none Population density and parasitism of larch casebearer, Coleophora laricella (Huebner), were monitored for 10 years before parasites introduced for biological control became abundant. Overwintering densities remained fairly stable at about 100 larvae per 100 buds. The factor most closely correlated with population change was something, as yet unknown, other than the native parasites, which tended to be inversely density-dependent. The data will serve as the "before" period in a before and after evaluation of the biological control program. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Ecology; Population dynamics; Land and freshwater zones; Nearctic region; North America; USA Coleophora laricella (Coleophoridae); Hymenopteran parasites; Agathis pumila & Chrysocharis laricinellae, population density & dynamics relationships; Prevalence; Agathis pumila & Chrysocharis laricinellae (Hymenoptera); Density & dynamics relationships; USA; Population dynamics; Population density; Monitoring & parasitism relationships; Oregon & Washington; Oregon; Washington; Blue Mountains; Population density & dynamics, parasitism relationships Agathis pumila (Braconidae ); Chrysocharis laricinellae (Eulophidae ); Lepidopteran hosts; Coleophora laricella; Prevalence; Coleophora laricella (Lepidoptera); Host dynamics & density relationships; USA; Oregon; Washington; Blue Mountains; Lepidopteran host dynamics & density, prevalence effects