Tamarix biocontrol in North America

Authors: Gaffke, Alexander; DUDLEY, TOM - University Of California; BEAN, DANIEL - Colorado Department Of Agriculture; DRUS, GAIL - St Francis University; JOHNSON, MATTHEW - Northern Arizona University; KNUTSON, ALLEN - Texas A&M University; WEAVER, DAVID - Montana State University; SING, SHARLENE - Us Forest Service (FS); ORR, BRUCE - Stillwater Sciences; THOMPSON, DAVID - New Mexico State University

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 5/31/2022
Publication Date: 6/1/2022
Citation: Gaffke, A.M., Dudley, T.L., Bean, D.W., Drus, G.M., Johnson, M.J., Knutson, A.E., Weaver, D.K., Sing, S.E., Orr, B.K., Thompson, D.C. 2022. Tamarix biocontrol in North America. Book Chapter. 329-355.

Interpretive Summary: The invasive weed tamarisk has proven to be a detrimental, and difficult weed to control since its introduction to the United States. Tamarisk has broadly impacted agriculture throughout the southwestern United States and is very costly to control through conventional means. Therefore, a biological control program using a host specific beetle was implemented to mitigate the negative economic and ecological impacts of the weed. This program has been very successful at controlling tamarisk in some areas but has failed in other areas. After 20 years of the program, a scientist at the USDA, ARS, Center for Medical Agricultural and Veterinary Entomology, Gainesville, Florida, in collaboration with multiple university, state, federal, and private industry colleagues, have reviewed the success and failures of the program. From this review, critical insights into the program were obtained and provided the basis for specific actions that are outlined for improved success of this biological control strategy in the future, ultimately yielding cost savings for weed control for agricultural producers.

Technical Abstract: The biological control program against Tamarix spp. (tamarisk/saltcedar; Tamaricaceae) was initiated in the 1970s to reduce negative impacts of this invasive Old World shrub to riparian biodiversity and ecosystem function in western North America. Field releases of host-specific leaf beetles (Chrysomelidae) in the genus Diorhabda were initiated in 2001, with significant establishment and widespread defoliation observed roughly two years after open releases. What followed were a variety of complex interactions among invasive Tamarix, its guild of herbivores including Diorhabda spp., and the physical and biotic environment, which varied across the western U.S. project area. Defoliation yielded sustained lower evapotranspiration and opened canopies, allowing increases in desired vegetation in some areas, while in other areas beetle establishment failed for reasons that included less-suitable host species, mismatches of environmental cues with diapause development of the beetle, and predation by generalist insectivores. In some regions, such as Texas, agent populations were short-lived, resulting in lack of sustained Tamarix suppression. In other areas, beetle populations reached initial epidemic densities but then declined to moderate levels with patchy subsequent defoliation and diminished target mortality. These short-term dramatic impacts to invasive Tamarix but limited sustained control suggest potential value in releasing additional host-specific agents, some of which have already been studied and readied for petitioning for release. Stakeholder enthusiasm for the biocontrol program was high in some locations and allowed reduced expenditures for conventional weed control. At the same time, perceived threats to sensitive wildlife species, particularly the endangered southwestern willow flycatcher (Empidonax traillii extimus) in Arizona, New Mexico, and Utah, resulted in legal actions terminating federal involvement in the biocontrol program nationally. A strategic approach to ecological restoration targeting ecosystems with high potential for enhanced wildlife habitat could help resolve these conflicting issues, but progress is currently inhibited by a lack of political support and financial resources, suggesting that renewed collaboration of interested parties across disciplines is needed to more fully achieve long-range benefits.