Differential defoliation and mortality of white spruce and balsam fir by eastern spruce budworm

Abstract

Eastern spruce budworm (Choristoneura fumiferana) is a native defoliating insect in Canada and the United States that has large impacts on forest health. Balsam fir (Abies balsamea) and white spruce (Picea glauca) are its primary host trees, where budworm larvae consume buds and needles during larval development. While Eastern spruce budworm consumes both species, it is generally reported to defoliate balsam fir at higher levels. Investigating patterns and conditions that differentiate species’ defoliation and mortality is informative for understanding susceptibility. We studied an outbreak of spruce budworm at plantations in northern Wisconsin, USA, with defoliation first detected in 2014 that appeared to have higher impacts on white spruce, which is counter to general expectations. In 2019, we quantified spruce budworm impacts on these two tree species and tested which tree characteristics were most associated with tree defoliation and mortality. We found mortality of 60% and 49% at the sites for white spruce trees, while 0% of the balsam fir trees surveyed were dead. We determined that tree species was the main factor related to spruce budworm defoliation, rather than tree diameter at breast height or canopy class and suggest that different timing of species-specific mast-seeding events were related to defoliation and tree mortality.