SAMPLING,DISTRIBUTION,DISPERSAL Distribution of the Common Tick, Ixodes ricinus (Acari: Ixodidae), in Different Vegetation Types in Southern Sweden ANDERS LINDSTRO ¨ M AND THOMAS G.T. JAENSON 1 Medical Entomology, Department of Systematic Zoology, Evolutionary Biology Centre, Uppsala University. Norbyva ¨ gen 18D, SE-752 36 Uppsala, Sweden J. Med. Entomol. 40(4): 375Ð378 (2003) ABSTRACT The aim of this study was to investigate whether differences in Ixodes ricinus (L.) nymphal relative density exist among different vegetation types in southern Sweden. Nymphal I. ricinus were sampled in southeastern Scania in southern Sweden during JuneÐAugust 1997. A total of 110 Ð180 25-m 2 samples were taken by blanket-dragging from each of nine different vegetation types. There was a highly signiÞcant difference in nymphal abundance between the open areas as one group and the forested areas as another group (P  0.0001). Vegetation types that differed signiÞcantly in median nymphal abundance from all other vegetation types were pine forest (16 nymphs/100 m 2 ) and beech forest (40 nymphs/100 m 2 ). No signiÞcant differences in median nymphal density were revealed among mixed deciduous forest, alder forest, oak forest, and hazel forest (28 Ð32 nymphs/100 m 2 ), or among dry meadow, meadow, and heath (0 nymphs/100 m 2 ). Forestation of open areas is likely to lead to increased I. ricinus abundance and disease risk. KEY WORDS biotopes, Ixodes ricinus, Sweden, tick sampling, vegetation types IN THE NORTHERN HEMISPHERE, Ixodes ticks are the most important arthropod disease vectors of pathogens from wild animals to humans. In Europe, the common tick Ixodes ricinus (L.) is the most important vector of diseases to humans. Such diseases include Lyme bor- reliosis (caused by Borrelia burgdorferi s.l. Johnson, Schmid, Hyde, Steigerwalt, and Brenner), tick-borne encephalitis virus (TBE), tularemia [Francisella tula- rensis (McCoy and Chapin)], babesiosis (Babesia di- vergens MÕFadyean and Stockman, B. microti Franca), and ehrlichiosis [Ehrlichia phagocytophila (Foggie)]. On a global scale, only mosquitoes are considered to be a medically more important group of arthropods. In view of the signiÞcance of tick-borne diseases, it is important to know where ticks are abundant. Access to relatively large blood hosts is essential for oviposi- tion and reproduction of I. ricinus. Presence of mam- malian, avian, or reptilian hosts and a suitable habitat with adequate vegetation cover are important for sur- vival and development of the tick. Elevation, temper- ature, rainfall, and humidity are key abiotic factors that inßuence the presence, development, activity, and longevity of this tick (Estrada-Pen ˜ a 2001). The vegetation, host fauna, soil, topography, and climate are interrelated, and extremes of any one factor may adversely affect the tickÕs ability to survive (Guerra et al. 2002). In an extensive study in Spain, Estrada-Pen ˜ a (2001) recorded the highest abundance of I. ricinus in oak (Quercus) and mixed old forests with many ecotones, whereas this tick was absent in open biotopes. Ticks seemed to prefer sites with much secondary plant growth, e.g., river canopies, mixed forests, and decid- uous heterogeneous woods. I. ricinus was absent in open habitats, homogenous young coniferous forests, and open hillsides. In southern Scotland, coniferous woodland was most infested with questing I. ricinus nymphs, deciduous woodland had slightly lower in- festation, and upland sheep pastures had much lower infestation (Walker et al. 2001). Of the three main vegetation types, Vaccinium myrtillus/Calluna vul- garis vegetation had the highest infestation, followed by Agrostis/Festuca/Holcus grassland and Pteridium vegetation with the least tick abundance (Walker et al. 2001). In Sweden, the abundance of I. ricinus was investigated by Mejlon and Jaenson (1993) in differ- ent biotopes at two localities in southcentral and southern Sweden. Mixed spruce/deciduous woodland had the greatest nymphal abundance, followed by spruce forest and cattle pasture with the least abun- dance. They concluded that tick survival was greater in woodland biotopes than in open biotopes because of the more optimal environmental and climatic con- ditions in the woodland. Gray et al. (1998) summa- rized data on B. burgdorferi s.l.-infected I. ricinus for 16 European countries: high risk situations, i.e., large numbers of infected nymphs, occur in heterogeneous woodland, usually with a recreational function and with a diverse fauna and usually including cervids. In some other habitats, tick abundance was also high, 1 E-mail: Thomas.Jaenson@ebc.uu.se. 0022-2585/03/0375Ð0378$04.00/0  2003 Entomological Society of America