By JOHN SCHMITZ

For the Capital Press

Biocontrol agents and proper management practices are the best defense against a reoccurring tansy ragwort problem, according to livestock and weed control specialists.

After being held in check for over 20 years, tansy ragwort, or “tansy,” a persistent weed that’s toxic to certain livestock, has been making a comeback in Western Oregon, said Oregon State University Extension weed management specialist Andy Hulting.

“We’ve gotten into this cyclic (situation) where the weed population builds up, then the biocontrol population builds up, and then the weed population crashes, and it goes back and forth,” Hulting said.

Hulting said that weather conditions the last several years have “promoted” tansy growth while at the same time not favoring biocontrol agents. “It’s just a matter of time until we get those insects built back up.”

Hulting said that letting biocontrols do their job, though it may take a few years for them to reach full force again, makes the most sense in pastures with hard-to-reach areas and in large fields where the cost of chemical sprays is prohibitive and there’s no grazing of susceptible animals.

Beginning in 1960, the Oregon Department of Agriculture introduced three insects that feed on tansy from its native habitat in Western Europe.

Over the next dozen years, ODA distributed the insects throughout the state, focusing on the most heavily infested areas. The descendants of these early populations are now in the process of rebuilding.

“I’ve been walking around looking at a lot of tansy plants this summer and noticed biocontrols on almost every (tansy) plant I’ve looked at,” Hulting said. “They’re out there.”

While changes in Willamette Valley weather several years ago began to favor the return of tansy, it’s been the last two years, and especially 2011, that tipped the scales, said Shelby Filley, OSU regional livestock and forage specialist.

Filley said that in fields where there’s a lot of tansy and little forage, ranchers have three options: fence off the infected area, remove susceptible livestock from the area or remove the tansy by hand.

When there is plenty of good forage available, livestock will usually avoid tansy, Filley said.

Chemicals are not a good alternative, Filley said, not only because most come with grazing and re-entry restrictions, but because they wilt the tansy, making the weed more palatable to livestock.

ODA entomologist Eric Coombs said that if tansy is treated with chemicals or mowed too late, seeds will mature, “which makes treatment a waste of time and money.” Mowing at the wrong time can also turn tansy, a biennial weed, into a short-lived perennial, he said.

Beneficial agents in the program serve mostly to weaken tansy plants, rendering them less able to survive competition from other plants, Coombs said.

Wilt is also a problem when tansy is cut and left in the pasture. Tansy should be cut before seeds mature, then removed from the pasture and the seedy portion handled as garbage and not burned.

Coombs said that he found cinnabar moths at most of the sites he visited, with flea beetles due in October.

Cattle are much more susceptible to tansy than horses and sheep, Filley said, with the lethal accumulative amount in cattle being 3.6 percent of body weight, and with horses 7.3 percent. “In lower amounts you have other problems.”

Online

For more information go to http//:extension.oregonstate.edu/Benton.