Numerical Behavior of Some Western Spruce Budworm (Lepidoptera: Tortricidae) Populations in Washington and Idaho

The numerical behavior of populations of the western spruce budworm, Choristoneura occidentalis Freeman, was analyzed by using life table data from north-central Washington and central Idaho. The survival rate of instars IV through VI was lowest at budworm densities below ca. 10 4th instars per m2 of foliage. This rate was fairly stable (0.2 to 0.6) across higher densities (10 to 40 per m2 of foliage). The survival rate of pupae from predation increased from ca. 0.2 among the least dense populations to ca. 0.8 among the most dense ones. Survival among the remaining pupae was systematically lower among the populations in Washington than among those in Idaho. The survival rate of eggs and instars I to III was not a function of budworm egg density, place, year, or host species. When density was low, observed egg density was sometimes greater than the ability of resident females to produce eggs. Conversely, at high densities observed egg density was always lower than the egg-laying potential of resident, emerging females. Adult female behavior, together with a generally positive relation between density and subsequent survival during the large larval and pupal stages, may profoundly influence the population dynamics of this pest.