Seasonal and diel activity of Ixodes ricinus (Acari: Ixodidae) subpopulations in Denmark. Aspects of size, physiological age, and malate dehydrogenase genotype in a forest site without any undergrowth

The underlying population dynamics and the behavioural patterns of the vectors are key issues in understanding the transmission of vector borne pathogens. For the tick Ixodes ricinus both seasonal and diel activity have been described as bimodal patterns, which in seasonal aspect has been interpreted as representing two cohorts. However, recent studies have shown that this interpretation may be incorrect. The aim of this study was to obtain more detailed information on nymph host seeking by studying subpopulations of ticks during the day and season. The study was designed to allow for comparisons of the diel variation and seasonal variation in their dependency in a number of tick characteristics. The study took place in a forest with planted beech trees without any undergrowth. Ticks were collected by flagging the dead leaves on the forest floor. For each nymph, a number of visual observations were made. The size and physiological age was observed and the nymphs were genotyped in the malate dehydrogenase locus (MDH, E.C. 1.1.1.37). Briefly the main results can be given as: (i) There were significant differences in the composition of size classes during the season, but only limited trends in time. (ii) The proportion of the small nymphs was highly variable, with a variation from 3% to 24% in October and September, respectively. (iii) The diel variation in MDH genotypes was significant in May and August. (iv) Nymph size classes and physiological age appeared to interact. The non-random interaction was caused by a relatively even distribution of small nymphs in all four age classes, while large nymphs tended to fall into age class 2 and 3. The length by age interaction for the individual months was noted to be significant in May, July, August and September, but not in June. Similarly the interaction was significant in the morning and afternoon, but not at midday. The overall results describe the seasonal and diel activity patterns as changing systematically for several characteristics under the influence of weather condition and population dynamics. In conclusion: The observations are best interpreted as being produced of a single cohort of ticks, but the revealed complexity of the host seeking activity suggest that measures of activity × abundance should be interpreted very cautiously in relation to population dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.