Overview

          Tamarisks are small trees or shrubs that belong to the family Tamaricaceae (order: Violales). These Eurasian natives were introduced in North America in the mid 1800’s, mainly to provide windbreaks and to prevent soil erosion in semi-arid and arid areas. The plants became naturalized in late 1800’s (Shafroth et al. 2005), and some species are still planted as ornamentals and shade trees. Approximately 1-1.6 million hectares of land in North America is infested today (Shafroth et al. 2005). The plants are considered to be among the world’s 100 top invaders and one of the most damaging invasive weeds in the western US. The species that are present in the U.S. include Tamarix ramosissima, T. parviflora, T. aphylla, T. chinensis and T. canariensis, and several hybrid forms between the different species (Gaskin & Schaal 2002). Similar to Arundo donax (giant reed), tamarisk invades riparian systems and responds positively to flooding which seems to facilitate establishment. The plants form dense stands, and invaded areas often become completely dominated by tamarisk with few other plant species present.

Tamarisk monoculture growing along the Virgin River, NV.

There are five primary ecological and economical effects of Tamarix infestations:

1) High rates of water use are especially important for the water limited Western U.S., lowering water tables, reducing stream flows, drying springs and resulting in annual water losses estimated in excess of $133 million (Zavaleta 2000).
2) Tamarisk is capable of utilizing deeper and more saline groundwater than most native plant species and may subsequently increase near-surface soil salinity (USDA APHIS 2005).
3) Tamarisk domination of riparian plant communities leads to decline in floral and faunal diversity (Dudley & DeLoach 2004). Fifty-one special-status species are considered to be negatively impacted by tamarisk (Dudley et al. 2000).
4) Accumulation of tamarisk litter and dead wood affects the frequency and intensity of fire in invaded communities, and subsequently exerts great influence over post-fire plant and animal community composition (USDA APHIS 2005)
5) Plants interferes with land use and access, particularly recreational uses such as fishing, birdwatching and wildlife photography (USDA APHIS 2005).

Biological control of Tamarix

          Tamarisk has proven extremely difficult and labor intensive to control with mechanical and chemical methods. Consequently, a tamarisk biocontrol program was initiated in the 1960’s. In North America, the plants are subject to very little herbivory. As with many introduced plant species, the full suit of plant feeding insects and pathogens that may feed on the plants in the native ranges was not imported at the time of introduction. One leaf hopper species, Opsius stactogalus (Homoptera: Ciccadelidae), and two scale insects, Chionaspis spp. (Homoptera: Diaspididae), do occur on tamarisk in North America, but these species do not exert significant pressure on the plant (Weisenborn 2001, Lewis et al. 2003). After extensive exploration of potential biocontrol agents in the plants native ranges (Europe, Asia and North Africa), and subsequent testing to ensure safety and efficacy (e.g. DeLoach et al. 2000, DeLoach et al. 2003, Lewis et al. 2003, Dudley & Kazmer 2005), the leaf beetle Diorhabda elongata (Coleoptera: Chrysomelidae) was released into the open in 2001 (DeLoach et al. 2003). The results of these releases were mixed (DeLoach et al. 2004), and the most successful establishment took place at five sites in Colorado, Nevada, Utah and Wyoming. At sites in northern Nevada, the beetles have defoliated vast stands with over 10 000 ha damaged at one site in 2004 (Geraci et al. 2006). By the 4th year of defoliation (2005), plant mortality reached 40%. Even without plant mortality, biocontrol may provide substantial benefits. Sap-flow measurements and evapotranspiration indicated that groundwater losses were reduced by over 70% during the first year of defoliation in Nevada, with greater savings in subsequent years (Pattisson et al.; unpubl. data). In addition, recent studies suggest that both avian (Hitchcock et al.; in review) and spider (Dalin & Dudley; unpubl. data) diversity and abundance have increased in stands where D. elongata is present.


Current Tamarix Biological Control Projects

          As described previously, the releases of D. elongata have not always been successful. In California, for example, despite cage trials at four locations, and open releases in three of those, there are no field populations of D. elongata in the State at the moment. There are three ecological reasons that might explain these establishment failures. First, at North American sites lower than 37-38° N, the original form of D. elongata (originally collected from Fukang, China, 44° N) was not successful because it responds to declining daylength by entering diapause too early in the season to successfully over winter (DeLoach et al. 2004). In incubator studies, the Fukang beetles cease reproduction at a critical daylength of 14.5 hours, meaning that they are only reproductive for one month at mid-latitudes (e.g. 37° N) and the critical day length is never achieved at southern latitudes (Bean et al. 2001). A second factor that might influence establishment is predation, primarily by harvester-type ants (Hymenoptera: Formicidae) which prey on both larvae and adult beetles. At one site in Oregon (43.5° N), which should have been ideal in terms of day length, the density of ant colonies was extremely high and near total reduction of released beetles was observed over the course of two days following release. A third factor is non-suitable host plants. One of the most invasive tamarisk species is T. parviflora and its related hybrids, which seem to be a poor host for the Fukang ecotype. Experimental planting of similar size T. ramosissima and T. parviflora plants were affected very differently by local high densities of beetles, with the T. parviflora largely avoided as opposed to near complete defoliation of the T. ramosissima plants. It may not be surprising that a central Asian herbivore (the Fukang ecotype) is not compatible with a west Asian/eastern Mediterranean host plant (T. parviflora).

Latitudial gradient experiment
          Together with collaborators in several states (see below), we are planning to test several other ecotypes of D. elongata for their responses to daylength. These beetle ecotypes represent Fukang, China (44.2° N), Turpan, China (42.9° N), Crete, Greece (35.8° N) and Sfax, Tunisia (34.7° N), and are currently held under quarantine in the U.S. Our purpose is to test which ecotype that is most suited for different latitudes. The beetles will be released onto plants in the field inside double secure cages. This is done to avoid unintended releases, since all ecotypes have to be approved first before any potential open-field releases can take place. Extensive host range testing has indicated that the various ecotypes show no differences in host specificity and potential risks to native plant species (Lewis et al. 2003, Herr & Carruthers 2005, Milbrath & DeLoach 2006). We have 18 field sites that will be included in the study, ranging from 33° to 48° N in latitude and the sites are distributed among three latitudial north-south gradients: pacific, desert, and mid-continent. Our prediction is that ecotypes will be reproductively active for the longest time period at those latitudes that match their latitudes of origin. We will also measure over wintering survivals of the ecotypes in these sites.

Predation studies
          We are currently studying generalist predator guilds that occur on tamarisk plants in North America. A ‘knock-down’ sampling technique is being used for comparing predator densities (e.g. ants, spiders, heteropterans) between sites where beetles have been released and non-release areas. The purpose is to determine whether predators have numerically responded to the release of beetles and, also, to investigate what predators are attacking D. elongata in the field. Preliminary results suggest that spider diversity and abundance is higher in release areas. It is, however, not clear if this is due to the presence of beetles or because of some other factors, such as higher abundance of alternative prey (e.g. leaf hoppers) or greater structural complexity of tamarisk stands, which will be investigated. Eric Knutson (New Mexico) is helping us with determining spider species. In addition, we plan to do experimental releases in areas with and without predators (ant exclusions) to evaluate the role of predators in establishment. This part is done in collaboration with Andrew Liebhold.

Host plant tests in common gardens
          To investigate what D. elongata ecotype that could be most suitable for biocontrol in relation to tamarisk species, we have established four common garden plots containing 12-14 plant genotypes. The plants represent genotypes of different tamarisk species (e.g. T. ramosissima, T. parviflora and T. aphylla) and hybrids between these species. The gardens are located in Las Cruces (New Mexico), Santa Barbara (California), Reno (Nevada) and Grand Junction (Colorado). The purpose will be to study the performance of the different beetle ecotypes (growth and survival rates) on the different host plant species. Based on current knowledge about host plants in native areas, we predict that beetles will perform best on those host plant species that they co-occur with in their native ranges and that performance will be intermediate on hybrid plants. The studies will be done in collaboration with Dan Bean in Colorado and Dave Thompson in New Mexico.


Collaborators
Gregg Abbott, USDA/APHIS, Richfield, UT
Earl Andress, USDA/APHIS, Phoenix, AZ
Bob Bartelt, USDA/ARS, Peoria, IL
Cameron Barrows, Center Natural Lands Management, Palms, CA
Dan Bean, Colorado Department of Agriculture, Palisade, CO
Matthew Brooks, USGS/BRD, Henderson, NV
Raymond Carruthers, USDA/ARS, Albany, CA
Brian Cashore, Inyo County Water Department, Bishop, CA
Anthony Chavez, USDI/BLM, Barstow, CA
Gretchen Coffman, UC Los Angeles, CA
Timothy Collier, University of Wyoming, Laramie, WY
Eric Coombs, Department of Agriculture, Salem, OR
Allard Cossé, USDA/ARS, Preoria, IL
Carla D’Antonio, UC Santa Barbara, CA
C. Jack DeLoach, USDA/ARS, Temple, TX
Debra Eberts, USDI/BOR, Denver, CO
John Gaskin, USDA/ARS, Sidney, MT
Richard Hansen, USDA/APHIS, Fort Collins, CO
Elisabeth Hebertson, US Forest Service, Ogden, UT
John Herr, USDA/ARS, Albany, CA
Denise Hosler, USDI/BOR, Denver, CO
Kirstin Johnson, Victor Valley State College, Victorville, CA
Dave Kazmer, USDA/ARS, Sidney, MT
Jeff Knight, Department of Agriculture, Reno, NV
Allen Knutson, Texas A&M Research and Extension Center, Dallas, TX
Eric Knutson, New Mexico State University, Las Cruces, NM
Andrew Liebhold, US Forest Service, Morgantown, WV
Bill Longland, USDA/ARS, Reno, NV
Jerry Michels, Texas A&M Research and Extension Center, Dallas, TX
Joseph Milan, USDI/BLM, Boise, ID
Patrick Moran, USDA/ARS, Weslaco, TX
Andrew Norton, Colorado State University, Forth Collins, CO
Robert Pattison, USDA/ARS, Reno, NV
Benjamin Rice, California Department of Forestry, Sierra Madre, CA
Monica Schwartz, Imperial Co. Water District, CA
Daniel Scharratt, Department of Agriculture, OR
Peter Weisberg, University of Nevada, Reno, NV
Brian Swedhin, Mesa State College, Grand Junction, CO
Lynne Silva, USDI/BLM, Vale, OR
Hillary Thomas, UC Davis, CA
Dave Thompson, New Mexico State University, Las Cruces, New Mexico
James Tracy, USDA/ARS, Temple, Texas
Livy Williams, USDA/ARS, Reno, NV



References

Bean, D., Chew, T., Li, B. and Carruthers, R.I. (2001) Diapause in relation to the life history of Dirohabda elongata (Chrysomelidae), a Eurasian leaf beetle introduced as a biocontrol agent of saltcedar (Tamarix spp.)(abstract) Ent. Soc. Am., San Diego.

DeLoach, C.J., Carruthers, R.I., Dudley, T.L., Lewis, P., Herr, J. and Tracy, J. (2000) New (Post-1994) host range testing of the leaf beetle, Diorhabda elongata deserticola, for biological control of saltcedar (Tamarix spp.) in the western United States. An addition to the petition to APHIS-TAGIBCAW of 14 March 1994 requesting an opinion on field release.

DeLoach, C.J., Carruthers, R.I., Dudley, T.D. and 18 others (2004) First results for control of saltcedar (Tamarix spp.) in the open field in the western United States. In: R. Cullen (ed.) XI International Symposium on Biological Control of Weeds, Canberra, Australia.

DeLoach, C.J., Lewis, P.A., Herr, J.C., Carruthers, R.I., Tracy, J.L. and Johnson, J. (2003) Host specificity of the leaf beetle, Diorhabda elongata deserticola (Coleoptera: Chrysomelidae) from Asia, a biological control agent for saltcedars (Tamarix: Tamaricaceae) in the Western United States. Biological Control 27: 117-147.

Dudley, T.D. and DeLoach, C.J. (2004) Saltcedar (Tamarix spp.), endangered species, and biological weed control – Can they mix? Weed Technology 18: 1542-1551.

Dudley, T.D., DeLoach, C.J., Lovich, J.E. and Carruthers, R.I. (2000) Saltcedar invasion of western riparian areas: impacts and new prospects for control. In: R.E. McCabe and S.E. Loos (eds.) Transactions of the 65th No. American Wildlife and Natural Resource Conference, Rosemont, Illinois. Wildlife Management Inst., Washington, DC. pp. 345-381.

Dudley, T.D. and Kazmer, D.J. (2005) Field assessment of the risk posed by Diorhabda elongata, a biocontrol agent for control of saltcedar (Tamarix spp.), to a non-target plant, Frankenia salina. Biological Control 35: 265-275.

Gaskin, J.F. and Schaal, B.A. (2002) Hybrid Tamarix widespread in U.S. invasion and undetected in native Asian range. Proc. Natl. Acad. Sci. USA 99: 11256-11259.

Geraci, C.C., Dudley, T. and Rundquist, B. (2006) Remote sensing assessment of widespread saltcedar infestation and biological control in NW Nevada. Prarie Perspect. (in press).

Herr, J.C. and Carruthers, R.I. (2005) Testing the saltcedar leaf beetle for potential impact to native non-target Frankenia spp. (poster) Ent. Soc. Am. annual meeting.

Hitchcock, D., Dudley, T.D. and Longland, W. Influence of biological control introductions on wildlife habitat quality in saltcedar stands. (In review for Gr. Basin Nat.).

Lewis, P.A., Herr, J.C., Dudley, T.D., Carruthers, R.I. and DeLoach, C.J. (2003) Assessment of risk to native Frankenia shrubs from an Asian leaf beetle, Diorhabda elongata deserticola (Coleoptera: Chrysomelidae), introduced for biological control of saltcedars (Tamarix spp.) in the western U.S. Biological Control 27: 148-166.

Milbrath, L.R. and DeLoach, C.J. (2006) Host specificity of different populations of the leaf beetle Diorhabda elongata (Coleoptera: Chrysomelidae), a biological control agent of saltcedar (Tamarix spp.). Biological Control 36: 32-48.

Schafroth, P.B., Cleverly, J., Dudley, T.L., Stuart, J., Van Riper, C and Weeks, E.P. (2005) Saltcedar removal, water salvage and wildlife habitat restoration along rivers in the southwestern U.S. Environ. Mgt. 35: 231-246.

USDA APHIS (2005) U.S. Department of Agriculture, Animal and Plant Health Inspection Service. Program for biological control of saltcedar (Tamarix spp.) in thirteen states: Environmental assessment, June 2005.

Zavaleta, E. (2000) The economic value of controlling an invasive shrub. Ambio 29: 462-467.

© 2006 Tom Dudley