Biological control of arundo, an invasive grass threatening water resources and national security

Authors: Moran, Patrick; Goolsby, John

Submitted to: Contributions of Classical Biocontrol to the U.S. Food Security, Forestry, and Biodiversity, 1985-2022
Publication Type: Book / Chapter
Publication Acceptance Date: 4/26/2022
Publication Date: 6/20/2022
Citation: Moran, P.J., Goolsby, J. 2022. Biological control of arundo, an invasive grass threatening water resources and national security. In: Van Driesche, R.G., Winston, R.L., Perring, T.M., Lopez, V.M., editors. Contributions of Classical Biocontrol to the U.S. Food Security, Forestry, and Biodiversity. FHAAST-2019-05. Morgantown, WV: USDA Forest Service. p. 373-389. https://bugwoodcloud.org/resource/files/23194.

Interpretive Summary: The USDA-Agricultural Research Service (ARS) developed a biological control program targeting the non-naive, invasive giant grass known as arundo or giant reed. Biological control of weeds is the process of discovering and testing insects from the weed's native range, then obtaining permits, releasing and evaluating insects that feed and reproduce only on the targeted weed, and cause a lot of damage. ARS undertook this project because arundo consumes and wastes water, spreads wildfires, destroys flood control structures and displaces native plants and animals in the southwestern U.S. ARS found two insects in the native range of this invasive grass in Mediterreanean Europe. A tiny black wasp that is harmless to humans, known as the arundo wasp, lays eggs in shoot tips, causing plant tumors, known as galls, to form, and the larvae eat the gall. Another almost microscopic insect, known as the arundo armored scale, sits immobile on the root or 'rhizome' of arundo and feeds on juices in the rhizome. Both agents were released in the Lower Rio Grande Basin of Texas and Mexico from 2009 to 2013, and since then in the Big Bend area of western Texas and in northern California. Along over 500 river-miles of the Rio Grande between Brownsville and Del Rio, the arundo wasp has reduced live weight of arundo shoots by up to 54%, and in the release plots, in a localized manner, the armored scale has reduced live weight by an additional 55%. Both insects are established in California as well. A third insect, a leaf-mining fly, has been permitted for release and studies to rear it are underway. The USDA-ARS arundo biological control project has reduced the ability of arundo to dominate river-side habitats along the Rio Grande, fostering increased plant diversity. Additional beneficial impacts are expected in the U.S.

Technical Abstract: A giant bamboo-like grass, known as arundo, giant reed, or carrizo cane in the United States, and scientifically as Arundo donax, was introduced from Mediterranean Europe for use in fences, roof construction, and for erosion control. It has invaded riparian habitats (areas close to water) along rivers, creeks, arroyos, and lakes in the dry southwestern United States. Arundo consumes and wastes billions of gallons of water per year in the Lower Rio Grande Basin of Texas and Mexico, as well as in the Central Valley of California. Arundo also displaces native plants and animals, harbors crop and veterinary pests, alters river flow patterns, blocks food control systems, and fuels wildfires. A biological control program directed by the U.S. Department of Agriculture, Agricultural Research Service has led to the release of two insects against this pest. The first is called the arundo gall wasp, a small black stingless wasp harmless to humans, that lays its eggs in arundo shoot tips, resulting in galls, inside which the wasp’s larvae develop. The arundo wasp has become established across over 25 sites along some 500 miles of the Rio Grande River in Texas and Mexico, as well as in Mexico, and at limited sites in northern California. By 2014, five years after its release in 2009, feeding by the arundo gall wasp had reduced live arundo shoot weight by an average of 22% across 10 sites, and by an additional 32% on average seven years after release across five sites checked at both time points, allowing a 1.8-fold increase in plant diversity along the Rio Grande and saving water valued at up to $10M per year. The other biological control agent released, the arundo armored scale, is a tiny, mostly immobile insect that feeds on plant juices inside the tuber-like rhizomes of arundo that sit on the soil, and on side shoots. It was first released in 2011, and by 2019 was established at over 25 sites in Texas and at eight sites in northern California. Plots with arundo scale at two sites along the Rio Grande had 55% lower live shoot weight on average than did neighboring plots with only the arundo wasp. In addition to the above two agents, one additional species has been approved for release, and studies on how to mass rear it are underway. The arundo biological control program has provided beneficial reduction of arundo reed stands, enhancing protection of water resources. Further improvements in control over expanding areas are expected as this project continues.