I have been hand picking Spotted Knapweed for more than five years near my home on Highway 20 in an attempt to control it without using any herbicides. While there are less plants than when I started my job will have to continue for many years because of an abundance of seeds that will persist in the soil for years.
I am sure the first plants came in with the road grader probably from Chimney Valley which has large populations of Spotted Knapweed along the road and in adjacent private properties. Last week I was visiting my friend Chris Easthope who has lived in the valley a few decades and he was showing me some of his fields that were once heavily invaded with Knapweed but are now showing a low percentage of the plants on fairly disturbed ground.
His conclusion is that the biological agents introduced a few decades ago are largely responsible for the reduction in the Knapweed populations in the valley. Chris was involved with the weed control program when he was working for the Ministry of of Forests 20-plus years ago and was trying to convince government and industry that the best long-term control was using biological agents. He did get some pressure to continue with the herbicide program but was convinced that the long-term, but slower biological approach, was the best solution. Now two decades later and with lots of proof at his home and many surrounding neighbours he is convinced that the insect approach is the best long-term and less expensive approach as well as less polluting.
I found the following research paper which supports Chris's view.
Impact of Biological Control on Two Knapweed Species in British Columbia by Don Gayton, D. and Val Miller. 2012. Impact of biological control on two knapweed species in British Columbia. Journal of Ecosystems and Management 13(3):1–14.
“Diffuse and spotted knapweed (Centaurea diffusa Lam and C. stoebe L.) are two closely related invasives found in many parts of British Columbia’s Southern Interior, causing substantial economic losses in rangelands. Beginning in 1970, the provincial government initiated a long-term biological control effort against the knapweeds, introducing 10 different insect agents from 1970 to 1987. In an effort to evaluate the efficacy of the program, archival (1983–2008) data was amassed from 19 vegetation monitoring sites that contained knapweed. In 2010, these sites were relocated and remonitored and cover values were analyzed. Diffuse knapweed showed significant declines at 14 of 15 sites; spotted knapweed declined at three of four sites. Possible alternative explanations for the decline are discussed. Evidence strongly points to a suite of biocontrol agents (seed feeders and root feeders) as the primary drivers of Knapweed decline in British Columbia’s Southern Interior”.
Chris still thinks that the use of herbicides should be minimized as it reduces the effectiveness of the introduction of the insects and is costly and can negatively impact many useful native species. A USDA research trial also supports the restriction of herbicides. Chris has also witnessed the useful nature of biological agents to control other weeds like Toad flax.
