Starch content of conifers defoliated by the Douglas-fir tussock moth

Abstract

Studies were conducted in British Columbia, Canada, and New Mexico in areas where the tussock moth (Orgyiapseudotsugata (McDunnough)) had heavily defoliated white fir (Abiesconcolor (Gord. and Glend.) Lindl.) and Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) to determine the relation between reserve energy and vigor of stressed trees. Starch, a measure of reserve energy, was found to be reduced in the twigs proportional to the estimated defoliation of sapling-sized trees; starch in the bark at breast height of large trees was also reduced with increased defoliation. Twig starch content did not differ for defoliated Douglas-fir and white fir growing on the same site. Defoliation by the tussock moth accelerated the normal seasonal loss of starch in twigs; this finding was confirmed with artificially defoliated seedlings. Additionally, starch in all vegetative parts of trees was reduced as defoliation intensified. Feeding by tussock moth larvae is expected to reduce reserve energy more if egg hatch closely follows bud burst.