Extreme infestations of winter tick (Dermacentor albipictus) on moose (Alces alces) have caused high calf mortality and population declines in the New England area. Increased hunter harvests have been proposed as a management strategy, following from a hypothesis that reduced moose populations will decrease questing tick abundances and facilitate recovery of moose populations. This management strategy relies on the existence of a host population threshold for tick persistence. Yet, field observations do not indicate a clear moose population threshold for tick persistence and it is uncertain that such a threshold even exists in this system. Management based on tick persistence thresholds is further complicated by the over-dispersion of tick burdens among moose, which creates strong heterogeneity in moose vital rates and production of ticks from infested moose. To address this complexity, we developed burden-structured models of moose-tick interactions which accommodate the impact of variation in tick burden on both host and parasite population dynamics. We then analyzed the model to determine the existence of moose population thresholds for ticks and assessed the feasibility of harvesting for winter tick management.
Results/Conclusions
We find theoretical evidence that moose population thresholds for tick persistence exist in this system. Harvesting can be used to reach thresholds and drive extirpation of winter ticks from the system. However tick-free moose populations are transient under constant harvesting; constant harvesting intensities sufficient for tick-extirpation eventually caused moose extirpation. But, since moose extirpation occurs after tick extirpation, state-dependent harvesting strategies could make harvesting feasible for winter tick control.
The feasibility of harvesting to control tick outbreaks also depends on the functional form between the abundance of questing tick larvae in the environment and attachment of larvae to moose. Positive density-dependent attachment can result in tick persistence thresholds and is expected if moose are the most important host for tick persistence. Density-independent attachment would not allow for tick persistence thresholds and could occur with sufficient magnitude of tick recruitment from alternative hosts, i.e., white-tail deer (Odocoileus virginianus). Future work will involve estimating the role of alternative hosts on winter tick persistence and the relationship between larval tick abundance and tick attachment, which is needed to produce data-driven projections of alternative management strategies.