Winter activity of questing ticks (Ixodes ricinus and Dermacentor reticulatus) in Germany − Evidence from quasi-natural tick plots, field studies and a tick submission study

•Tick winter activity in Germany was studied systematically by three different approaches.•Ixodes ricinus and Dermacentor reticulatus were monitored in.•Semi-natural field plots and flagging showed winter activity of I. ricinus and D. reticulatus.•Dogs and cats were infested with I. ricinus, D. reticulatus and I. hexagonus in the winter months.•An effective year-round tick control is strongly recommended. Changing climatic conditions and other anthropogenic influences have altered tick distribution, abundance and seasonal activity over the last decades. In Germany, the two most important tick species are Ixodes ricinus and Dermacentor reticulatus, the latter of which has expanded its range across the country during the past three decades. While I. ricinus was rarely found during the colder months in the past, D. reticulatus is known to be active at lower temperatures. To quantify tick appearance during winter, specimens were monitored in quasi-natural tick plots three times a week. Additionally, the questing activities of these two tick species were observed throughout the year at nine field collection sites that were regularly sampled by the flagging method from April 2020 to April 2022. Furthermore, tick winter activity in terms of host infestation was analysed as part of a nationwide submission study from March 2020 to October 2021, in which veterinarians sent in ticks mainly collected from dogs and cats. All three study approaches showed a year-round activity of I. ricinus and D. reticulatus in Germany. During the winter months (December to February), on average 1.1% of the inserted I. ricinus specimens were observed at the tops of rods in the tick plots. The average questing activity of I. ricinus amounted to 2 ticks/100 m² (range: 1-17) in the flagging study, and 32.4% (211/651) of ticks found infesting dogs and cats during winter 2020/21 were I. ricinus. On average 14.7-20.0% of the inserted D. reticulatus specimens were observed at the tops of rods in the tick plots, while the average winter questing activity in the field study amounted to 23 specimens/100 m² (range: 0-62), and 49.8% (324/651) of all ticks collected from dogs and cats during winter 2020/21 were D. reticulatus. Additionally, the hedgehog tick Ixodes hexagonus was found to infest dogs and cats quite frequently during the winter months, accounting for 13.2% (86/651) of the collected ticks. A generalized linear mixed model identified significant correlations of D. reticulatus winter ac