Intratree Variation in the Seasonal Distribution and Mortality of Spruce Budworm (Lepidoptera: Tortricidae) from the Peak to Collapse of an Outbreak

Abstract

Abstract A 5-yr field study was carried out to assess intratree variations in the distribution, abundance, and mortality of immature spruce budworm (Choristoneura fumiferana Clemens) (Lepidoptera: Tortricidae) from the peak to collapse of an outbreak. In most years, the highest density of overwintering second-instar (L2) larvae (per square meter of foliage) was located in the lower crown, whereas all subsequent stages (third- to sixth-instar larvae, pupae, and eggs) were at relatively higher densities in the upper crown. In contrast, overall abundance (per branch) throughout the season tended to be highest in the mid-upper to mid-lower crown. Mortality associated with 16 different parasitoid species varied significantly among years but varied among crown levels for only a few species. In particular, Apanteles fumiferanae (Viereck), Glypta fumiferanae (Viereck), Smidtiafumiferanae (Tothill), and Trichogramma minutum (Riley) all caused higher mortality in the upper crown of trees. Although infection associated with Nosema fumiferanae (Thomson) and mortality associated with fungal and viral pathogens often varied among crown levels, there was no clear or consistent trend from year to year. In general, trends in spruce budworm density and mortality within the crown were similar throughout all years of our study, despite significant variations in herbivore density, foliage availability, and parasitoid and pathogen impact. Our study indicates that intratree patterns of spruce budworm distribution and mortality are likely to remain consistent during an outbreak and further emphasizes the importance of intratree heterogeneity in shaping interactions within plant-herbivore-parasitoid communities.