Before and after evaluation of biological control of the larch casebearer (Lepidoptera: Coleophoridae) in the Blue Mountains of Oregon and Washington, 1972-1995

The larch casebearer, Coleophora laricella (Hubner), caused severe defoliation in the Blue Mountains in the 1970s. A biological control program using introduced parasitoids resulted in the establishment of 2 species. Agathis pumila (Ratzeburg) and Chrysocharis laricinellae (Ratzeburg). This article adds to earlier evaluations of 13 Blue Mountain plots by contrasting casebearer densities and dynamics before and after the introduced parasitoids became established. In the 10 yr before parasitism by A. pumila exceeded 10%, mean moth density averaged 50.03/100 buds and dynamics were characterized by nondelayed density dependence of large larvae. After a 5-yr transitional period for the populations to stabilize at new levels, moth density averaged 1.63/100 buds over the next 10 yr. A. pumila was associated with reduced casebearer density on all plots. The initial decline on 2 plots was coincident with a large impact by native parasitoids; A. pumila then assumed its apparent regulatory role. In the after period, mortality caused by A. pumila was delayed density-dependent based on significant, positive logistic regressions with casebearer larval density in the previous generation for 8 of the 13 plots. The casebearer population at the reduced density caused little or no significant defoliation. Aside from moderate parasitization levels in a few years at a few plots, C. laricinellae remained at low density, or was not recorded. A. pumila played the major role in this successful biological control