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Spotted Knapweed

Weed Calculator Available

Tuesday, June 15th, 2010

Have you ever wondered what the real impact is of weeds on your property?

Dr. Matthew Rinella has completed a project that will allow you in about 30 minutes with a pencil, clipboard, yardstick and a homemade measuring frame to figure out how weeds are impacting you. Dr. Rinella’s weed calculator will tell you how many pounds of weeds are on your property and if they were gone how much more good forage you would have.

This tool is great for ranchers and small acreage owners. If you are raising cattle, sheep, a few horses or like to watch elk graze you will find the results valuable.

The tool currently only will provide information on leafy spurge and spotted knapweed. Dr. Rinella hopes to expand this tool for use with other weeds. It is available free online.

Posts related to: Leafy Spurge, Spotted Knapweed
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Compatibility of Seed Head Biological Control Agents and Mowing for Management of Spotted Knapweed

Monday, April 5th, 2010

Authors: Jim M. Story, Janelle G. Corn, and Linda J. White

Summary: Seed head insects, primarily the seed head fly, Urophora affinis, and the seed head weevils, Larinus spp., are reducing spotted knapweed seed production by about 94% in most areas of western Montana. Studies were conducted on the compatibility of seed head biological control agents and mowing for management of spotted knapweed. Our study demonstrated that mowing of spotted knapweed in the spring and early summer can result in the growth of secondary flower buds which escape attack by seed head biocontrol agents, thereby allowing the knapweed to produce a nearly-normal complement of seed. Therefore, we conclude that the historical practice of mowing spotted knapweed during the spring and early summer with no follow-up mowings should be avoided if large populations of seed head biocontrol agents are present.

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Influence of Seed Head-Attacking Biological Control Agents on Spotted Knapweed

Wednesday, March 24th, 2010

Complete Title: Influence of Seed Head-attacking Biological Control Agents on Spotted Knapweed Reproductive Potential in Western Montana over a 30-year Period

Authors: Jim M. Story, Lincoln Smith, Janelle G. Corn & Linda J. White

Summary: Studies were conducted on the impact of seed head insects on spotted knapweed reproductive potential in western Montana over a 30-year period. Results indicated that seed head insects are reducing seed production by about 94% in many areas of western Montana. The reduction of knapweed seed production has resulted in a 98% reduction in the knapweed seed bank which, in turn, has contributed to the decline of knapweed in these areas. Of the seed head insects, the fly, Urophora affinis, and the weevils, Larinus obtusus and L. minutus, are having the greatest impact on knapweed seed production. The current decline of spotted knapweed in many areas is likely due to the impact of the seed head insects on the knapweed seed bank and the effects of the root weevil, Cyphocleonus achates, on mature plants.

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Decline of Spotted Knapweed Density in Montana with Large Populations of the Root Weevil

Monday, March 15th, 2010

Full Article Name: Decline of spotted knapweed density at two sites in western Montana with large populations of the introduced root weevil, Cyphocleonus achates (Fahraeus)

Authors: J.M. Story, N.W. Callan, J.G. Corn, L.J. White

Summary: Spotted knapweed plant density was monitored over a 11-year period (1993-2004) at two sites in western Montana where the root weevil, Cyphocleonus achates was released. Spotted knapweed density declined significantly over time at both sites (99% and 77%, respectively), after C. achates numbers increased dramatically at both sites. The current decline of spotted knapweed in many areas is likely due to the effects of C. achates on mature knapweed plants and the impact of the seed head insects on the knapweed seed bank.

Click here to download and read the complete article.

Transportation of Spotted Knapweed Seeds by Vehicles

Wednesday, January 27th, 2010

Vehicles have long been suspected of being a major distributor of spotted knapweed seed. The purpose of this experiment was to determine how many seeds could be disseminated by vehicles and to measure how far the seeds would travel. Sites were chosen at the Story Hills in Bozeman, the old Milwaukee Road rail yard in Deer Lodge and a site near the Bauxendale Volunteer Fire Department west of Helena. At each site, twelve plots each measuring twelve by forty feet were staked and the surrounding area was mowed. A late model pickup truck was driven 40 feet into each plot and backed out. The vehicle was then driven 25 mph for distances of 0, 0.1, 1.0, and 10.0 miles. The vehicle was placed on a large tarp and the entire undercarriage was vacuumed to collect spotted knapweed seeds and plant material. The collected material was bagged to await seed counting. At the Deer Lodge site, over 1633, 510, 226 and 138 seeds remained on the vehicle after traveling 0, 0.1, 1.0, and 10.0 miles. The results indicate that spotted knapweed seed is readily disseminated by motor vehicles for long distances.

Prepared by:

Phil Trunkle and Pete Fay
Department of Plant and Soil Science
Montana State University
Bozeman, Montana 59717

Download a PDF version of this document here.

Posts related to: Spotted Knapweed
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Impact of Knapweed on MT Economy

Tuesday, July 23rd, 1996

Steven A Hirsch and Jay A Leitch

The economic impact of thee invasive, exotic weeds–diffuse, spotted, and Russian knapweed (Centaurea diffusa, C. maculosa, and Acroptilon repens)–on Montana’s economy was estimated using a procedure developed for another invasive weed species. Published data and that from a survey of county weed boards were used to estimate direct negative impacts of over $14 million annually due to infestation of over 2 million acres of rangeland and wildland. This amounts to about $10.63 on each infested grazing land acrea nd $3.95 on each infested wildland acre. Direct plus secondary economic impacts, estimated using an input-output model, are about $42 million annually,, which could support over 500 jobs in the state’s economy. This first approximation suggests the knapweed infestation problem in Montana deserves attention, although more work could be done to refine these estimates and to allow estimation of he impacts at sub-state levels.

Link to Full Article

Montana Weed Control Association

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