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Managing the Spread of Invasive Weeds

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ARS is a global leader in limiting the spread of invasive species, with more than 12 laboratories in the United States and overseas facilitating the discovery and safety testing of new biological control agents. Globally, invasive weeds are the single largest and most damaging group of invasive species, costing an estimated $137 billion per year. On western Federal lands alone, the spread of invasive weeds is estimated at 2,300 acres per day. The following FY 2019 accomplishments illustrate ARS successes in limiting and recovering from the spread of invasive species. Hyperlinked accomplishment titles point to active parent research projects.

New biological control agent for Brazilian pepper tree. Brazilian pepper tree originated in South America and then became one of the most widespread and destructive invasive species in the Florida Everglades. Efforts to control and eradicate this weed have not been effective and it continues to spread. ARS scientists in Fort Lauderdale, Florida, in collaboration with Argentinian collaborators, recently obtained authorization for field release of a thrips (the insect Pseudophilothrips ichini) biocontrol agent. Research has shown that thrips that fed under greenhouse conditions reduced Brazilian pepper seedling growth by 80 percent. The new biocontrol agent may provide land managers and farmers with a cost-effective means of controlling Brazilian pepper tree by reducing the current and expensive reliance on herbicidal control.

Reducing an invasive weed in the cattle fever tick quarantine zone. The giant reed is an invasive weed in the cattle fever tick Permanent Quarantine Zone, where it clogs portions of the Rio Grande River and reduces border visibility. Two biological control agents of the giant reed, the arundo wasp and the arundo scale, were released in 2009 and 2010, respectively. Nine years after the release of the scale, ARS scientists in Kerrville, Texas, documented 55 percent less above-ground biomass of stands of the giant reed in areas where both the arundo wasp and scale were used as a biocontrol agent compared with areas where the wasp alone was used. Despite its low dispersal rate, the impact of the arundo scale as a biological control agent was augmented when used together with the wasp. Reducing stands of giant reed reduces the ideal habitat for southern cattle fever ticks, which helps maintain the cattle fever tick quarantine.

Kudzu invasion and impact will expand northward with climate change. Kudzu, an invasive group of vines, currently infests nearly 8 million acres, much of that in commercially owned forests. Treatment costs commonly exceed the economic value of the timber harvest. Kudzu is a carrier of Asian soybean rust, a fungus that can damage soybeans. Kudzu also is known to increase emissions of greenhouse gases such as nitrous oxide that further exacerbate climate change. To determine the role of rising temperatures on kudzu distribution, ARS scientists in Beltsville, Maryland, in conjunction with university partners examined different populations of kudzu in relation to minimal freezing temperatures. The data indicate that kudzu has a greater potential to migrate northward as temperatures rise, and that it has not yet reached its biological northward limit. Any expansion of the range of kudzu has potentially devastating consequences, and this research provides new information that may help in identifying new areas at risk for kudzu invasion and associated negative impacts. Identifying areas most at risk for kudzu invasion helps in developing targeted management to minimize damage.

New transplanting and seeding methods help restore native rangelands. Degradation of rangelands from wildfires has led to millions of acres of native rangelands being dominated by cheatgrass, an invasive and exotic annual grass, causing significant loss of critical browsing plants for wildlife and livestock. Scientists in Reno, Nevada, initiated and tested new transplanting and seeding methods to reestablish antelope bitterbrush, a critical browse plant, following an extensive wildfire in northern Nevada. Transplanting resulted in an initial establishment of more than 100 new antelope bitterbrush per acre while seeding had initial establishment of more than 15,000 antelope bitterbrush seedlings per acre. The research was successful in demonstrating cost-effective techniques that significantly increased the establishment and recruitment of this critical species, improved nutritional forage for numerous wildlife species and domestic livestock, and prevented conversion of the area to cheatgrass dominance.

Reseeding cheatgrass-infested rangelands. In the Great Basin, invasive annual cheatgrass is estimated to have displaced approximately 25 million acres of native perennial vegetation, leading to catastrophic and costly wildfire cycles. Land management agencies spend millions of dollars in reseeding efforts, but restoring rangelands to a diverse, healthy, perennial-dominated ecosystem can be difficult with native grasses, and most studies don’t examine the long-term effect of reseeding efforts. ARS scientists in Logan, Utah, studied seedling establishment and plant persistence over 5 years for native grasses compared with the typical use of crested and Siberian wheatgrasses at four locations in Idaho, Utah, and Wyoming. Native grass seedling establishment of bottlebrush squirreltail, bluebunch, slender, and Snake River wheatgrasses were similar to that of Siberian wheatgrass; however, by year 5, western, Snake River, and thickspike wheatgrasses were the only native grasses to have plant densities similar to those of Siberian and crested wheatgrasses. In these cheatgrass-infested regions, many native grasses can establish but may not persist, leading to continued cheatgrass and wildfire. Seed mixes that include combinations of species that establish quickly, persist, and compete against invasive annual grasses are necessary for rangeland restoration. The research provides important information for land managers making decisions concerning species selection for rangeland revegetation projects.