An assessment of the susceptibility and vulnerability of eastern white pine (Pinus strobus L.) stands to pine false webworm (Acantholyda erythrocephala (L.)) in northern New York

This study assessed the impact of pine false webworm (Acantholyda erythrocephala (L.), Hymenoptera: Pamphiliidae) defoliation on eastern white pine (Pinus strobus L.) volume growth, as well as the site/stand conditions under which defoliation and damage occur in northern New York. Stem analysis of sawtimber-sized white pines revealed that mean annual volume increment was reduced by more than 50% and 95% by the third and fifth years of defoliation, respectively. Dramatic growth reduction occurred along the entire stem but began in the lower and mid bole. Missing and discontinuous rings were common in the lower bole but became less frequent with increasing height on the stem. Latewood tracheid reduction did not occur in the first year of defoliation and is not a reliable indicator of the first year of pine false webworm outbreaks. Increment core analysis revealed a temporal progression of radial growth impact that tracked the expansion of the webworm outbreak from its epicenter.; Stand susceptibility (as measured by pine false webworm density and defoliation) was a positive linear function of stand size. Holding stand size constant, susceptibility decreased as the A-horizon became finer-textured. Tree-species diversity and the relative dominance of white pine in the stand were negatively and positively correlated, respectively, with stand susceptibility. Population collapse between 1998 and 1999 was more pronounced as both elevation and soil nitrogen increased. Holding elevation or soil nitrogen constant, population collapse was more pronounced as the number of years of defoliation in the stand increased. On average, white pine stands located on glacial lake shoreline/delta sand deposits in the region were more susceptible to pine false webworm than stands on other landforms. Stand vulnerability, as measured by white pine mortality and missing ring frequency, decreased as the mean live crown ratio of the stand increased. Vulnerability to radial growth loss decreased as B-horizon carbon percentage and forest floor thickness increased. The possible mechanisms behind these susceptibility and vulnerability trends are discussed.