Recovery Plan for the Northern Spotted Owl - DRAFT

Insects cause many problems in Pacific Northwest forests and there are thousandsof species (Furniss and Carolin 1977). Only a few, however, have majorimpacts on forests. Insects in Table F.4 are listed according to their activity inforests; e.g., defoliators, terminal miners, bark beetles, aphids and scale insects,and wood borers. Other parts of trees, such as cones, are affected, butthese are not included in Table F.4 because they have little impact on foreststructure. Note that there are generally fewer insect species having a majorimpact on forests in the West Cascades subregion than in the other two subregions.Defoliators and bark beetles are important in the East Cascades andKlamath subregions. There, they can create large landscape level disturbancecausing tree mortality over thousands of acres. In the West Cascades subregiondisturbances are smaller, but occasionally large epidemics of defoliatorsoccur; e.g., the western hemlock looper which had outbreak periods in 1889-1991, 1911-1914, 1929-1932, 1943-1946, and 1961-1963. These outbreaksgenerally occurred in extensive old-growth hemlock stands (Furniss andCarolin 1977).Each insect has its own ecological niche and function in western forests andeach kind of tree is host to many insects. Some tree species, however, aremore attractive to insects than others. For example, Pines and oaks are infestedby far more insects than are redwoods and yew. Some insects play amajor role in the life cycle and structure of extensive forest areas, such as thewestern pine beetle in ponderosa pine forests (Furniss and Carolin 1977). Withforest harvesting and management many of these insects became destructiveand now are controlled artificially.Most western forest insects are native, although a few, such as the balsamwooly adelgid, have been introduced. Numbers of insects tend to fluctuatewidely and some periodically become epidemic, such as the Douglas-fir tussockmoth and spruce budworm. A number of bark beetle species also has becomeepidemic, especially in drier areas. Fire control practices and early forest harvestingpractices in the last 75 years are generally thought to have contributedto increases in bark beetle populations by creating populations of stressedtrees. Bark beetles tend to cause the highest amounts of insect-related mortality.In healthy ecosystems insects generally remain in small numbers withoutbreaks often precipitated by plant stress (e.g., caused by drought, nutrientdeficiencies, and air pollution). With predicted global climate change, manyplant species could become stressed as temperature and precipitation changebeyond their tolerance level (Perry and Borchers 1990). This, plus the fact thatinsect populations usually grow faster in warmer, drier climates, could lead tolarge outbreaks. However, climate changes may suppress some insects.Defoliating insects cause reduced radial growth and height increment, topkilling,and reduced regeneration; they also sometimes kill extensive stands(Swetnam and Lynch 1989). Heavy defoliation by Douglas-fir tussock moth ofDouglas-fir and grand fir in north central Idaho caused growth of host speciesto decrease 75 to 90 percent in 1 year (Brubaker 1978). Normal growth ratesreturned 3 to 4 years after maximum defoliation, however. Some researchersreport increased radial growth of trees after defoliation (Stoszek 1988.Wickman 1990). Lowest levels of defoliation usually are associated with laterstages of succession (old-growth) (Stoszek 1988).Many of the trees killed by bark beetles and defoliators also are being attackedby root diseases and it is sometimes difficult or even impossible to separate theeffects of insects and diseases, especially in drier forest types. Snags and downwood from insect-killed trees increase forest-fire hazard but provide importantwildlife habitat.437