Forest Science Program

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The decline of diffuse knapweed in British Columbia

Author(s) or contact(s): R.F. Newman, S. Turner, B.M. Wallace, and S. Cesselli
Source: Forest Science Program
Subject: Pest Management, Range Management
Series: Technical Report
Other details:  Published 2011. Hardcopy is available.


There are literature reports of a decline in diffuse knapweed population beginning in the early 2000s at several locations on western North American rangeland. To document changes in certain diffuse knapweed populations in British Columbia, we selected five previously monitored diffuse knapweed-invaded sites located on low-elevation grasslands in the Bunchgrass and Ponderosa Pine BEC zones in the southern interior and sampled these for plant species cover, and abundance of biological control insects. Diffuse knapweed populations and soil seed reserves were shown to decline by an average of 74% and 78%, respectively, at five sites in British Columbia from the 1990s to 2009. Three factors were discussed as possible causes for the decline of diffuse knapweed at the five sites. Climate warming/drying was shown to have occurred over the same period as the reported decline in diffuse knapweed at three Kamloops sites and is a possible contributing factor. Increased plant competition, particularly from bluebunch wheatgrass, may also be a contributing factor; however, the small number of sampling sites in this study cannot provide firm conclusions in this regard. The ubiquitous nature of biological control insects at the five sites, combined with their known abilities to damage knapweed, also places biological control as a possible contributing factor for the decline of diffuse knapweed. It is also possible that two or more of the three factors are acting in concert to reduce diffuse knapweed. The demonstrated decrease in diffuse knapweed at the five sampled sites provides baseline data that may contribute to a better understanding of how biological control agents, climate warming/drying, and improved grazing management may be interacting on weed-invaded sites in British Columbia.

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Updated October 18, 2011