Evidence of negative density‐dependent dispersal in an invasive forest pest

Understanding the mountain pine beetle's dispersal patterns is critical for evaluating its threat to Canada's boreal forests. It is generally believed that higher beetle densities lead to increased long‐distance dispersal. One possible explanation is that beetle aggregation pheromones become repellent at high concentrations, causing beetles to fly above the forest canopy where wind currents may carry them great distances. Using helicopter surveys of infested trees from 2010 to 2020 in Alberta, we modeled the spatial relationship between successive annual infestations as a proxy for beetle dispersal. Instead of the expected positive association, we observe a weak negative association at all spatial scales: At higher infestation densities, new infestations appear closer to previous years' infestations. A possible explanation is that at low population densities, beetles cannot successfully attack healthy trees and must travel farther to find weakened hosts. Even so, the influence of beetle density on population spread is minor compared to the spatiotemporal variation in the overall (density‐independent) scale of spread, with spatiotemporal heterogeneity accounting for 81% of the variance in our models. This spatiotemporal heterogeneity explains mountain pine beetle's erratic range expansion across western Alberta, which varied from 20 to 220 km annually.