Over 300 insects feed on tamarisk in Eurasia, but few native organisms attack it in North America, allowing the weed to spread unchecked. Biocontrol reunites a plant with its natural enemies, selected to exert damage but feeding ONLY on that plant. The tamarisk leaf beetle, Diorhabda elongata, is from central Asia and satisfies those criteria.
Early 1800s - Tamarisk imported from Eurasia for flood control
1970s - overseas exploration beginning in the 1970s resulted in Diorhabda Carinulata being selected as a viable biological control method
- This tamarisk leaf beetle defoliates tamarisk - scraping the foliage and causing it to dry out. after repeated years of exposure to these beetles, tamarisk dies off.This has been shown to be the most effective method for destroying tamarisk in the long-term, as other efforts result in regrowth and reestablishment.
1996 - This species of beetle was approved for field release biocontrol methods by the USDA Animal and Plant Health Inspection Services (APHIS)
- The Tamarisk Biocontrol Project is one of the most intensively studied invasive plant control programs ever undertaken. With over 10 years of testing to ensure Diorhabda was not only effective, but also would not feed on native plants nor crops, the U.S. Department of Agriculture authorized its release, with input from a technical advisory group that included the U.S. Fish and Wildlife Service. While long-term benefits of tamarisk control are anticipated, there is always some risk when releasing a foreign organism. (from virgin river tab on existing website)
1995 - The Southwestern sub-species of Willow Flycatcher (Empidonax traillii extimus) (SWFL) was listed as an endangered species by the USFWS and was reported to use Tamarisk as nesting habitat.
- The primary point of contention surrounding the SWFL was whether or not defoliation of the Tamarisk by the beetle would occur during nesting times, which would expose the SWFL to excessive predators and heat during its late breeding season. (Sogge et al. 2003, 2008). (Dudley & Bean, 2012)
2001- The tamarisk leaf beetles were released in central Utah,
1993-2003 - 10 years of testing on the tamarisk leaf beetle revealed that the larvae only completed their development on the tamarisk trees and did not harm native vegetation.
2006 - Local managers moved the beetles to the Virgin River near St. George in southern Utah.
2010- The beetles were dispersed down the Virgin River, defoliating plants through the Virgin Valley,
2011-2013 - It has been documented that the beetles are capable of tolerating and establishing further south than the original insects released. The beetles contiued moving to the Momon Mesa area in 2011 and in 2012 to Lake Mead
- The beetles are slowly increasing their habitat to a more southeastern extent and now have passed beyond the Virgin River towards the Upper Gila River.
- The beetles became established 250 kilometers north of the border with Mexico.
2014 - The beetles have colonized as far south as Mohave Valley in California
Looking Forward - Current research is being done to determine the range of the tamarisk leaf beetle.
- It is possible that the beetles may colonize to the Colorado Delta of Mexico. Further research is being done to test genetic selection of the adaptation and establishment capacity of the beetle in the Lower Colorado River. (Tom Dudley, U.C. Santa Barbara; Dan Bean, Colorado Dept of Agriculture; Kevin Hultine, Desert Botanical Garden, AZ).
- Monitoring further dispersal and establishment is being achieved through pheromone baited sentinel traps placed along potential colonization sites.
- Tom Dudley is a Collaborator on the 2014 Weaver Proposal for this pheromone based monitoring.
Effects of this Bio-Control Method
Myriad benefits have resulted for the surrounding ecosystems - water savings of 2,500 acre-feet/year from reduced evapotranspiration in northern Nevada, reduced wildfire threats, and positive signs of recovery of native and beneficial vegetation.
Introduction of the beetles has resulted in successful destruction of a large amount of Tamarisk. However, the endangered SWFL began to use Tamarisk as its habitat, and concern grew that as the beetles moved closer to primary SWFL habitat, destroying the Tamarisk would also destroy SWFLs and other bird species. To some degree, the issues of climate change have mooted this conversation as drought and water demands have negatively affected Tamarisk and SWFL populations alike.
In most riparian systems, native populations are sufficiently established to aid in restoration after Tamarisk invasion and reduction, while few riparian systems are too degraded for these native plants to provide habitat for the flycatcher within an appropriate time frame.
The presence of the beetles has created a more balanced ecosystem for birds, with remaining Tamarisk providing more structural complexity to the treescape and the beetles becoming a prominent food source. Due to this, limiting weed treatments is ideal, in order to prevent secondary invasion by other invasive species such as Russian thistle.
In Arizona, federal regulators funded a media campaign condemning the use of the beetles. However, stopping biocontrol is not the issue at hand, but rather the incorporation of such methods into larger ecosystem dynamics. Promoting native vegetation, both passively and actively, is at the forefront of this dynamic focus.
With a recent turn in focus to water conservation, the tamarisk leaf beetles is receiving increasingly positive attention as people desire the removal of water-intensive tamarisk. However, water savings vary between regions , climates, sub and surface water sources as well as water transport measures.
In 2014, John McCain called upon the USDA and the USDI to have a plan that maximizes the benefits of the beetle's presence.
Successful restoration requires the diligent monitoring and research into the efficacy of biological control to be coupled with equally effective restoration measures. In the Colorado Basin, this has resulted in two types of revegetation practices - rapid response (short term plantings followed by irrigation, etc. to ensure growth) and sustainable (lending itself more towards improving natural recruitment of plant species)
Moving forward, research will focus on the multiple evolutions of both host and insect.
Visit our page on the Colorado Delta in Mexico for a look at more current research.