A. Milne

The seasonal and diurnal activities of individual sheep ticks (Ixodes ricinus L.)

1. Activity in Ixodes ricinus was studied by laying down newly emerged ticks in natural clumps of vegetation from which all hosts (with the possible exception of ‘mice’) were excluded. After a period of quiescence the ticks climbed to the ‘active’ position at the vegetation tips. Activity was assessed either by simple observation (in adults which were marked individually) or by ‘brushing’ the vegetation with the hands (nymphs and larvae). 52 % of the adults and 44 % of the nymphs were recovered. 2. The seasonal activity behaviour was followed in three series of ticks set out during May, July and October 1945. The results were used in interpreting the time relations of seasonal activity in ‘wild’ populations. The timing of the activity cycle appears to be largely determined by the availability of unfed ticks rather than by the prevailing meteorological conditions. 3. The diurnal pattern of behaviour in active ticks was closely examined during a 24 hr. period. Many remained at the tips without moving. The others began or ended a phase of activity by night or by day; but among these, the proportion ending a phase at night was significantly greater. This is the main reason for the previously reported diurnal fluctuation of activity in a tick population. 4. Two hundred and seven adult ticks spent, on an average, a total of 9 days at the vegetation tips (individual limits 1 and 54 days). This activity was spread over an ‘active period’ (interval between first and last appearances) averaging 30 days. Very active ticks rarely remained continuously at the tips. The mean number of visits to the tips was 4 (limits 1 and 19), each lasting for an average of 2·5 days. Where long spells of activity alternate with short periods of quiescence, the behaviour is probably regulated by the humidity reaction, the quiescent phase providing an opportunity for restoring the depleted water balance. 5. Newly moulted adult ticks possess reserves of fat adequate for many months of quiescence. During activity these reserves are exhausted in a few weeks or even days. Duration of survival mainly depended on whether the onset of activity was immediate or delayed. Some adults remained quiescent for 1 year before becoming active for the first time. 6. Several types of orientations were observed in the field. In attaining the position at the tips favourable for encountering a host, the gravity response (upward-turning near the tip) is of major importance. The tick avoids wind and direct sunlight by sheltering behind its supporting stem. In sensing the approach of a ‘host’ (the observers finger) the perception of eddies of warm air is particularly significant. The response (questing, then orientation) is elicited much less readily if the stimulus is applied from the leeward.

The ecology of the sheep tick, Ixodes ricinus L. Microhabitat economy of the adult tick.

Since it is concerned with filling in certain details of a picture, long since sketched in outline, the new work in this paper permits only a somewhat disjointed summary. From a study of Ixodes ricinus L. in nature, the main new facts emerging are: (I) Unfed adults in a deep vegetation layer such as presented by rough hill or moorland pasture dominated by bents, heather or bracken. (i) Practically all inactive unfed ticks are in the underlying mat, and nearly all of these in the upper portion of the mat. This distribution does not change, summer or winter. (ii) When the tick is active its progression is practically confined to the vertical, i.e. between its niche in the mat and the vegetation tips immediately above. Random undirected movements due to certain circumstances may achieve a 0–8 in. (average 2 in.) change of position in the horizontal plane; but there is apparently no ruling urge to move in the horizontal plane. (iii) The unfed tick comes to the vegetation tips to await a host for only a limited time during the 3-month activity season. Commonly this amounts to about five periods of 4–5 days each if no host is forthcoming. Between the periods of activity it returns to the upper mat. If it stayed constantly at the tips its survival time would be much shorter than normal. (iv) From nymphs engorged in spring, adults emerge in autumn but remain inactive for a considerable period. About half of them may perish within the first 100 days after emergence. Survivors become active during the following March–June. Effective life ends when the tick, if still unfed, finally stops coming to the vegetation tips, the only position from which a host can normally be achieved. After emergence, the effective life of the unfed adult deprived of a host is less than one year; and the actual life is not much more than one year at best. Thus adults not finding a host in spring will be ineffective, i.e. die without reproducing before the next activity season.

The ecology of the sheep tick, Ixodes ricinus L.; spatial distribution.

Spatial distribution is understood to connote distribution of density (including zero density) over the land surface at a given time. Combining fact and conjecture, the following is a theory of spatial distribution of Ixodes ricinus in Britain. The tick passes practically the whole of its 3-year life span on the ground, spending a total of only 3 weeks on hosts. A considerable amount of vegetational cover is necessary for efficient survival on the ground; the more important limiting circumstances in lesser cover seem to be humidity (conditioned by temperature) in summer, perhaps temperature (alone) in winter, and the activity of predators. A host is necessary for mating as well as food purposes. Though small at best, the ticks chances of achieving a host are greater the more hosts on the ground. In practice this means the more sheep, since the latter are by far the main hosts. For the most part, spatial distribution to-day can be explained adequately in terms of interplay between amount of ground-cover and of host-potential, with cover the master factor. Thus poor cover keeps the heavily stocked lowland pastures free of permanent tick population even when opportunity of colonization is offered. The tick is confined to the hills and moorlands with their deep vegetation layers. There, within limits, it is more numerous on a pasture the more universal the distribution of adequate cover (because it drops at random from the host) and the higher the host-potential. Among hill and moorland pastures, however, there are some cases where, despite ample cover and hostpotential, either (1) no ticks are present, or (2) the tick population is lower than it ought to be. These cases can be explained on grounds of lack of opportunity for colonization (1); or colonization so recent that population has not attained the level of ‘dynamic equilibrium’ with the environment (2). The question of ‘spread’ (colonization) is discussed fully in the text. The sheep flock is the main agent in spreading ticks. Spread is comparatively slow and relatively small in extent nowadays. There has very probably been considerable increase both in tick population densities and in infested areas since the advent of intensive hill and moorland grazing with farm stock some centuries ago.

The ecology of the sheep tick, Ixodes ricinus L. Distribution of the tick in relation to geology, soil and vegetation in northern England

The present findings apply to a particular area, the four northern counties of England, and must be viewed within the limits of the data presented. Only one tick, Ixodes ricinus L., the so-called sheep tick, infests sheep and cattle in northern England. I. canisuga Johnston, alone or with I. ricinus , has been found on sheep dogs and foxes. The distribution of the sheep tick in this region is markedly discontinuous, the total infested areas including only about one-fifth of the hilly country, to which the tick is almost exclusively confined. In north-west Northumberland, tick distribution shows a significant degree of correlation with certain factors inherent in or influenced by the surface geology. Broadly speaking, where surface geology lends itself to relatively good natural drainage and soil, the grazing is relatively good and ticks are absent; and where surface geology results in relatively poor natural drainage and/or soil, the grazing is relatively poor (‘rough’) and ticks are present. No consistent correlations between soil characteristics—i.e. p H, available phosphates and potash, soil textures, mechanical analysis and soil depth—and tick distribution have been found. Nor is there a consistent correlation- between quality of natural drainage and tick distribution. The interaction of soil and natural drainage factors, however, profoundly influences the character of the vegetation layer. In the four northern counties, the same plant dominants occur on tick-infested and tick-free hill lands. Where grazing is rough ticks are usually present irrespective of whether the dominant plant is one of the rough grasses ( Nardus, Molinia, Agrostis, Aira ), bracken or heather; where grazing is relatively good (i.e. not rough), ticks are invariably absent. Observations on a smaller scale show that the thicker, i.e. rougher, the vegetation layer, the denser the tick population and also the thicker the vegetation layer, the thicker the basal mat. On five plots it has been shown that there is a consistent positive correlation between mat thickness and tick population density: the thicker the mat,’ the denser the tick population irrespective of whether bracken or grass is the dominant. Thus the chief controlling factor in tick distribution is the physical character of the vegetation layer.

The ecology of the sheep tick, Ixodes ricinus L. Host relationships of the tick: Part 1. Review of previous work in Britain

On the whole published data on the host relationships of Ixodes ricinus L. in Britain are rather inadequate, but the following points emerge: The tick (stages unspecified) has been found on twenty-three mammal, twenty-one bird and one reptile species. These include both wild and domestic animals. There are no records of non-hosts. The host list is incomplete. Its length and variety suggest, however, that the tick will feed on practically any mammal or bird it may meet. Tick stages found on fifteen mammal, twenty-one bird and one reptile species have been recorded. For a number of reasons (see text) ‘recorded’ absence of any tick stage from an animal is not necessarily reliable. This, together with the incompleteness of the host list, detracts a little from the assertion of several authors, namely, that in general larvae and nymphs occur on both mammals and birds, but adult ticks on mammals only. No precise host-potentials for any wild species relative to sheep (or other important farm stock) have been published in Britain. But, in one area, Scottish mountain hare, red deer and roe deer have been slaughtered on a large scale because of their alleged importance as hosts. Very inadequate data from two authors seem to suggest that as normally stocked populations ( a ) grouse are negligible compared with sheep as hosts to female ticks, and ( b ) mountain hare and red deer may play quite an important part in maintaining female ticks. Suggestion ( b ) at least should not be taken seriously until comprehensively investigated. One author shows that an ‘appreciable’ tick population was maintained on a hill grazing by a reduced wild fauna in the absence of sheep. Another author shows that the tick population of a grazing was considerably reduced by acaricidal treatment of sheep, together with almost complete extermination of a section of the wild fauna. Unfortunately, the two factors cannot be disentangled. We would like to thank Mr Charles Elton, Director, Bureau of Animal Population, Oxford, and Dr J. MacLeod of the Cooper Technical Bureau, Berkhamsted, for criticism of the MS.; Mr F. Wallace for information on red deer stocking; and Dr V. B. Wigglesworth, F.R.S.,. Director, Agricultural Research Council Unit of Insect Physiology, Cambridge, within whose organization the work was done.

The ecology of the sheep tick, Ixodes ricinus L. Host availability and seasonal activity.

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.