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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