A modified matrix model to describe the seasonal population ecology of the European tick Ixodes ricinus

The sheep tick Ixodes ricinus is the most multicompetent vector in Europe, which is responsible for significant diseases of humans and livestock throughout the northern hemisphere. Modelling the tick's complex seasonal dynamics, upon which pathogen transmission potential depends, underpins the analysis of tick-borne disease risk and potential tick control. We use laboratory- and field-derived empirical data to construct a population model for I. ricinus. The model is a substantially modified stage-classified Leslie matrix and includes functions for temperature-dependent development, density-dependent mortality and saturation deficit-meditated probability of questing. The model was fitted to field data from three UK sites and successfully simulated seasonal patterns at a fourth site. After modification of a single parameter, the model also replicated divergent seasonal patterns in central Spain, but any biological factors underlying this geographical heterogeneity have not yet been identified. The model's applicability to wide geographical areas is thus constrained, but in ways that highlight gaps in our knowledge of tick biology. Sensitivity analysis indicated that the model was generally robust, particularly to changes in density-independent mortality values, but was most sensitive to changes in parameters related to density-dependent mortality. 5.Synthesis and applications. Vector population models allow investigation into the effects of individual environmental factors on population dynamics in ways not easily possible by experimental manipulation of in situ populations. Our model can be used to evaluate public health risk, tick management strategies and potential effects of future environmental change.