Douglas-Fir Tussock Moth- and Douglas-Fir Beetle-Caused Mortality in a Ponderosa Pine/Douglas-Fir Forest in the Colorado Front Range, USA

An outbreak of the Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, occurred in the South Platte River drainage on the Pike-San Isabel National Forest in the Colorado Front Range attacking Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. Stocking levels, species composition, and tree size in heavily and lightly defoliated stands were similar. Douglas-fir tussock moth defoliation resulted in significant Douglas-fir mortality in the heavily defoliated stands, leading to a change in dominance to ponderosa pine, Pinus ponderosa Lawson. Douglas-fir beetle, Dendroctonus pseudotsuqae Hopkins, populations increased following the defoliation event but caused less mortality, and did not differ between heavily and lightly defoliated stands. Douglas-fir tussock moth-related mortality was greatest in trees less than 15 cm dbh (diameter at 1.4 m above the ground) that grew in suppressed and intermediate canopy positions. Douglas-fir beetle-related mortality was greatest in trees larger than 15 cm dbh that grew in the dominant and co-dominant crown positions. Although both insects utilize Douglas-fir as its primary host, stand response to infestation is different. The extensive outbreak of the Douglas-fir tussock moth followed by Douglas-fir beetle activity may be associated with a legacy of increased host type growing in overstocked conditions as a result of fire exclusion.