Questing behaviour of Ixodes ricinus ticks(Acari: Ixodidae)

H. A. Mejlona and T. G. T. Jaensonb*aDepartment of Zoology, Uppsala University, Villavagen 9, S-752 36 Uppsala, Sweden

bMedical Entomology, Department of Entomology, Swedish University of Agricultural Sciencesand Zoological Museum, Uppsala University, Villavagen 9, S-752 36 Uppsala, Sweden

(Received 19 April 1997; accepted 22 June 1997)

ABSTRACT

The vertical distribution in the vegetation of questing Ixodes ricinus ticks was investigated in twodifferent vegetation types (‘high’ and ‘low’ vegetation) at two localities in south–central Swedenduring 1992–1993 (Toro) and 1995 (Bogesund). Significant correlations were found between thevertical distribution of immature ticks and the height of the vegetation. The greatest meanavailabilities of the larvae and nymphs in low vegetation were in the intervals 0–9 and 30–39 cm,respectively. The larval numbers were greatest close to the ground (0–29 cm) in both high andlow vegetation. The larval : nymphal ratio, at ground level at localities free of ground vegetation,varied between 8 : 1 and 32 : 1. In high vegetation, the greatest mean numbers of nymphal andadult ticks were at height intervals of 50–59 and 60–79 cm, respectively. These ranges are withinthe estimated height interval (40–100 cm) of the main part of the body surface of their ‘preferred’host, the roe deer (Capreolus capreolus). The presence of most questing I. ricinus larvae atground level would favour the transmission of Borrelia burgdorferi s.l., since this is where thehighly reservoir-competent rodents and shrews usually occur.

Key words: Ixodes ricinus, Borrelia burgdorferi, vertical distribution, host-seeking activity,questing behaviour, host.

INTRODUCTIONThe vertical distribution of questing Ixodes ricinus is influenced by many factorsincluding the height and other physical properties of the vegetation. For instance,investigations by Lees and Milne (1951) on the vertical distribution of questing I.ricinus ticks in natural vegetation, showed that the majority of ticks were questingclose to the tips of the vegetational parts. The gravity, humidity and temperaturewill also influence the movement and vertical distribution of questing ticks (Lees,1948; Belozerov, 1982). The desiccation tolerance generally increases with tick age(stage). Therefore, the larvae tend to quest lower in the vegetation than the nymphsor adults (Gigon, 1985). However, such a distribution pattern could also be due toa ‘preference’ of particular stages of I. ricinus to quest for hosts of certain sizes.Talleklint and Jaenson (1994) recorded that near Stockholm approximately 70% ofall larval engorgements took place on small mammals such as shrews (Sorex spp.)

*To whom correspondence should be addressed at: Fax: +46 18 559888;e-mail: Thomas.Jaenson@zoologi.uu.se

Experimental & Applied Acarology, 21 (1997) 747–754

0168–8162 © 1997 Chapman & Hall

and rodents (Apodemus spp., Clethrionomys glareolus and Microtus agrestis),whereas the majority of nymphal and female ticks fed on larger mammals such asroe deer (Capreolus capreolus) and hares (Lepus spp.). Thus, in order to maximizethe likelihood of encountering the main part of their ‘preferred’ hosts, nymphs andadults should be questing higher above ground than larvae.

The aim of this study was to investigate whether questing I. ricinus ticks tend tooccur in relatively distinct, tick stage-associated vertical zones in the vegetation andif such a potential distribution pattern may be related to one or more of thefollowing factors: (1) meteorological variables, (2) vegetation structure and (3) sizeof the ‘preferred’ mammalian hosts.

MATERIALS AND METHODS

Study localitiesThe main field investigations were carried out at Toro (58°50'N, 17°51'E), an islandsituated 57 km south of Stockholm (June–August 1992 and 1993) and at Bogesund(59°25'N, 18°10'E), 10 km north of Stockholm (June and September 1995). Tickswere collected once monthly. Additional studies were performed at three localitiesin south-western Sweden: Dagsås (57°04'N, 12°30'E) and Anggårdsbergen(57°41'N, 11°57'E) in July 1995 and Hallands Vadero (56°26'N, 12°34'E) in August1995. The study area at Toro was located in a mixed forest clearing made forelectrical power lines. The vegetation was dense and consisted mainly of youngalder bushes (Alnus glutinosa) approximately 2 m high and grasses and ferns up toapproximately 1 m high. At Bogesund, two different vegetation types were studied:a meadow at a forest edge and a herbaceous pine forest. The herbal layer in the pineforest at Bogesund was considered to represent the ‘low’ vegetation type (i.e.generally 0–50 cm high but occasionally reaching 80 cm). The vegetations studiedat Toro and the open site at Bogesund were considered to represent the ‘high’vegetation type (i.e. 0–150 cm and occasionally higher). The study sites in broad-leaf forests at Dagsås, Anggårdsbergen and Hallands Vadero were, in general,almost devoid of any ground vegetation but covered with dead beech (Fagussylvatica) leaves.

Environmental variables and tick abundanceFor both the high and low vegetation types, the vertical distribution of thevegetation, i.e. the potential tick questing sites, was estimated by visual quantifica-tion for each 10 cm increment. This measure, i.e. the distribution of the vegetationalapices, is henceforth referred to as the height of vegetation.

The air temperature and relative humidity (RH) were measured at Toro andBogesund. At Toro, these variables were measured 10 cm above the ground. AtBogesund, the temperature was measured at 0, 10, 50 and 100 cm above the groundand the RH at 10 and 50 cm. Sampling was not conducted during or shortly afterrainfall.

To sample the ticks, a dress of white cotton flannel cloth was used. The dresstotally covered the collector between neck and ankles and was marked at every

748 H. A. MEJLON AND T. G. T. JAENSON

10 cm from 10 to 140 cm above the ground. The collector walked slowly with hisarms raised through the vegetation. Ticks that attached to the cloth were collectedat approximately every 15 m. The stage of each tick and its location on the dresswas recorded. The ticks were subsequently released. In each biotope, eight to 18stops to collect ticks were made. At Toro and Bogesund it was, in general, notpossible to record ticks at the 0–10 cm level. However, in the pine forest atBogesund, where the ground cover was relatively sparse, the tick availabilitybetween 0 and 10 cm was estimated using a small flag (10 3 20 cm).

To estimate the ratio between the abundance (availability) of the different activetick stages, blanket-dragging with a 1 3 1 m white flannel cloth was performed atDagsås, Anggårdsbergen and Hallands Vadero in the forested study areas where theground cover was very sparse or absent. The tick availability is defined here as thenumber of questing ticks that attached to the cloth per unit of walking distance.

To calculate the effective area sampled on each occasion and at each of the 13different height intervals, we estimated the collector’s horizontal body coverage tobe 30 cm, i.e. approximately the hip width of the collector. The total area coveredper height interval per sampling occasion thus ranged from 42 to 88 m2.

The surface areas of the different potential mammalian host species wereestimated from drawings of museum specimens. A height range of 0–10 cmrepresents the surface areas of Sorex minutus, Sorex araneus, M. agrestis, C.glareolus, Apodemus flavicollis and Apodemus sylvaticus, a range of 10–30 cmLepus timidus and a range of 40–100 cm roe deer. To calculate the likelihood ofquesting ticks to encounter hosts at Bogesund and Toro, we used previouslypublished data on the density of the main tick hosts at Bogesund (Talleklint andJaenson, 1994). We considered the vertical distribution of the host surface areas asapproximately the same in all biotopes investigated. This approximation wasdeemed acceptable since, in general, small mammals tend to be more abundant thanmedium-sized or large hosts in most mainland biotopes in southern and centralSweden, even if the species composition of the potential hosts for I. ricinus mayvary among biotopes.

Statistical methodsEach tick collected was treated as a separate case and assigned a value correspond-ing to the height interval where the tick was collected. Comparisons of the heightlocations between the tick stages were made using the Kruskal–Wallis test (Sokaland Rohlf, 1981). Spearman rank order correlations were used to compare thefrequency of the host surface area and height of the vegetation with the mean tickavailability.

RESULTS

Vertical distribution of ticks in high vegetationAll stages of I. ricinus were found in all 13 different height intervals from 10 to140 cm above ground level, except adult ticks, which were absent from one interval(Table 1). The intervals with the greatest availability were 10–19 cm for the larvae,

749QUESTING BEHAVIOUR OF IXODES RICINUS

50–59 cm for the nymphs and 60–79 cm for the adults (Table 1). In the height range10–140 cm the mean heights (6 S.D.) of occurrence of the larvae, nymphs andadults were 50.2 6 28.9, 58.8 6 29.9 and 66.0 6 31.3 cm, respectively.

The questing heights recorded in high vegetation differed significantly among thelarvae, nymphs and adults (Kruskal–Wallis test, H(2,1381) 5 33.1 and p , 0.0001).This difference was mainly due to the larvae questing significantly lower thannymphs (Kruskal–Wallis test, H(1,1346) 5 27.4 and p 5 0.0001). There was nodifference in questing heights between the nymphs and adults.

Vertical distribution of ticks in low vegetationIxodes ricinus subadults were absent from at least two different height intervals(Table 1). The greatest mean availabilities of the larvae and nymphs were in theintervals 0–9 and 30–39 cm, respectively. No adult ticks were found here. Themean questing heights for the larvae and nymphs were 17.6 6 10.0 and27.0 6 16.4 cm (only five ticks), respectively. These means are not significantlydifferent.

Vertical distribution of ticks in relation to vegetation and mammalian hostsThe vertical distributions of the I. ricinus larvae, nymphs and adults in relation tothe height of the vegetation and host surface areas are shown in Fig. 1(a) and (b).

TABLE 1

Estimated proportion of host target area (% host; 100% 5 7740 cm2/hectare), vegetational heights (% veg)and estimated density (mean numbers) of I. ricinus larvae, nymphs and adults per 100 m2 in different verticalzones between 0 and 140 cm above ground level

High vegetation Low vegetationHeight interval(cm) % host % vegetation Larvae Nymphs Adults % vegetation Larvae Nymphs Adults

0–9 41.5 11.0 – – – 15.0 61.9 2.4 0.010–19 5.4 14.0 20.0 6.3 0.5 25.0 35.7 1.3 0.020–29 5.4 14.0 19.3 9.7 0.5 25.0 50.3 0.0 0.030–39 1.1 13.0 14.6 10.2 0.2 13.0 4.0 2.6 0.040–49 9.9 12.0 12.7 10.4 0.5 7.0 1.3 1.3 0.050–59 11.0 9.4 15.7 11.5 0.3 7.0 1.3 0.0 0.060–69 11.0 5.7 13.0 10.1 1.0 4.0 0.0 0.0 0.070–79 11.0 4.9 9.7 8.1 1.0 4.0 0.0 0.0 0.080–89 1.7 3.5 7.5 6.2 0.3 NV NV NV NV90–99 1.7 2.6 5.7 5.2 0.6 NV NV NV NV100–109 0.3 2.2 7.3 4.5 0.5 NV NV NV NV110–119 NH 1.6 2.2 1.8 0.2 NV NV NV NV120–129 NH 1.1 1.4 3.6 0.0 NV NV NV NV130–139 NH 1.1 0.4 1.0 0.2 NV NV NV NV

–, no data; NH, no hosts; NV, no vegetationTwo different vegetational types were sampled: high (semi-open bush/meadow) and low (herbaceous pineforest). Ticks were collected at Toro (June–August 1992–1993) and Bogesund (June and September 1995)near Stockholm, Sweden.

750 H. A. MEJLON AND T. G. T. JAENSON

Fig. 1. Estimated proportion of main hosts’ surface areas (shaded bars) and vegetation heights(open bars) in (a) high and (b) low vegetation types. The lines represent the density, i.e. the meannumbers per 100 m2, of questing I. ricinus larvae (dotted line), nymphs (broken line) and adults(solid line) at the vertical range 0–140 cm. The ticks were sampled near Stockholm on eightoccasions in 1992 (Toro) and 1995 (Bogesund).

751QUESTING BEHAVIOUR OF IXODES RICINUS

In high vegetation, within the interval 10–140 cm, the larval and nymphal distribu-tion patterns were similar to that of the height of the vegetation. They were clearlydissimilar from the distribution pattern of the host surface areas. In addition, in lowvegetation the larval tick distribution data fitted the distribution of the height ofthe vegetation. The nymphal data were considered inadequate for statisticalevaluation.

Correlations between the tick numbers and the estimated proportions of theheight of the vegetation gave significant positive coefficients in three out of fivepossible combinations, whereas the correlations between the tick numbers and thehost surface area variable were non-significant in all combinations (Table 2). Thetemperature and RH varied only slightly between the different levels (0, 10, 50 and100 cm) and were not significantly associated with tick vertical distribution.

Proportion of questing larvae to questing nymphsIn each biotope, the larval to nymphal availability ratio was estimated from the totalnumbers of each stage recorded for the whole range of vegetation sampled, i.e.10–140 cm (high vegetation) or 10–80 cm (low vegetation). For comparison, thefollowing data from the blanket drag samplings were used: (1) areas free of groundvegetation but covered with dead beech leaves (Anggårdsbergen, Dagsås andHallands Vadero) in 1995 and (2) low vegetation biotopes at Bogesund and highvegetation biotopes at Toro in 1991–1992.

By dress sampling, the larval : nymphal ratios were 1.4 : 1 (high vegetation,620 m2 sampled) and 19 : 1 (low vegetation, 80 m2 sampled). The larval : nymphalratios based on previous blanket dragging (H. A. Mejlon unpublished data) in theseareas were 7.5 : 1 (1800 m2), and 25 : 1 (500 m2), respectively. Finally, the areasfree of ground vegetation yielded larval : nymphal ratios of 18 : 1 (HallandsVadero, 40 m2), 8.3 : 1 (Anggårdsbergen, 100 m2) and 32 : 1 (Dagsås, 100 m2).

TABLE 2

Spearman rank correlations between tick availability, i.e. the numbers of questing I. ricinus collected and theestimated vertical proportion in 10 cm increments of host surface area or vegetation heights

Host surface area Vegetation height

Tick stage Vegetation Number of ticks rs p value n rs p value n

Larva High 796 0.37 NS 10 0.96 0.0001 13Nymph High 550 0.60 NS 10 0.80 0.001 13Adult High 35 0.47 NS 10 0.34 NS 13Larva Low 96 –0.14 NS 8 0.92 0.01 8Nymph Low 5 –0.17 NS 8 0.45 NS 8Adult Low 0 No ticks observed

The ticks were sampled in the interval 0–140 cm above ground level in high and low vegetation types at twolocalities (Toro and Bogesund) near Stockholm. rs, Spearman rank correlation coefficient. n, number ofvertical increments.NS, not significant at the 0.05 level.

752 H. A. MEJLON AND T. G. T. JAENSON

Thus, in high vegetation, the larval : nymphal ratio based on dress sampling(1.4 : 1) was considerably less than that of both blanket dragging (7.5 : 1) andvegetation-free ‘control’ areas (8.3 : 1–32 : 1). In low vegetation, the larval : nym-phal ratios seemed less variable between the dress sampling (19 : 1), blanketdragging (25 : 1) and control areas (8 : 1 –32 : 1).

DISCUSSION

Not surprisingly, in high, partly open vegetation, the distribution of larval andnymphal ticks appeared strongly associated with the vegetation structure, i.e. theheight of the vegetation (Table 2). Nevertheless, both larvae and nymphs wererecorded at all height intervals between 10 and 140 cm. However, the mean heightat which the larvae were present was significantly lower than those of nymphs oradults. In low vegetation, only the larval vertical distribution appeared to becorrelated with the vegetation structure. The vertical distribution of the adult ticksappeared to be less dependent on the vegetation structure, although larger samplesare needed to confirm this.

The vegetation structure affects the microclimate in which the ticks live and willtherefore influence their water balance. According to Lees and Milne (1951), adultI. ricinus spend only approximately 30% of their time questing above ground. Theremainder is spent on the ground where the humidity is usually high, whereby thewater balance can be restored. The same should apply to immature ticks, but sincenymphs and in particular larvae are more sensitive to desiccation than adults, thesestages are likely to quest lower and to spend less time above ground level. Thework by Gigon (1985) on Swiss I. ricinus populations in artificial arenas showedthat subadults generally quest at lower heights (7–11 cm) than adults (10–50 cm)and that this pattern of tick vertical distribution is related to habitat type.

Because I. ricinus larvae mainly infest small mammals while nymphs and adultsusually feed on medium-sized and large mammals (Talleklint and Jaenson, 1994),a lower mean height of distribution of questing larvae compared to that of questingnymphs and adults conforms to their partly different host associations. This issupported by the fact that the larval density in both high and low vegetation wasgreatest at the lowest intervals sampled (10–19 and 0–9 cm, respectively).

The mean questing heights for larvae in high (50.2 cm) and low (17.6 cm)vegetation types indicate that the host size (small mammals) as well as thevegetation structure and the relatively low degree of desiccation tolerance of thelarvae may influence their vertical distribution in the habitat. Since they presumablyspend most of their time at ground level, they would actually have greateropportunities to contact small mammals here. The mean questing heights recordedat Toro and Bogesund are biased since ticks questing at 0–10 cm could not berecorded. It is reasonable to assume that the majority of questing tick larvaeoccurred below 10 cm. Therefore, we estimated the larval and nymphal avail-abilities below 10 cm by blanket dragging in vegetation-free areas. It should beemphasized that the ratio of questing larvae to questing nymphs at a particularlocality is likely to vary during the season because of their varying seasonal

753QUESTING BEHAVIOUR OF IXODES RICINUS

densities. As expected, at ground level, the larval : nymphal ratios were8.3 : 1–32 : 1 by blanket dragging compared to 1.4 : 1 by dress sampling invegetation of .10 cm.

Approximately 80% of the feeding nymphs are expected to take their meals fromhares and cervids (Talleklint and Jaenson, 1994). This means that an optimalquesting site for these nymphs would be located approximately 10–100 cm aboveground. Our data conform to this and show that the greatest mean abundance of thenymphs was in the height range of 30–39 cm in low vegetation and at 50–59 cm inhigh vegetation (Table 1 and Fig. 1). Approximately 45% of feeding adult I. ricinusfemales will take their blood meal from roe deer (Talleklint and Jaenson, 1994).The proportion of adult I. ricinus questing in the ‘roe deer zone’ (40–100 cm) was63%. Thus, for the adult ticks our data also conform to the ‘host size-dependentquesting strategy hypothesis’.

Other investigations on the host-seeking behaviour of ticks include that of Loyeand Lane (1988). They recorded that adult Ixodes pacificus were mainly questingclose to the tips of 25 and 50 cm wooden dowels, while 75 cm dowels were usedless often. Cervids and lagomorphs are major hosts of adult I. pacificus. The bodysizes of these hosts correspond well to the mean questing height of the adultticks.

ACKNOWLEDGEMENTS

We are very grateful to Jeremy Gray and Lars Talleklint for valuable comments onan earlier version of this paper. This work was supported by grants from theSwedish Natural Science Research Council to T. G. T. Jaenson.

REFERENCES

Belozerov, V.N. 1982. Diapause and biological rhythms in ticks. In Physiology of ticks, F.D.Obenchain and R. Galun (eds), pp. 469–500. Pergamon Press, New York.

Gigon, F. 1985. Biologie d’Ixodes ricinus L. sur le Plateau Suisse – une contribution a l’ecologiede ce vecteur. Doctoral thesis, Faculty of Sciences, University of Neuchatel.

Lees, A.D. 1948. The sensory physiology of the sheep tick Ixodes ricinus L. J. Exp. Biol. 25:145–207.

Lees, A.D. and Milne, A. 1951. The seasonal and diurnal activities of individual sheep ticks(Ixodes ricinus L.). Parasitology 41: 189–208.

Loye, J.E. and Lane, R.S. 1988. Questing behavior of Ixodes pacificus (Acari: Ixodidae) inrelation to meteorological and seasonal factors. J. Med. Entomol. 25: 391–398.

Sokal, R.R. and Rohlf, F.J. 1981. Biometry, 2 edn. W.H. Freeman and Co, New York.Talleklint, L. and Jaenson, T.G.T. 1994. Transmission of Borrelia burgdorferi s.l. from mammal

reservoirs to the primary vector of Lyme borreliosis, Ixodes ricinus (Acari: Ixodidae), inSweden. J. Med. Entomol. 31: 880–886.

754 H. A. MEJLON AND T. G. T. JAENSON