Biological controls increase weed management options

When I started my job as the weed management projects coordinator for the Powell County Weed Board several years ago, I inherited a project that began with my predecessor in cooperation with the (now defunct) MSU Sheep Institute. This project involved importing sheep to prescriptively graze with cattle across several contiguous livestock operations in the Deer Lodge valley, primarily targeting the noxious weed leafy spurge. Data collected by the coordinator(s), extension agents and cooperators and analyzed at the institute confirmed the decrease of spurge and the increase of desirable grasses and other vegetation under the combined sheep and cattle grazing pressure.

A high protein content plant, spurge was largely preferred by the ewe and lamb pairs and disdained by most cattle. It actually took twice as many trucks to export the sheep in the fall than it had taken to bring them in during spring when the lambs were young due to their exponential weight gain. In the meantime, grasses and other vegetation favored by cattle, horses and wildlife increased with the reduction of this aggressive source of resource competition.

The sheep in this project were being employed as a form of biological control. Biological control can be generally defined as the beneficial action of parasites, pathogens and predators in managing pests and their damage. More simply put, this type of management approach involves the use of the activity of one species to reduce the adverse effects of another.

In the case of the imported sheep, predation of the plant leafy spurge (so to speak) was the mode of action. Biocontrol provided by living organisms, collectively called "natural enemies" is an important component of integrated (or the combination of several) weed management practices aimed at reducing plant population sizes and therefore the negative impacts of rangeland and wildland non-native, noxious weeds.

By far the most common form of biocontrol and one which many of us are familiar with, involves the use of insects. Before coming to Montana, I worked for a Native American tribal program in Idaho that collected, mass reared and released species specific, natural enemy insects imported from the continents where the exotic noxious weeds originated.

Recently, I was able to work with Dr. Jeffry Littlefield, Research Scientist, and Jennifer Birdsell, Research Associate, both of MSU Land Resources and Environmental Sciences, on insect release projects in Powell County. Before insects are released into open environments, several years of extensive research, and many steps involving testing, documentation and federal and state authorization protocols are conducted first oversees and then in several western states including Montana. Littlefield and his staff refine the process in preparation for potential general release by working to determine (1) host specificity, ensuring the insect will not move onto any native plant species and cause damage, (2) bionomics, the relationship of the insect with its environment (across the life stages of both the insect and its host plant) and (3) field ecology, or the comparative analysis of insect establishment success in variable environmental conditions.

While biocontrol insects for noxious weeds such as knapweed and leafy spurge distributed by the Weed Board for many years are fairly abundant and widely established, we often receive queries about the availability of insects for other invasive plant species. Unfortunately, effective biocontrol agents have not been found for every noxious weed in Montana. Further, many noxious weeds are too closely related to native and other beneficial plant species and it is difficult to find biocontrol agents that will not attack these non-target, desirable plants.

We were particularly excited to be offered the opportunity to participate in the release of two "new to us" insects for hoary cress, or whitetop, and Russian knapweed.

Russian knapweed is a perennial similar in appearance to the more common biennial to perennial spotted knapweed with prominent white tips on purple to pink flowerheads and blue-green leaves. This knapweed species forms dense colonies due to a vigorously spreading root system rather than a deep taproot and the bracts below the flower petals are smooth and papery versus spiny and "spotted." The Russian knapweed gall wasp we released forms galls via larval stage feeding and weaken the plant as well as reducing flower and seed production.

Whitetop is also a perennial with an extensive spreading root system. While similar in appearance to another spring blooming white flowered mustard family plant, annual field pennycress, whitetop has arrowhead shaped leaves with toothed edges that are grayish green in color and dense flower clusters that create a flat-top appearance. Mites feeding on developing stems and branches cause the formation of galls which may stunt the plant and suppress flower and seed production.

Biological control insects are released in well-established weed infestation sites that are large enough to support enough population growth to significantly impact the target weed. When this is not the case, it's best to use other methods such as pulling, mowing and herbicide applications. We look forward to monitoring the release sites to determine establishment. I'm certainly hopeful as each additional tool in the control methods toolbox increases our success rates.

You may find the following links useful: Montana Biocontrol Coordination Project http://www.mtbiocontrol.org, iBiocontrol http://www.ibiocontrol.org; and Dr. Jeffery Littlefield https://landresources.montana.edu/directory/faculty/1524325/jeffrey-littlefield