Weed-Suppressive Bacteria – Testing a Control Measure for Invasive Grasses in the West

The Exotic Problem and a Potential Biological Solution

Across sagebrush landscapes in the western United States, exotic annual grasses such as cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caputmedusae) have invaded fifty to 70 million of acres of rangeland in 13 western states. These invasive species thrive in disturbed areas and significantly alter landscapes, affecting soil properties, wildlife habitat, and livestock forage. Importantly, these plants also change natural fire regimes, fueling large wildfires when they die in late summer and fall, then crowding out native plants in the aftermath of the fire with their superior ability to establish and prosper in burned areas. See Cheatgrass and Medusahead for more information.

Land managers are searching for efficient and effective solutions to combat these invaders. Active approaches, such as using herbicides or targeting grazing to eliminate invasive grasses, have had limited success. In the late 1900s and early 2000s, researchers began to experiment with using biological approaches to selectively eliminate or inhibit growth of exotic annual grasses. One biological approach uses Weed-Suppressive Bacteria, which are isolated strains of the soil bacterium Pseudomonas flourescens that are developed to target invasive weeds with minimal impact to non-target plant species, such as native or agriculturally important plants.

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Is WSB Effective in Controlling Invasive Grasses?

Three major strains or products containing WSB became available to land managers in the 2010s: D7, ACK55, and MB906. Managers began to take notice. One appeal of using WSB is that the bacteria are considered native to rangelands, therefore it is a seemingly natural control measure that is more appealing than using a chemical herbicide. WSB can also be dried and rehydrated for relatively inexpensive spraying across large landscapes. Yet key questions about its effectiveness need to be answered before managers invest in this weed-control strategy. For example:

• Are positive effects of WSB reproducible and predictable?
• What are the effects to non-target native plants?
• How does WSB behave at sites with different soil, weather, topography, or aspect?
• Is WSB effective on large scales?

More Detailed Research Provides Evidence

In 2015, USGS and collaborating researchers and managers began studying WSB to address these important questions above. Developing studies with controls, replicates, and sufficient monitoring were crucial to impartially evaluate whether WSB was truly effective against cheatgrass and medusahead. Below are summaries of completed research involving USGS scientists who tested the effectiveness of WBS for controlling invasive grasses on rangelands in Idaho, Montana, Washington, and Wyoming. These studies and several others published with them are summarized in the following synthesis:

Germino, M.J., Lazarus, B.E., 2020, Synthesis of weed-suppressive bacteria studies in rangelands of the western USA: A special section of articles in Rangeland Ecology Provides Little Evidence of Effectiveness: Rangeland Ecology and Management. https://doi.org/10.1016/j.rama.2020.02.007

Study #1: Testing ACK55 and D7 in the Lab and in the Field

Researchers tested effects of ACK55 and D7 on cheatgrass both in the laboratory and ACK55 at field sites in Montana and Wyoming. The bacteria strains reduced cheatgrass germination and root and shoot lengths in petri-dish cultures in controlled lab settings, but had no effect on plants in field experiments. Findings contribute to growing evidence that these strains do not reliably control cheatgrass in the Northern Great Plains, Central Rocky Mountains, and elsewhere.

Reinhart, K.O., Carlson, C.H., Feris, K.P., Germino, M.J., Jandreau, C.J., Lazarus, B.E., Mangold, J., Pellatz, D.W., Ramsey, P., Rinella, M.J., Valliant, M., 2019, Weed-suppressive bacteria fails to control Bromus tectorum under field conditions: Rangeland Ecology and Management, https://doi.org/10.1016/j.rama.2019.07.006

Study #2: Testing Effects of D7 and MB906 Across Different Sites

USGS researchers evaluated the effects D7 and MB906 on exotic annual grasses at three sagebrush-steppe sites with contrasting soils and climate. Neither bacteria strain affected exotic annual grasses, perennial bunchgrasses, or total community cover, either applied alone or in combination with herbicides or discing. Results indicate a low likelihood of these strains to reduce annual grasses.

Germino, M.J., Lazarus, B.E., 2019, Weed-suppressive bacteria have no effect on exotic or native plants in sagebrush-steppe: Rangeland Ecology and Management, https://doi.org/10.1016/j.rama.2019.10.004

Study #3: Testing D7 as a Spray or in a Seed Mixture

USGS and USFWS researchers tested the ability D7 to control cheatgrass in south-central Washington in two case studies: one involved mixing D7 with native seeds sown after a fire and the other involved spraying D7 on a native community with high abundance of cheatgrass. D7 applied as a spray or seed mixture did not significantly affect cover, biomass, or density of cheatgrass.

Pyke, D.A., Shaff, S.E., Gregg, M.A., Conley, J.L., 2019, Weed-suppressive bacteria applied as a spray or seed mixture did not control Bromus tectorum: Rangeland Ecology and Management, https://doi.org/10.1016/j.rama.2019.11.001

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Study #4: Evaluating MB906 Effects over Nearly 10,000 Acres Sprayed by Managers

To accurately assess responses of both native and non-native grasses to MB906, land managers applied MB906 alone and in combination with the herbicide imazapic on three sagebrush-steppe landscapes burned several months prior. This study, co-produced by land managers and the USGS, shows how proper controls, dispersed replication, and quantitative monitoring can generate needed information for invasive grass management. Results will be published soon.

Lazarus, B.E., Germino, M.J., Brabec, M.A., Peterson, L., Walker, R.N., Moser, A.M., 2020, Post-fire management-scale trials of bacterial soil amendment MB906 show inconsistent control of invasive annual grasses: Rangeland Ecology and Management, https://doi.org/10.1016/j.rama.2020.03.005

Study #5: An Additional Laboratory Study on the Mechanisms Underlying Failure of D7 and ACK55 to be Effective in the Field

In petri dishes and pot studies, researchers tested ACK55 and D7 at different concentrations, and with or without bacterial cells or just the active ingredients produced by bacteria in isolation, and with or without removal of other bacteria that could compete with ACK55 and D7. While some of the desired target effects were observed in petri dishes, the effects could not be replicated in soils, and the failures in soil were not due to presence of other, presumably competing bacteria. 

Lazarus, B.E., Feris, K., Germino, M.J. 2020. Weed-suppressive bacteria effects differ in culture compared to in soils and with or without microbial competition and separation of active ingredient: Biological Control, https://doi.org/10.1016/j.biocontrol.2020.104422

Take-Home Message: WSB Does not Effectively Control Cheatgrass or Other Exotic Annual Grasses

Collectively, these studies show that the Weed-Suppressive Bacteria P. flourescens — strains ACK55, D7, and MB906 — are not likely to be effective in controlling invasive exotic grasses in western U.S. rangelands. There were no negative effects to exotic annual grasses, perennial bunchgrasses, or total community cover within three or four years of treatment when WSB was applied in the field alone or in combination with herbicides. It is possible that new formulations or application techniques could lead to more consistent, desired effects; however the studies described above tested three strains across a wide range of conditions, and yet no consistent effects were observed.

Coordination is Key

The spread of invasive grasses has diminished ecosystem services on about half of sagebrush steppe rangelands, effecting both private and public lands. As landowners, state and federal land managers, and researchers search for solutions, coordinating work across affected areas is needed to efficiently address the problem.

WSB is an example of tool that was reported to solve a complex and wide-spread ecological problem without harmful side effects. Concern amongst scientists and land managers about the lack of peer-reviewed research to support claims of success motivated a variety of independent studies to test effects. Establishing a science-management collaboration to enable learning from manager’s trials with WSB or other emerging tools can be critical for allowing learning and adaptive management of the tools.