Medusahead and Ventenata in the Northern Great Plains Ecoregion: Invasion History and Management Efforts

By Brian Mealor, Beth Fowers and Luke Sander. Presented at the Western Society of Weed Science annual meeting (2018)


The invasive winter annual grasses medusahead (Taeniatherum caput-medusae) and ventenata (Ventenata dubia) have a relatively long history of spread and impact in the Intermountain West. In 2016, self-sustaining populations of both species were documented in Sheridan County, Wyoming, representing the first known populations of each species in the Great Plains region.

The Northeast Wyoming Invasive Grasses Working Group formed in direct response to these new invasive grass populations with a primary goal of minimizing impacts to rangelands for wildlife and agriculture by reducing, containing, or eradicating medusahead and ventenata in northeast Wyoming. The working group is implementing an EDRR approach by collecting and sharing distribution data, strategically implementing control actions, and monitoring efficacy of treatments.

In 2017, more than 22,000 acres were intensively surveyed for presence of medusahead and ventenata, with significantly more acreage informally added to the species distribution via collaborators and citizen-scientists. While the current known distribution of medusahead is relatively restricted, the outer boundaries of the known ventenata range in Wyoming went from one observation prior to 2016 to well over 1 million acres of gross acres in February 2018.

Observations from the collaborative working group emphasize the importance of education and outreach in EDRR programs to the contributions of diverse partnerships in such an effort. Future efforts will incorporate vector-pathway analysis coupled with remote sensing to prioritize high-likelihood sites of future invasion for medusahead.

Click here to see a poster describing survey methods, distribution, and management of invasive grasses in the project area.

The following expanded information was provided by Dr. Brian Mealor, Director and Associate Professor, Sheridan Research and Extension Center, Sheridan, Wyoming.

  • Herbicide treatments developed by the working group focused primarily on medusahead locations as part of an eradication goal. As more distribution data is gathered, the focus of the program may shift from eradication to containment and control.
  • All treated areas had both medusahead and ventenata; however, medusahead has been the primary target.
  • The combination of Milestone® specialty herbicide at 7 fluid ounces per acres (fl oz/A) in combination Plateau at 7 fl oz/A was selected for application based on:
  1. Grazing flexibility
  2. Inconsistent control results with Plateau alone across annual grasses, although field research conducted in the West shows good efficacy on medusahead.
  3. Field trials conducted in California with Milestone indicate medusahead suppression.
  4. Significant infestations of sulfur cinquefoil (Potentilla recta) occur in combination with medusahead and ventenata. Milestone provides excellent control of sulfur cinquefoil.
  • Aerial application volume was 5 gallons per acre applied prior to annual grass emergence or early post emergence.

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Milestone specialty herbicide is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.

When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. Consult the label before purchase or use for full details. Always read and follow label directions.

Developing a Detection Method for New Invaders at the Landscape Scale

By Lisa C. Jones and Timothy Prather, University of Idaho. Presented at the Western Society of Weed Science Annual Meeting, 2018.

Abstract: The ability to predict plant invasions and detect them early in the process are important considerations for invasive plant management. While agencies and landowners typically take the approach of on-the-ground searches and some may utilize habitat suitability models, these tools may not facilitate detection of incipient infestations when the species is unknown. We set out to develop a method to identify where to look for a new invader to assist managers in focusing search efforts to areas more prone to invasion. We used habitat suitability models (also referred to as species-specific susceptibility models) of seven plant species to investigate whether creating weed “hotspots” of overlapping models was an effective tool to infer areas more invaded within the boundaries of a 4,200-ha ranch in southern Idaho. We tested this by sampling vegetation cover by species, in five, 0.125 m2 quadrats placed along each of 24 transects located in areas modeled to be suitable habitat for either zero, two, four, or six weed species located in the northeast section of the ranch. Since it is well-documented that roads and trails provide corridors for dispersal, we located transects either near (within 60 m) or far (more than 60 m) from unimproved roads. We hypothesized that non-native species richness and/or cover would be higher in hotspots where a greater number of suitability models overlapped closer to roads. Of the 46 unique species in our quadrats, five species (11%) were non-native, of which Japanese brome (Bromus japonicus) and downy brome (Bromus tectorum) were the most abundant. Among non-native species, there was no significant difference in richness or foliar cover between hotspots or proximity to roads. Among native species, richness and foliar cover were not significantly different between hotspots, but they were curiously greater in transects closer to roads. To further aid the development of a detection method for new invaders, we examined indicator species that are positively or negatively associated with Japanese and downy brome. Notably, when downy brome cover was high, two perennial native forbs were in greater abundance, and when downy brome was not present, Sandberg’s bluegrass (Poa secunda) cover was high. There were no positive indicator species for Japanese brome, though there were 11 native species negatively associated with it. Overall, our initial foray to develop a detection method using existing weed habitat suitability models was not successful in identifying areas at greater risk of invasion as evidenced by current diversity and cover of non-native species. However, we recognize the limits of our small sample size and narrow extent of the area surveyed (15% of the ranch). Identifying sites at high risk to invasion when the life history traits and environmental niche of the invader is unknown is a complex challenge, but one that has the potential to help land managers prioritize areas for invasive plant monitoring. Future tests will investigate if there are specific modeled weed species combinations that are suggestive of areas generally susceptible to invasion; for example, more non-native species were along transects located where leafy spurge (Euphorbia esula) habitat was predicted. Further, indicator species may be used to reveal which models are better candidates for estimating invasibility.

Timing Aminopyralid to Prevent Seed Production Controls Medusahead (Taeniatherum caput-medusae), and Increase Forage Grasses

Medusahed Grass.jpg

By Matthew J. Rinella, Josh S. Davy, Guy B. Kyser, Fadzayi E. Mashiri, Susan E. Bellows, Jeremy J. James and Vanelle F. Peterson

Presented at Western Society of Weed Science Annual Meeting, 2018.

Abstract: Exotic annual grasses such as medusahead [Taeniatherum caput-medusae (L.) Nevski] and downy brome (Bromus tectorum L.) dominate millions of hectares of grasslands in the western United States. Applying picloram, aminopyralid, and other growth regulator herbicides at late growth stages reduces seed production of most exotic annual grasses. In this study, we applied aminopyralid to T. caput-medusae to determine how reducing seed production in the current growing season influenced cover in the subsequent growing season. At eight annual grassland sites, we applied aminopyralid at 55, 123, and 245 [ 1 ] g ae ha −1 spring just before T. Caput-medusae heading. The two higher rates were also applied pre-emergence (PRE) in fall to allow comparisons with this previously tested timing. When applied in spring during the roughly 10-d period between the flag leaf and inflorescence first becoming visible, just 55 g ae ha −1 of aminopyralid greatly limited seed production and subsequently reduced T. caput-medusae cover to nearly zero. Fall aminopyralid applications were less effective against T. caput-medusae, even at a rate of 245 g ae ha −1. The growing season of application, fall treatments, but not spring treatments, sometimes reduced cover of desirable winter annual forage grasses. The growing season after application, both spring and fall treatments tended to increase forage grasses, though spring treatments generally caused larger increases. Compared with other herbicide treatment options, pre-heading aminopyralid treatments are a relatively inexpensive, effective approach for controlling T. caput-medusae and increasing forage production.

Invasive Plant Science and Management abstract available here.

[ 1 ] Aminopyralid rates of 55, 123 and 245 grams ae/ha are equivalent to Milestone® specialty herbicide at 3, 7 and 14 fluid ounces per acre respectively.

Canada Thistle Affects Herbage Production in the Northern Great Plains

Canada Thistle (Cirsium arvense) Affects Herbage Production in the Northern Great Plains (2017). Travis R.Carter and Rodney G.Lym. Invasive Plant Science and Management.  

canada thistle flower.jpg

Abstract: Canada thistle can cause greater than 50% yield loss in small grain crops, but little is known about production losses when the weed invades pasture and wildlands. Change in grass, forb, and woody species production from Canada thistle infestations was evaluated in two separate studies in North Dakota. The first measured change in production following aminopyralid applied at 120 g ha−1 to control Canada thistle at two prairie sites. In general, grass, broadleaf, woody, and total plant yields were similar between treated and untreated prairie, regardless of the near-complete control of Canada thistle following aminopyralid application. Grass yield increased by 365 kg ha−1 the year after treatment at one location, with no change in forb or woody species production. Plant production was also estimated at 20 ungrazed wildland preserves located within two Major Land Resource Areas (MLRAs). Similar to the prairie sites, minimal differences in production between Canada thistle–infested and noninfested sites were observed. The only exception was an increase in grass production of 425 kg ha−1 at one of the MLRAs, with no change in broadleaf or woody species production between the Canada thistle–infested and non-infested sites. In contrast to cropland, pasture and wildland production of other species was not consistently reduced by Canada thistle.


economic threshold for control of canada thistle with aminopyralid

 FIGURE: Economic threshold for control of Canada thistle with aminopyralid applied at 120 g ha-1 based on forage response at Fargo and the Rolling Soft Shale Plains in North Dakota in 2015 and 2016.  Maximum stem density at the study sites was 27 stems m-2 in Fargo and 35 stems m-2 in the Rolling Soft Shale Plains. Gray segments were an extrapolation beyond observed Canada thistle densities to estimate cost-effective stem density required for treatment based on forage response. The standard error of the mean was 10.1 and 8.3 at the Fargo and Rolling Soft Shell Plains locations, respectively (Rod Lym, personal communication 2018).

FIGURE: Economic threshold for control of Canada thistle with aminopyralid applied at 120 g ha-1 based on forage response at Fargo and the Rolling Soft Shale Plains in North Dakota in 2015 and 2016.  Maximum stem density at the study sites was 27 stems m-2 in Fargo and 35 stems m-2 in the Rolling Soft Shale Plains. Gray segments were an extrapolation beyond observed Canada thistle densities to estimate cost-effective stem density required for treatment based on forage response. The standard error of the mean was 10.1 and 8.3 at the Fargo and Rolling Soft Shell Plains locations, respectively (Rod Lym, personal communication 2018).

For information on the Impact of Canada Thistle Cover on Plant Community Structure in Early Stage Prairie Restoration go to

Constructing Standard Invasion Curves from Herbarium Data—Toward Increased Predictability of Plant Invasions

by Pedro M. Antunes and Brandon Schamp. Invasive Plant Science and Management, 10(4):293-303. 2017.


Is it possible to predict which nonnative plant species will become invasive weeds and when? Authors explore challenges related to developing invasion curves for plants using herbarium data.  The goal is to better position herbaria and researchers to assist natural resource managers in prioritizing needs, supporting management decisions and developing prevention and monitoring programs.  


Secondary Invasion and Reinvasion after Russian-Olive Removal and Revegetation


Erin K. Espeland, Jennifer M. Muscha, Joseph Scianna, Robert Kilian, Natalie M. West, and Mark K. Petersen. Invasive Plant Science and Management October-December 2017 Vol. 10, No0. 4: 340-349.

Cut-stump application of triclopyr provided 96% control of Russian olive the year following treatment.  Seeded native species did not have trouble establishing once adequate spring moisture occurred in the second growing season after Russian-olive removal, indicating that removal did not present substantial obstacles to successful revegetation. Follow-up control of Russian-olive is critical after initial treatment. [ READ FULL ABSTRACT. ]

Forest Roads Facilitate the Spread of Invasive Plants

 Photo by Chuck Bargeron, University of Georgia,

Photo by Chuck Bargeron, University of Georgia,

David A. Mortensen and others. Invasive Plant Science and Management 2(3):191-199.

This large-scale survey highlights the importance of roads in predicting the presence of invasive plants, also revealing that one invasive plant, Japanese stiltgrass (Microstegium vimineum), has spread rapidly since its introduction. READ FULL ABSTRACT HERE