Inventory Monitoring for Estimating Impact of Insects on Seed Production in a Douglas-Fir Seed Orchard in Western Oregon

Abstract

Impact of various factors on seed production was examined in a Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) seed orchard in western Oregon during 1984. Cones were examined monthly between April and September for mortality or evidence of insect damage. Loss of immature cones began early in the growing season and had a substantial impact on seed production. A newly discovered cone-feeding weevil, Lepesoma lecontei (Casey), caused a large proportion of this mortality. Unexplained cone mortality (possibly frost damage), undeveloped seed, Douglas-fir cone gall midge ( Contarinia oregonensis Foote), and Douglas-fir seed chalcid ( Megastigmus spermotrophus (Wachtl)) were other major causes of seed loss. Crown elevation was important for an accurate estimate of the importance of some seed-loss factors. An inventory monitoring system for Douglas-fir seed production based on early monitoring of immature cones, identifying specific seed loss factors, stratified crown sampling, and an adequate sample size, is suggested.