Risk rating systems for the Douglas-fir beetle in the interior western United States

Douglas-fir beetle (DFB), Dendroctonus pseudotsugae, risk rating systems were developed for four USDA Forest Service ranger districts in northern Idaho. Risk rating systems were developed using a geographic information system, USDA Forest Service aerial detection survey maps, and nearest neighbor distances. Years where epidemic populations of DFB occurred over the four ranger districts were selected and risk rating systems were developed for each district. Nearest neighbor distances for between-year (t + 1) and between-2-year (t + 2) infestations were measured. Percentiles of nearest neighbor distances were determined and used to estimate distances at which 50, 75, and 90% of infestations occurred. On average, there was a 50% chance of an infestation occurring within 176 in from previous year infestations, 75% chance from within 517 m, and 90% chance from within 1,188 m. The between-2-year risk rating determined that there is a 50% chance of infestation occurring on average within 249 m, 75% within 627 m, and 90% within 1,332 m from infestations that occurred 2 years earlier.