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As you likely know, cinnabar moths and a flea beetle were introduced to our region about 40 years ago in order to limit tansy ragwort which, at that time, was a widespread invasive weed. Even though the project succeeded, populations of both the weed and its natural enemies have waxed and waned which, in the overall scheme of things, is to be expected. It goes like this: When tansy does particularly well due to beneficial weather, the plants supply more food for moths and beetles, which also do well. Then as the insects gain the upper hand, the number of weeds decline.
Here in the Portland area, the tansy is in full bloom and starting to set seed. If the plants are doing similarly where you are, physically removing as many plants as possible will decrease the amount of seed for next year’s crop.
A 2011 news release from the Oregon Department of Agriculture is still pertinent today and lists a contact person. http://www.oregon.gov/ODA/pages/news/110810tansy.aspx Notice the final two paragraphs, namely this: “Pulling or mowing are always available options, but the latter only leads to the weed growing back stronger and heartier next year. Herbicides can be used to control tansy ragwort but need to be applied in early spring before the stalks are formed or late fall after some re-growth of seedlings and rosettes.
“Until then, the best advice is to practice patience and wait for the good bugs to beat the bad weeds.”
Thanks you for the information. Hope the ideas presentated about the current methods of control work. In our area, it seems like the weed in the last few years, has won the battle.
Yes I've also seen quite a bit of tansy ragwort this year here in Washington County. Even so, the infestation seems far less than during the previous resurgence during 2011. Recall that it's an up and down action in the populations of both the weeds and their insect pests.
After reading your orginal post, I went out yesterday and took a look at some tansy plants and saw a few very small cinnabar moths on a few plants. So it does look like they are still around. Guess I better leave a few plants for them to eat.