Tansy Ragwort: A success story

Kudzu, Japanese Beetles, Asian Longhorned Beetles, Zebra Mussels, Asian Carp, Varroa Mites.

This is of course but a short list of invasive species. All of these were brought to the U.S., either by accident or intent, where they found ecosystems that were poorly adapted to control their proliferation. Eventually, we can expect the populations of such exotics to balance out. Some local plants will develop a resistance to their attacks, and some local predators will develop a taste for them. Unfortunately, natural balancing is a slow process by human standards, so other solutions are attractive. One such solution is to intentionally import another species which will keep the exotic species in check. This is called a biological control.

Wait a minute … isn’t that how we got into this trouble in the first place?

Most scientists are understandably wary of biological controls. Ecosystems are big complex things, and it’s hard to be certain that there will be no unwanted side effects. In a recent post, I’ve mentioned the blackberry rust which is spreading across Oregon. Our forages class covered another case of biological control in Oregon, and a few days later we found an example on our property.

This is Tansy Ragwort:

It is a biennial plant native to Europe. Tansy contains high levels of alkaloids which cause irreparable liver damage and death in horses and cattle, but have no effect on sheep. It was first found in Oregon in 1922, and by the 1970s it was causing heavy losses in the beef and dairy industries. Sheep were used as a limited control, as they will readily eat the foliage, but a more effective solution was sought. From among the 60 insect species that feed on Tansy in it’s native ecosystems, 3 were approved for import into Oregon as biological controls. Of these, two have been extremely effective. The Cinnabar Moth and the Tansy Flea Beetle. The larvae stage of the Cinnabar Moth is the most high profile of the two, feeding exclusively on Tansy during the summer. You can at times barely see the plant for all the orange and black striped caterpillars. But if the moth larvae weakens the plant while it is trying to set seed, the Flea Beetle larvae truly kills it. The larvae stage of these beetles feed on the root systems of Tansy, killing many plants outright. Those that survive then become a food source for the adult form of the Flea Beetle. Both of these insect controls work well, because Tansy is their exclusive food source.

When we found this specimen of Tansy Ragwort growing in the disturbed soil of the chicken coop tracks, we looked at it very closely and …

Have at it little guys. This plant is for you.

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2 Responses to Tansy Ragwort: A success story

  1. Michelle says:

    In my experience, the beetles ravage but do not kill tansy; the plant still matures and produces flowers and seeds. Here (we have horses as well as sheep) I must pull it up, or it spreads.

  2. lee says:

    Hi Michelle – Our class instructor said that it was the larvae stage of the beetles that did the most damage. Those are the plants that simply don’t make it. The adult form of the beetle puts holes in the leaves, but doesn’t prevent flowering. Biological controls are not 100%, and in fact a certain population of Tansy is necessary to sustain the population of predatory insects. The Oregon fact page has an older photo with huge expanses of solid Tansy. If it spread to that density today, the subsequent boom in insect population would almost eradicate it from the local area. Some accounts talk of cattle ranchers suffering 10% losses yearly from Tansy poisoning in the 70s. The biological controls have been very successful at preventing that level of loss.

    We’re planning to raise sheep, so it doesn’t worry me too much, but I’ll still cut down the plant before it sets seed. No need to spread something that might hurt my neighbor’s horses.

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