The near-eradication three decades ago of tansy ragwort, one of Oregon’s most problematic invasive species, was considered a crowning achievement because of innovative biological control methods.

But this year, tansy ragwort has flourished throughout western Oregon through a combination of weather conditions and a “down cycle” for the three insects used to control this toxic weed, continuing a disturbing recovery trend that began six years ago. Suddenly, a new generation of Oregonians, including numerous owners of small farms, ranches and other acreage, are being forced to deal with tansy before it kills their cattle or horses.

A moderate La Niña event followed by a cool, wet spring has created a double-whammy in the state, according to Andrew Hulting, an Oregon State University Extension weed management specialist.

“The same weather conditions that promote the growth of tansy also have limited the number of beneficial insects used to control the weed,” said Hulting, a faculty member in OSU’s Department of Crop and Soil Science.  “The insects are in a down cycle and they may take a few years to recover.”

Tansy ragwort, a member of the sunflower family, was introduced into Oregon in the early 1920s and within three decades became a major problem. It contains toxins that killed thousands of livestock animals – mostly cattle and horses – and contaminated pastures and hay, according to Eric Coombs, a biological control entomologist with the Oregon Department of Agriculture. By the 1970s, Coombs said, many western Oregon pastures, hillsides and logged areas were heavily infested with the plant.

Beginning in 1960, the Oregon Department of Agriculture introduced three insects that feed on the toxic weed – the cinnabar moth, the tansy ragwort flea beetle and a seed head fly – all imported from tansy’s homeland in Western Europe. Over the next dozen years, ODA distributed the insects throughout the state, focusing on the most heavily infested areas.

By the mid-1980s, tansy ragwort was more or less under control, Coombs said, and that added some $5 million annually in economic benefits to Oregon agriculture.

“Tansy maintained a low profile in Oregon until 2005, when a winter drought was followed by a warm, wet spring, which created the conditions for resurgence of this pernicious weed,” Coombs said. “Because tansy populations were low, so were the biocontrol agent populations that depend on the weed. This ‘boom and bust’ cycle in a natural cycle and it will take several years for the insects to build up and re-control the weed.”

The tansy ragwort flea beetle lays eggs on or near the plants and larvae burrow into and feed on the roots, killing or injuring the weed. Cinnabar moths also deposit eggs on tansy, and when their offspring reach the caterpillar stage, they can defoliate the plants. The ragwort seed fly is less effective, but still aids in tansy control when their larvae feed on seed heads.

Timing is key

There are, however, things that landowners can do on a short-term basis to combat tansy, according to Hulting. The key, he adds, is timing.

“In addition to the biological controls, tansy can be controlled by chemical applications and manual removal,” Hulting said. “But it has to be done at the right time of the year. Late fall or early spring is actually the best time to spray herbicides, when new seedlings are in the rosette stage and the plants have fresh growth. Herbicides applied in the summer, when the tansy is blooming, are less likely to be effective.”

Mowing pastures after seeds have matured on the plant is not recommended, Hulting says, because the seeds can spread – and they have a long life in the soil. Seeds can lie dormant for as long as 10 years, then germinate. Mowing also limits the food source for the biocontrol insects.

“There is a background population of tansy that’s always out there,” he said. “If the acreage you’re trying to control tansy ragwort in is small, you can dig it out and burn it, bury it, or seal it and take it to a landfill. Composting generally doesn’t work because the average compost pile doesn’t get hot enough to kill the weed seeds and then you’ve just created a new tansy patch.”

The OSU Extension Service and the Oregon Department of Agriculture have collaborated on a guide to tansy ragwort biology and management that offers suggestions on how to control it. The publication is available online at: http://bit.ly/n5B7O5

Aurora Villarroel, an Extension veterinarian with the OSU College of Veterinary Medicine, said Oregon already has experienced cases of cattle this year that have been stricken with toxicity from ingesting tansy ragwort. “Tansy can kill horses and cattle, and it can trigger abortions in pregnant animals,” she said. “That happened in a case earlier this year in the Willamette Valley.”

Animals that have eaten tansy may appear listless, they may quit eating and drinking, Villarroel said. Often they will lie down and are too weak to get up.

“If you have 2-3 animals displaying the same signs,” Villarroel said, “always call your veterinarian, because you may be dealing with some kind of poisoning.”

Interestingly, sheep are not affected by the toxins in tansy, and are used by some farmers to clear fields of tansy before cattle go to pasture.

In the long term, biocontrols may offer the best approach to getting tansy ragwort under control once again, Hulting said. Buying the insects can be expensive – and probably isn’t necessary, he added.

“If you do nothing, the insects eventually will show up,” he said. “But if you can’t keep your animals from a pasture that has tansy in it, you may want to use chemical controls, or manually remove it. Fortunately, tansy is easy to kill. But it also spreads easily, so it’s important to do it right.”