Baculoviruses in populations of western spruce budworm

•Density and generational mortality in western spruce budworm were tracked over 14 years.•A nucleopolyhedrosis virus (ChocNPV) was ubiquitous in wild populations.•Multiple infections with granulosis (ChocGV) and cypoviruses (CoCPV) were common.•Mortality caused by viruses was correlated to generational rates of mortality.•Impact on western spruce budworm was greatest at highest densities.

Population studies of western spruce budworm, Choristoneura occidentalis, revealed that a baculovirus, ChocNPV, was widespread in outbreak populations over a broad geographical area of British Columbia, Canada although the rate of mortality was usually low (<5%). Elevated levels of ChocNPV-related mortality (≈20%) were found when western spruce budworm populations reached high densities (≈300 larvae per kg of Douglas-fir foliage) and contributed to declines in population densities in these areas. A subsample from budworm collections examined using a multiplex-PCR assay showed ChocNPV was the most prevalent virus but also often occurred in combination with a granulovirus, ChocGV and a cypovirus, CoCPV.

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