Quantifying Dispersal of Southern Pine Beetles with Mark-Recapture Experiments and a Diffusion Model.

Pest management decisions should take into consideration quantitative information on dispersal of insect pests, but such information is often lacking. The goal of this study was to measure intraforest dispersal in the southern pine beetle (SPB). We developed an analytical formula for interpreting data from mark-recapture studies of insect dispersal. The formula is obtained by postulating a simple model of diffusion with disappearance (e.g., as a result of death) for the spread of marked insects from the release point. Parameters of the model (assumed to be invariant in space) are estimated by fitting the curve to the cumulative number of recaptured insects as a function of the distance from release. We also derived an expression for the distribution of dispersal distances in terms of the fitted parameters. The proposed methodology was applied to a mark-recapture study of SPB dispersal Statistical analysis of recaptures-with-distance curves obtained in 11 replicate releases indicated that the proposed formula provided an accurate description of the data. There were no systematic departures from the functional relationship prescribed by the formula, and the model consistently outperformed another commonly used formula for fitting data on dispersal distances, the exponential curve. We explored the effect of spatial heterogeneity in the host distribution on SPB movement by regressing the deviation from the recapture rate predicted by the model in each trap on the pine basal area around the trap. This correlation was significantly greater than zero, indicating that beetles tended to aggregate in localities where pines were dense. This result suggests that a diffusion model with spatially varying parameters may provide a more accurate description of the redistribution process in the SPB. Quantitative results on SPB intraforest dispersal were summarized by calculatingradii of circles enclosing a given proportion of SPB dispersal distances. For example, we estimated that one half of released beetles dispersed >0.69 km. This result has important implications for evaluating the area-wide consequences of current or novel control tactics that rely on disrupting SPB movements, e.g., cut-and-leave and treatments with anti-congregation pheromone.