Winter activity of Ixodes ricinus in a Berlin forest

Nymphs and adults of the hard tick Ixodes ricinus typically quest in Central Europe from March/April to October/early November. The recent trend towards milder winters raises the question whether I. ricinus will gradually extend its seasonal activity and use mild winter periods for questing. The present study followed the host-seeking activity of I. ricinus from early September 2006 to early March 2007 in a Berlin forest. Field-collected larval and nymphal ticks were allowed to take a full blood meal in the laboratory and to moult. The resultant unfed nymphs (n=125) and adults (n=149) were released onto three field plots (1×1 m²) in early September 2006. A second group of field-collected unfed I. ricinus nymphs (n=79) and adults (n=47) were released onto a field plot in early September 2006. Each plot was covered by an approximately 10 cm thick layer of leaf litter (mostly of oak and beech) to provide a humid microclimate refuge and shelter for non-active ticks. Wooden rods (60 cm high) arranged in a 6×8 grid on these field plots were climbed by questing nymphs and adults. Active ticks were observed on each of 33 observation dates. The winter activity revealed by this experiment was confirmed by flagging for ticks on two occasions in January and February 2007 with remarkably high numbers of collected ticks. This appears to be the first time that extended or even continuous winter activity of I. ricinus nymphs and adults has been demonstrated in Central Europe, most probably caused by the extremely mild winter of 2006/2007. The fact that I. ricinus now can be active during the whole winter, a time of the year when these ticks historically have been dormant when the weather is normal, is relevant to the public because people who enter forest areas should be well aware of the risk of getting tick bites and tick-borne infections in mild winter periods.