher than that of the natives. As the health and weight of the experimental sheep improved, the average infestations of these and of the native stock approximated.

12. Reports on the geographical distribution of I. ricinus in various districts in Scotland, received from sheep farmers and others, are recorded.

13. There is a distinct seasonal periodicity in the activity of I. ricinus. It appeared on sheep early in March, and became increasingly numerous during April-May. In June the numbers began to decrease, and steadily diminished until late August, when there occurred a slight increase in average infestation of sheep. This increase was of short duration and by the middle of November no ticks were found on sheep. The same periodicity was observed in the following spring and summer. An occasional tick was found on sheep during the winter, in mild warm weather. Adult ticks first appeared in early March. Nymphs appeared by the middle of March. Larvae were apparently most numerous in May–June.

14. The seasonal incidence of I. ricinus on sheep in 1930 bore no apparent relation to rainfall.

15. When the variations in average weekly air temperature for 1930–31 are charted in relation to the seasonal incidence of ticks on sheep for that period and the curves compared, there seems to be a correlation between temperature and tick activity. The data indicate that the tick infests sheep in the greatest numbers at 45–60° F. The lower critical temperature for tick activity in nature appears to be 40–45° F., and the higher 60–70° F.

16. Over a thousand gorged nymphs and many larvae have been collected in the field and bred out in the laboratory; in none of them has there been any evidence of hyperparasitism.