Relations among stand structure, dispersal of second-instar western spruce budworm, defoliation, and height growth of young conifers

Abstract

Local stand structure had little influence on dispersal of second-instar western spruce budworm (Choristoneuraoccidentalis Freeman). Numbers of dispersing larvae caught on traps averaged 37 larvae/m2 in the cut stands, as high as were found in adjacent uncut stands, and were not related to basal area of overstory in harvested stands, distance to the adjacent uncut stands, or budworm populations in the uncut stands. Despite high numbers of dispersing larvae, defoliation of host regeneration in cut stands was low (87% of the nonlarch host incurred less than 25% defoliation) and was not predictable (p ≤ 0.05) from dispersing larvae. Defoliation of mature trees in the adjacent uncut stands averaged 20% and ranged to 45%. Three-year height growth of postharvest host conifers in the cut stands increased with smaller overstory basal area, greater initial height, and greater crown ratio, but was not affected by the small amount of budworm defoliation. Regression models of 3-year height growth were similar among host western larch (Larixoccidentalis Nutt.), grouped nonlarch hosts (Douglas-fir, Pseudotsugamenziesii var. glauca (Beissn.) Franco; grand fir, Abiesgrandis (Dougl.) Forbes; and subalpine fir, A. lasiocarpa (Hook.) Nutt.), and grouped nonhosts (ponderosa pine, Pinusponderosa Dougl.; and lodgepole pine, P. contorta var. latifolia). Larval dispersal may be influenced more by forestwide conditions and spring weather than by local stand factors. Larvae reaching target seedlings likely are removed by predators such as birds and ants. Vigorous small host trees appear to be poor habitat for budworms; the insect had little effect on height growth of 5- to 20-year-old seral conifer stands of western Montana.