Dispersal from outbreak core areas may overwhelm management efforts at invasion fronts

Understanding the characteristics of insect dispersal events across large landscapes is difficult when individual insects are too small to easily track over the course of their flights. Nevertheless, management programs designed to slow the spread of invasive or harmful insects depend critically on understanding the timing, directions, distances, and spatial characteristics of dispersal. The specifics of dispersal and population growth, for example, may determine whether managers choose to prioritize management at the leading edge or at the core of pest invasions. We fitted dispersal kernel models for directed dispersal of the mountain pine beetle (Dendroctonus ponderosae Hopkins) and the black headed budworm (Acleris gloverana Walsingham). Outbreaks of both species were heavily managed, but quantitative demographic assessments of management effects permitted their reversal so that we could use dispersal models to simulate population spread in the absence of management. We also compared the simulated effect of a management strategy aiming to slow the spread of the pest population at the invasion front with one focused on the core of invasion. In both the mountain pine beetle and in the black-headed budworm, we find that management that diminishes dense source populations well behind the leading edge of invasions more effectively reduces spread rates than management that focuses on eradicating populations at the invasion front. Our analysis suggests that this pattern is driven by the nature of directed dispersal that allows dispersers to leap-frog over newly established populations coupled with high population growth and dispersal rates behind the invasion front.