Concerns on the RVC Spruce Budworm Report
Shorter Version 2 to HCN
In the December 2020 edition of HCN, Councillor Hanson writes that a November 30, 2021 motion was passed by RVC Council “directing Administration to continue monitoring and assessing the spruce budworm infestation in Bragg Creek and the surrounding area to determine if and when a more active management strategy becomes beneficial.” In apparent support of the motion is the 2021 report commissioned by RVC, “Spruce Budworm Report For Rocky View County” prepared by Toso Bozic of ATTS Group.
Two management strategies over the next 2-5 years are identified in the ATTS report, 1. Continued Monitoring and Assessment, and 2. Aerial Spraying with BtK (Bacillus thuringiensis var. kurstaki) and the chemical tebufenozide.
RVC has chosen strategy 1. Continued Monitoring and Assessment. This strategy is apparently based on the statements in the report that predatory birds, insects, and a variety of viral, bacterial and fungal diseases can help reduce or even collapse the budworm infestation. The ATTS report says “Studies in BC show that upwards of 75% of spruce budworm mortality occurs due to naturally occurring pathogens and parasites. Weather events such as cold springs with late frosts and/or cold and wet summers can greatly reduce budworm populations. In all likelihood, allowing nature to run its course will result in the collapse of spruce budworm populations (7).” These statements are attributed to reference 7, Nealis and Regniere, 2021. “Ecology of outbreak populations of the western spruce budworm”. Ecosphere 12(7):e03667.
The above quote from the ATTS report suggests that naturally occurring pathogens, parasites and weather events (e.g., late frosts, wet summers, cold springs) are likely to result in the collapse of our budworm infestation. However there is a key finding from Nealis and Regniere 2021 that is missing from the ATTS report and needed for infestation collapse. Nealis and Regniere examined a massive forest budworm outbreak affecting Douglas Fir between 1997 and 2016 in the interior of British Columbia. “Parasitoids and pathogens can cause sudden declines in densities, but [budworm] populations inevitably wane as a result of cumulative degradation of their resource.” Resource here is identified as the conifer needles that budworms feed and overwinter on.
In other words, the primary determinant of the collapse of budworm infestations is the loss of food supply and overwintering habitat caused by budworm defoliation. This is the critical factor that is missing from the ATTS report and which is needed for RVC to fully understand the nature of a budworm infestation and collapse.
The study did say that the combination of both defoliation (food and habitat loss caused by budworms) and natural enemies are the ultimate cause of population collapse. Although natural enemies may be the ultimate agent that brings about the collapse, defoliation is an important cofactor in this. As noted by the authors, “systemic negative effects of host damage [defoliation] on budworm survival occur earlier in the life cycle, in the non-feeding stages.
These effects are evident as gradual but sustained decreases in survival of the early-larval stages”.
I think it is important to clarify the statement by ATTS that B.C. studies have shown up to 75% budworm mortality can be attributed to naturally occurring pathogens and parasites. Nealis and Regniere 2021 are clear that this finding is based on budworm larvae and pupae harvested in the field and artificially kept alive in the laboratory until death. Parasites that emerged “from budworm cadavers were recorded as the cause of mortality.” How this methodology relates to a real forest habitat is unclear and not discussed in the study.
Monitoring and Assessment may indeed be the way to go, but the suggestion in the ATTS report of relying on weather, predation and disease is neither transparent to or supported by the work of Nealis and Regeniere who clearly show many years of defoliation and the loss of budworm food and habitat are critically necessary for a natural collapse. The Douglas fir forest outbreak in the interior of BC lasted for 10 to 15 years before naturally collapsing… Is this what we want?
Dennis Stefani
