CORVALLIS, Ore. -- 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." Read More Here

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