Noxious weed re-emerges in Oregon

Tansy ragwort re-emerges in Oregon, but still controllable
Biocontrol should keep the noxious weed in check
By Oregon Department of Agriculture

Farmers and ranchers in Western Oregon this summer are noticing patches of the telltale bright yellow flowers of tansy ragwort- a noxious weed once the scourge of the 1970s. But the experts at the Oregon Department of Agriculture say it’s no cause for alarm as successful biological control agents should keep the plant from making an unwelcome comeback to prior levels.

The equation is simple- as the tansy ragwort population grows, so do the populations of flea beetles and cinnabar moths that feed off the weed. It’s all part of a natural cycle, and ODA’s Noxious Weed Control Program believes the good insects will maintain the upper hand.

In the last couple of weeks, ODA has been receiving calls from landowners or their neighbors  anxious about the return of tansy ragwort, now in a very visible stage. The outbreaks are spotty and localized. Still, many Oregonians remember the bad old days when tansy was so invasive in Western Oregon that cattle and horse owners reported more than $4 million in losses each year as their animals grazed on infected pastures. Too often, the leaves of tansy grew among the grasses consumed by livestock in the spring, leading to sickness and death.

At this point in the growing season, the tansy flowers are in bloom and the weed is tall enough for animals to generally avoid by eating around it.

“The late spring and early summer rain has led to a resurgence in some areas,” says ODA entomologist Eric Coombs. “It’s counterintuitive to just let it go right now, but the whole premise of biological control is to allow the insects present to naturally build up on their own.”

Coombs has personally visited many of the sites where tansy has popped up this year and has found the good bugs- the biocontrol agents that help kill the weedy plant- present in all cases. This comes after intensive efforts years ago to release insects in infested areas where the flea beetle and cinnabar moth are now established as part of the natural environment. Due to the cool, wet spring, cinnabar moth populations are very low this year. However, the flea beetles are still active.

“I think we’ve done our job, now it’s time to wait and let the insects to their job,” says Coombs. “It’s all a natural cycle. We will get these flare-ups of tansy ragwort that will move around from field to field depending on factors like the weather or how the field is used. It might be another year or two before the insects build up in numbers again and knock the weed back down. It would take three to five years if the natural enemies had to be reintroduced.”

Tansy ragwort has the distinction of being the only weed for which a Governor’s Task Force was created, leading to a control program housed in ODA that has made effective use of biological control.

“Since the mid-1980s, there has been an estimated $5 million annual benefit from the biological control of tansy ragwort throughout Western Oregon,” says Coombs. “All in all, for every dollar spent in our biocontrol program, the public gets about $13 back in benefits due to the impact of reuniting a noxious weed with its natural enemies.”

The cinnabar moth eats the leaves of tansy ragwort. With the flea beetle working on the roots and ragwort seed fly eating the seeds, the fearsome threesome has worked wonders. It has been almost too good. Much of the weed has been destroyed over the last 25 or so years that there hasn’t been enough tansy to maintain high populations of the bugs. The result this summer has been a sporadic but definite reappearance of the poisonous weed. While ODA would prefer a complete eradication of tansy, realistically, it is not in the biocontrol agent’s best interests to eat up every last plant.

“As long as we can suppress the weed below an economically damaging level, we’ll be satisfied,” says Tim Butler, supervisor of the Weed Control Program.

Tansy ragwort contains poisonous alkaloids that can kill livestock if ingested. Three decades ago, when much of Western Oregon was covered with the weed, cattle and horses were dropping in alarming numbers. Oregon doesn’t appear to be returning to that scenario despite this year’s resurgence of tansy. ODA will be working with Oregon State University’s Veterinary Diagnostic Laboratory to see if any documented cases of tansy poisoning of livestock are reported. The last documented case goes back to 1995.

ODA officials continue to say as long as livestock are not dying due to the poisoning, the insects are doing their job at controlling tansy- even though there have been some very noticeable flare-ups. Patiently waiting for the established biological control agents to build up is still the best course of action. That’s not easy for some farmers and ranchers, and it doesn’t mean there is nothing they can do in the meantime to help.

“If they can maintain good pasture management techniques- fertilization, prevention of overgrazing,  and irrigation to help maintain the competitive advantage of desirable plants species- that all plays a key role in minimizing soil disturbances that lead to emergence of tansy ragwort from the seed bank in the soil,” says Butler.

In addition to overgrazing, factors leading to the resurgence of tansy include construction, logging, fires, floods, and other events or practices in which the soil is disturbed.

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.

For more information, contact Tim Butler at (503) 986-4621.

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