Radial growth impact of pine false webworm defoliation on eastern white pine

Abstract

The impact of pine false webworm (Acantholyda erythrocephala (L.), Hymenoptera: Pamphiliidae) defoliation on the radial growth of mature eastern white pine (Pinus strobus L.) during an outbreak was assessed using a stem analysis comparison of two stands (defoliated vs. control) and increment cores collected from 21 defoliated stands and 5 control stands in northern New York State. Stem analysis revealed that whole-stem standardized annual volume increment (AVI) in a defoliated 67-year-old white pine stand (n = 10 sample trees) was reduced significantly below the AVI in the nondefoliated control stand (n = 8 sample trees) by the second year of moderate to heavy defoliation, and AVI was reduced by 97% by the fifth year of defoliation. No time lag between upper-bole and lower-bole impact was observed, and annual growth rings were more frequently missing or discontinuous at lower stem heights. The standardized latewood tracheid index was not reduced significantly below control stand levels until the third year of defoliation. Increment-core analysis revealed growth losses that corresponded with reported periods of defoliation in nearly all stands; sustained suppressions (5–16 continuous years) below a growth index of 0.5 occurred in over half of all defoliated stands. These results are discussed in relation to pine false webworm biology, comparisons with other conifer defoliators, environmental factors, and methods employed.