Development of a qPCR-based method for counting overwintering spruce budworm (Choristoneura fumiferana) larvae collected during fall surveys and for assessing their natural enemy load: a proof-of-concept study.

BACKGROUND: In eastern Canada, surveys of overwintering 2nd instar spruce budworm (Choristoneura fumiferana) larvae ('L2s') are carried out each fall to guide insecticide application decisions in the following spring. These surveys involve the collection of fir and spruce branches in selected stands, followed by the mechanical/chemical removal of larvae. The latter then are counted manually on filter papers, using a stereomicroscope. Considering the significant effort and difficulties which this manual counting entails, we developed a quantitative (q)PCR-based 'molecular counting' approach designed to make this step less tedious. RESULTS: Using the C. fumiferana mitochondrial cytochrome c oxidase 1 (COI) gene as a target for qPCR DNA quantification, we show that the amount of DNA in a larval extract is strongly correlated with the number of larvae used to generate that extract, and that molecular estimates of L2 counts are comparable to those generated using the manual approach. In addition, we used the same DNA extracts to monitor the microsporidian pathogen Nosema fumiferanae, and the hymenopteran parasitoids Glypta fumiferanae and Apanteles fumiferanae in overwintering L2s employing a subset of a TaqMan assay developed by Nisole et al. (2020) for the identification of budworm natural enemies. We show that the proportion of individuals affected by each natural enemy in samples containing a known number of larvae can be estimated from presence/absence data through the binomial probability distribution. CONCLUSION: The present proof-of-principle study shows that a molecular approach for counting L2s and assessing their natural enemy load is clearly possible and is expected to generate reliable results. © 2021 Her Majesty the Queen in Right of Canada. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. Reproduced with the permission of the Minister of Natural Resources Canada.

Wood losses and economical threshold of Btk aerial spray operation against spruce budworm.

BACKGROUND: Spruce budworm, Choristoneura fumiferana (Clem.), causes cumulative defoliation and hence annual growth loss of the balsam fir, Abies balsamea (L.) Mill, host tree. Annual growth increments of mixed balsam fir stands were measured by stem analysis over a 9 year period (1994-2002), when Bacillus thuringiensis ssp. kurstaki (Btk) was applied to control spruce budworm defoliation. With this approach, it was possible to quantify the change in stand volume growth after aerial spray applications of Btk. RESULTS: Differences between the periodic volume increment of protected and unprotected plots were statistically significant, while differences between protected and budworm-free plots were not significant. After 9 years, the difference in periodic increment between protected and unprotected plots was 20 m(3) ha(-1), and the difference in periodic mortality was 20.5 m(3) ha(-1). CONCLUSION: An economic assessment of Btk treatments indicates that biopesticide aerial spraying operations are justified, as they prevented substantial balsam fir mortality and growth losses over the 9 year study.