Yellow Starthistle

Photo: Yellow StarthistleCarpets of yellow flowers have appeared on the hillsides around the county, have you noticed? They are buttercups, but the yellow glow sets me to thinking about yellow starthistle. Yellow starthistle was the bane of my existence as a county weed supervisor. It tended to leapfrog ahead of any efforts I made to control its spread. It has devastated thousands of acres of rangeland along the foothills of the Blue Mountains in the surrounding areas.

Yellow starthistle is highly adapted to our local climate conditions. It invades and dominates the annual grasslands common to our region. The plant uses every chance it can to get ahead. Yellow starthistle assaults and dominates south slopes and disturbed sites. In the absence of competitive perennial vegetation, yellow starthistle forms dense stand on vacant lots, pastures and roadsides.

Competitive perennials as a control has been researched by Oregon State University faculty in the Rogue and Grande Ronde Valleys. The most promising solution shown is to replace yellow starthistle and associated weedy annuals with competitive perennials, particularly grasses. Perennials shade the soil surface and weaken starthistle seedlings by limiting access to sunlight's energy and to the deep soil moisture they need for midsummer growth and seed production. Site preparation, seeding, seedling care and supplemental herbicide use are all critical to the successful establishment of perennial grasses.

Biological control insects can contribute to the potential success of a perennial grass community by reducing the seed production of remaining or reintroduced yellow starthistle plants. Six insects- three weevils and three flies- have been established in Umatilla County for biological control of yellow starthistle:

  • Bud weevil, Bangasternus orientalis
  • Hairy weevil, Eustenopus villosus
  • Flower weevil, Larinus curtus
  • Gall-fly, Urophora sirunaseva
  • Larve of two peacock flies, Chaetorellia australis and C. succinea

All of these insects feed on the seedhead. I have seen sites where the hairy weevil has significantly reduced seed production. As of yet the current complex of insects has not demonstrated wide-reaching acceptable control levels.

So keep in mind that next flush of yellow you see on the hillsides around the area will not be cute and cuddly buttercups but the dastardly villain yellow starthistle. Make your plans before the annual invasion begins. Your local extension office and county weed control program can assist you in making effective plans. For additional information on yellow starthistle visit the OSU Extension web site http://www.cerealcentral.com. To signup for releases of biological control agents call 278-5462.

Photo: Yellow StarthisleIdentification

Annual, fall and spring seed germination. Yellow flower with long stiff spines at the end of its bracts. Some seeds have parachute hairs and some don't, resulting in a distribution pattern that produces dense stands and rapid spreading. Toxic to horses.

Economics

Aggressive, adaptable weed that inhabits the growth of desirable plants in pasture, rangelands, and waste land. May become a problem in CRP ground where grass stand is week. Has been found in wheat crops where waste lands are heavily infested.

Chemical Control

Several selective herbicides are available. Residual chemicals are preferred due to the long germination period. Contact your local weed control supervisor or chemical consultant for specific recommendations.

Biological Control

Image: Drawing of Yellow Starthistle Hairy WeevilTwo seed head weevils and a seed head fly have been released in Eastern Oregon in the past 5 years. Contact your county weed control program for availability in your area. Competitive grass plantings and range management practices are key components to a successful yellow starthistle control program.

The possibility of bugging starthistle to death would delight the hearts of weed warriors across the west and here in Umatilla County. The yellow starthistle hairy weevil, Eustenpus villosus, is currently making an effort to do just that - bugging starthistle to death!

Stop and look at the next yellow starthistle patch you drive by. It is easy to check for the presence of the weevil by looking for characteristic damage of the immature seedheads. (See photo.) The adult weevils feed externally on the young flowers and larvae feed within mature flowers.

The hairy weevil was released in the United States in 1990 and in Umatilla County in 1993-1995 by the County Weed Control program. Numerous release sites were made across the county. Each release site received 50-100 weevils. Sites were selected where herbicide control was not being used and where insecticide use was not planned. I visited an area on upper Pine Creek this week within a mile of where the weevil had been released 6 years ago. The weevil was well established in the area. The weevils had significantly impacted the number of flowers per plant and were easy of find on the plants. Year 7 is the year we would expect to start seeing plant stand reduction in the area of the initial release.

If you would like more information on hairy weevils contact Matt Voile, Umatilla County Weed Supervisor, 278-5462, or your local extension office.

More Information

Yellow starthistle (Centaurea solstitialis) made Umatilla County's Dirty Dozen List as the # 6 weed problem in the county. Yellow starthistle has been recognized since the 1920s as a noxious weed in the Pacific Northwest. Infestations were estimated at over 8 million acre, primarily in California, Oregon, Washington, Idaho, in 2001.

Photo: ®2002 Molly Elizabeth Bagley

A Umatilla County weed survey conducted ten years ago identified about 290,000 acres infested in the county. The invasion has not stopped. Yellow starthistle continues to invade rangeland, pastures and roadsides, competing with native plant communities, while forming a monoculture.

As an annual, yellow starthistle relies exclusively on seeds for reproduction. Rangeland plants may average 10,000 to 40,000 seeds per m2, most of which germinates or is lost to predation or decay within three years.

Competitive perennials as a control has been researched by Oregon State University faculty in the Rogue and Grande Ronde Valleys. The most promising solution shown is to replace yellow starthistle and associated weedy annuals with competitive perennials, particularly grasses. Perennials shade the soil surface and weaken starthistle seedlings by limiting access to sunlight's energy and to the deep soil moisture they need for midsummer growth and seed production. Site preparation, seeding, seedling care and supplemental herbicide use are all critical to the successful establishment of perennial grasses.

Biological control insects can contribute to the potential success of a perennial grass community by reducing the seed production of remaining or reintroduced yellow starthistle plants. Six insects- three weevils and three flies- have been established in Umatilla County for biological control of yellow starthistle:

  • Bud weevil, Bangasternus orientalis
  • Hairy weevil, Eustenopus villosus
  • Flower weevil, Larinus curtus
  • Gall-fly, Urophora sirunaseva
  • Larvae of two peacock flies, Chaetorellia australis and C. succinea

All of these insects feed on the seedhead. There are sites where the hairy weevil has significantly reduced seed production in Oregon. As of yet the current complex of insects has not demonstrated wide-reaching acceptable control levels. To inquire about releases of biological control agents call Umatilla County Weed Control at 541-278-5462.

Herbicides can effectively control starthistle. Products listed in the Pacific Northwest Weed Management Handbook are 2,4-D LV, Tordon, Telar, Curtail, Redeem R&P and Stinger or Transline. Several of these products have plant-back restrictions and can harm desirable broadleaf plants. Read and carefully follow the herbicide labels and listed restrictions. The Weed Management Handbook is now available on-line at http://weeds.ippc.orst.edu/pnw/weeds

Some day a creative Molecular Biologist may develop transgenic starthistle plants that contain a controllable gene system that will cause yellow starthistle to self destruct. But until then an integrated approach using several of these management technique will continue to be our best option.

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