Southern Pine Beetle: Factors Influencing the Growth and Decline of Summer Infestations in East Texas

Abstract

Abstract The influence of selected forest site and stand factors on the rate of expansion of individual infestations (spots) of southern pine beetle (Dendroctonus frontalis) was studied during the summer months of 1975-77 in east Texas. In small spots (≤20 active trees), further expansion seldom occurred, presumably due to an insufficient quantity and distribution of brood to perpetuate necessary pheromone production. Similarly, the proportion of spots that had declined to less than 20 active trees after 1 month (day 30) was inversely related to number of active trees at first visit (day 0). In expanding spots, the number of additional trees killed per day was significantly and positively correlated with number of active trees per spot and stand basal area. In addition, mean rates of summer spot growth varied among years in direct relation to areawide beetle population levels in east Texas which increased to outbreak levels and then collapsed over the 3-year study period. This emphasizes the need to include some measure of beetle population level and/or aggressiveness in future spot growth models. No correlation was found between number of brood trees per spot at the first visit and stand basal area at the spot origin. These findings support the following hierarchy of priorities for direct control: Priority 1--spots with >100 active trees, even in sparse stands; Priority 2--spots with 20-100 active trees in high basal area stands; Priority 3--spots with 20-100 active trees in low basal area stands; Priority 4--spots with ≤20 active trees. During the summer, small spots, particularly those lacking freshly attacked trees (pheromone source), have a high probability of soon going inactive without control. Forest Sci. 25:547-556.