[PDF] Longitarsus Jacobaeae Swiss Biotype Effect On Tansy Ragwort Senecio Jacobaea In Naramata British Columbia eBook
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The flea beetle, Longitarsus jacobaeae (Swiss biotype), exhibited a classical biological control pattern on its host plant tansy ragwort (Senecio jacobaea L.) in Naramata, B.C. by rapidly increasing in number after its release and causing the decline of the plant. L. jacobaeae's presence was identified by approximately 1.6-mm2 feeding holes in the ragwort's leaves. L. jacobaeae fed preferentially on rosettes (p=0.0333) as opposed to bolted plants (p=0.4621). It caused a visibly significant decrease in the number of rosettes and bolted plants combined at the trial sites in 3-4 years (p=0.0221). From the point of release, L. jacobaeae dispersed 100 m over the study site in 4 years.
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.