PRODUCTION AND PERSISTENCE OF THE NUCLEAR POLYHEDROSIS-VIRUS OF THE DOUGLAS-FIR TUSSOCK MOTH, ORGYIA-PSEUDOTSUGATA (LEPIDOPTERA, LYMANTRIIDAE), IN THE FOREST ECOSYSTEM

The production and persistence of the nuclear polyhedrosis virus of the Douglas-fir tussock moth, Orgyia pseudotsugata, has been determined by periodic sampling of a series of natural and induced epizootics. We have demonstrated that low prevalence rates during the early instars result mainly in larval mortality of older instars which ultimately leads to the greatest production and persistence of polyhedra in the forest ecosystem. Infections in the last two larval instars are responsible for the majority of the polyhedra produced. In addition, virus produced during the early instars is largely inactivated before it becomes incorporated into duff or soil. Active virus once incorporated in the duff layer is subject to little vertical distribution in the soil and remains active for at least 11 years. Direct control measures, applied before nuclear polyhedrosis develops naturally, may prevent the accumulation and persistence of virus in forest duff and soil. The nuclear polyhedrosis virus of the Douglas-fir tussock moth can be reintroduced into the forest canopy through contamination of foliage by dust-borne virus. © 1979.