Saltcedar Biological Control

Overview

One biological control agent for saltcedar has been approved by APHIS. It is a leaf beetle originally known as Diorhabda elongata (Coleoptera: Chrysomelidae).

Scientists collected the leaf beetle from several areas in Eurasia, looking for biotypes adapted to different latitudes and discovered that there were multiple cryptic species. The original population from northern China is now called D. carinulata, and one from Crete is called D. elongata (Fig. 1, Tracy and Robbins 2009). Two additional species (D. carinata and D. sublineata) are established east of the Rocky Mountains, but are not discussed here.

Leaf beetle adults become active when green foliage appears in the spring, feeding and laying eggs on the leaves (Fig. 2, Deloach and Carruthers 2004). The larvae also feed on leaves and pupate on the soil surface and in leaf litter (Fig.3). The beetles can have more than one generation per year, depending on species and latitude, but adults that emerge in late summer are in reproductive diapause, and will not lay eggs until the next spring. Adults spend the winter hiding in leaf litter and soil, often near saltcedar trees. The adults are good dispersers and tend to aggregate, which causes patches of saltcedar to become completely defoliated (Fig. 4). Trees that have access to sufficient moisture may regrow leaves later in the summer, otherwise not until next spring. Repeated defoliations can cause tree mortality.

For areas in California north of the Tehachapi Mountains, both D. elongata and D. carinulata are CDFA-approved biological control agents. Unfortunately, most of the saltcedar in central and northern California is Tamarix parviflora, which is not the preferred host of these Diorhabda beetles.

The Northern China leaf beetle (Diorhabda carinulata) was released in 1999-2002 in the Owens Valley area (Tinemaha Reservoir, Inyo County) on Tamarix ramosissimaand T. parviflora, and this is the only area in Northern California where it has established (Pratt et al. 2019). This beetle population has persisted at extremely low densities. This beetle was also released in 2001 in Lovelock, Nevada and widely in Colorado and Utah starting in 2005, causing spectacular defoliation of saltcedar (T. ramosissima) (Carruthers et al. 2008, Bedford et al. 2018). The beetle has gradually moved down the Colorado River into southwestern willow flycatcher (SWFL) habitat in New Mexico, Arizona and southern California (Dudley and Bean 2012). Defoliation of saltcedar poses a risk to SWFL, which is a federally endangered subspecies, because it nests in saltcedar in areas where willows no longer occur. Restoration of cottonwood-willow vegetation in the wake of saltcedar defoliation will be important for improving the prospects of SWFL. A lawsuit to help protect SWFL resulted in APHIS revoking the permit for Diorhabda beetles in 2010. Meanwhile the beetle has spread all the way down the Colorado River and is dispersing into the Mojave Desert of southeastern California. California Department of Food and Agriculture (CDFA) does not make any releases of the leaf beetles south of the Tehachapi Mountains to avoid SWFL habitat. Biological control researchers are monitoring the beetles in southern California, but no agents can be released in this area. In 2018, D. carinulata was collected along the Mojave River in San Bernardino County and released on T. parviflora in Butte, Kern, and Fresno Counties; however, none of the releases resulted in established populations.

The Crete leaf beetle(Diorhabda elongata) was released in 2003-2005 on T. parviflora-dominated saltcedar patches in the Cache Creek (Yolo County) and Pope Creek (Napa County) valleys, initially causing widespread defoliation (Fig. 4). The beetles are persisting and have dispersed about 9 miles/year but are not causing widespread mortality of saltcedar trees (Pratt et al.2019). Initial releases of this beetle species at other sites in northern California (for example Stony Creek in Glenn County or Los Gatos Canyon in Fresno County) failed to establish. Beetles collected at Cache Creek were released in Colusa, Kern, Fresno, Napa, and Yolo counties, but all these populations also failed to establish.

Heavy feeding by the leaf beetles can completely defoliate trees in early summer, as observed at Cache Creek in 2006-2007 (Carruthers et al. 2008). When this happens, the beetles leave the area in search of healthy trees. They may return to defoliate regrowth later the same year or next spring, but if beetles have left the area, trees can recover from the damage. In western Colorado D. carinulata caused 15% to 56% tree mortality after six years (Kennard et al. 2016). Unfortunately, this level of impact has not been observed for D. elongata on T. parviflora in northern California (Pratt et al. 2019). Pheromones of the beetle have been used experimentally in traps to monitor populations and even to attract beetles to attack specific trees.

The splendid tamarisk weevil, Coniatus splendidulus (Coleoptera: Curculionidae), is an accidental introduction that has been found in Arizona, California, Colorado, Nevada, Oklahoma, Texas and Utah, as well as in Mexico (Hassenflu et al. 2018, Silva et al. 2018) (Fig. 5). It is a small weevil (1/8 in. long), and its larvae feed on the leaves of several Tamarix spp. Its geographic distribution and ability to cause damage to saltcedar in California are unknown.

The tamarisk leafhopper, Opsius stactogalus (Hemiptera: Cicadellidae) is another accidental introduction that is widespread in southern California (Fig. 6). The leafhopper has three to four generations per year, and it overwinters in the egg stage in Tamarix spp. shoots and woody stems. It can occur in very high numbers and result in reduced plant growth. Heavy feeding results in a yellowing of the leaves and early leaf drop, a condition called “hopper burn” (Nissen et al.2009). High leafhopper populations produce large amounts of honeydew, which may attract ants that disrupt feeding by the leaf beetles but also coat leaves with a sooty mold that may increase shoot mortality (Siemion and Stevens 2015).

The splendid tamarisk weevil and tamarisk leafhopper are not permitted for use as biological control agents in California.

Biological Control Agents

Species	Common name	Distribution	Impact	Notes
Coniatus splendidulus	Splendid tamarisk weevil	limited	unknown	Accidental introduction, found in CA in 2010. Not a permitted agent.
Diorhabda carinulata	Northern China leaf beetle	limited	unknown	First released in CA in 2001 but failed to establish except at one site (Owens Valley, Inyo Co.); dispersing from AZ into Mojave Desert and San Bernardino Co.
Diorhabda elongata	Crete leaf beetle	limited	moderate	First released in CA in 2003, established in only one area (Cache and Pope Creek Valleys, Yolo/Napa Co.)
Opsius stactogalus	Tamarisk leafhopper	wide	moderate	Accidental introduction, found in US in 1907. Not a permitted agent.

How the Technique Is Employed

Survey your site for the Diorhabda spp. leaf beetles. If they are already present, there is no benefit to releasing more, as the beetles already present will build their populations. To survey, examine saltcedar trees that show partial or complete defoliation damage (Fig. 2). Defoliation is most likely to occur between June and September. Examine stems for adult beetles (Fig. 2) and larvae. Defoliated stems will have cast larval skins and frass (feces) from larval beetles. In the absence of defoliation, sweep-net or beat a few trees (to knock insects into an open sheet or net) at your site to confirm that the beetles are absent. Note that damage from the tamarisk leafhoppers resembles beetle defoliation from a distance. Close examination of the leaves and stems will reveal the planthoppers and the yellow and possibly sooty appearance of 'hopper-burned’ leaves (Fig. 4). Close examination of branches is also required to observe the splendid tamarisk weevil and/or its basket-like pupal cases.

For additional information see Nissen et al. (2009).

Special Tips

Pheromones of the beetle have been used experimentally in traps to monitor populations and even to attract beetles to attack specific trees.

Caveats

It is not permitted to release these leaf beetles in California south of the Tehachapi Mountains. Leaf beetle larvae, pupae and diapausing adults are susceptible to predation by ants. Flooding can drown insects in the ground (pupating during the summer, hibernating during the winter). The species of saltcedar that is most common in northern California (Tamarix parviflora) is not the most preferred or suitable species of saltcedar for the leaf beetles. The leaf beetles can attack a nontarget, non-native shade tree, athel (T. aphylla, Deloach et al. 2003, Moran et al. 2009), although they do not persist on it. They can also reproduce on a native plant, alkali heath (Frankenia salina), under confined conditions, but not when the beetles are free to disperse (Dudley and Kazmer 2005, Milbrath and DeLoach 2006, Herr et al. 2009).

Where Can I Get These?

The California Department of Food and Agriculture has a special use permit to move Diorhabda beetles from the Mojave River to saltcedar infestations in northern California. Unfortunately, recent efforts doing this have not been successful in obtaining sustained populations of beetles on T. parviflora, the predominant species in northern California. Future efforts are investigating better release methods, such as using aggregation pheromones and cages. These methods are still under development.

There are no known commercial vendors of the leaf beetles.

Redistribution of the splendid tamarisk weevil and tamarisk planthopper are not permitted in California.

References

Bedford, A., T.T. Sankey, J.B. Sankey, L. Durning, and B.E. Ralston. 2018. Remote sensing of tamarisk beetle (Diorhabda carinulata) impacts along 412 km of the Colorado River in the Grand Canyon, Arizona, USA. Ecological Indicators 89: 365-375. doi.org/10.1016/j.ecolind.2018.02.026.

Carruthers, R.I., C.J. Deloach, J.C. Herr, G.L. Anderson, and A.E. Knutson. 2008. Salt cedar areawide pest management in the western USA. In K. Opender, G. Cuperus, & N. Elliott (Eds.), Areawide pest management: theory and implementation. CABI, Wallingford, UK, pp. 271-299.
https://digitalcommons.unl.edu/usdaarsfacpub/655

Deloach, C.J. and R.I. Carruthers. 2004. Saltcedar, pp. 311-316. In: Coombs, E.M., J.K. Clark, G.L. Piper and A.F. Cofrancesco, Jr. (eds.), Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.

DeLoach, C.J., P.A. Lewis, J.C. Herr, R.I. Carruthers, J.L. Tracy, and J.L. Johnson. 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: theory and applications in pest management 27: 117-147. doi:10.1016/S1049-9644(03)00003-3

Dudley, T.L. and D.W. Bean. 2012. Tamarisk biocontrol, endangered species risk and resolution of conflict through riparian restoration. BioControl 57: 331-347. doi.10.1007/s10526-011-9436-9

Dudley, T.L. and D.J. Kazmer. 2005. Field assessment of the risk posed by Diorhabda elongata, a biocontrol agent for control of saltcedar (Tamarix spp.), to a nontarget plant, Frankenia salina. Biological Control 35(3): 265-275. doi:10.1016/j.biocontrol.2005.05.002

Hassenflu, A.M., N. Taylor, C.M. Ritzi. 2018. New locality records of splendid tamarisk weevil, Coniatus splendidulus(Fabricius), and Chionaspis scale in West Texas and New Mexico. Southwestern Entomologist 43: 527-530. doi.org/10.3958/059.043.0224

Herr, J.C., R.I. Carruthers, D.W. Bean, C.J. DeLoach, and J. Kashefi. 2009. Host preference between saltcedar (Tamarix spp.) and native non-target Frankenia spp. within the Diorhabda elongataspecies complex (Coleoptera: Chrysomelidae). Biological Control 51(3): 337-345.doi:10.1016/j.biocontrol.2009.07.015

Kennard, D., N. Louden, D. Gemoets, S. Ortega, E. González, D. Bean, P. Cunningham, T. Johnson, K. Rosen, and A. Stahlke. 2016. Tamarix dieback and vegetation patterns following release of the northern tamarisk beetle (Diorhabda carinulata) in western Colorado. Biological control 101: 114-122.
dx.doi.org/10.1016/j.biocontrol.2016.07.004

Milbrath, L.R., and C.J. DeLoach. 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.

Moran, P.J., C.J. DeLoach, T.L. Dudley, and J. Sanabria. 2009. Open field host selection and behavior by tamarisk beetles (Diorhabda spp.) (Coleoptera: Chrysomelidae) in biological control of exotic saltcedars (Tamarix spp.) and risks to non-target athel (T. aphylla) and native Frankenia spp. Biological Control 50: 243–261.

Nissen, S., Sher, A., and Norton, A. (eds.). 2009. Tamarisk: Best Management Practices in Colorado Watersheds. Colorado State University, Ft. Collins, CO. 80 pp. https://www.riversedgewest.org/sites/default/files/resource-center-documents/CSUtamariskBMP_lowres.pdf.

Pratt, P.D., J.C. Herr, R.I. Carruthers, M.J. Pitcairn, B. Viellgas [sic], and M.B. Kelly. 2019. Release, establishment and realized geographic distribution of Diorhabda carinulata and D. elongata (Coleoptera: Chrysomelidae) in California, U.S.A. Biocontrol Science and Technology 29:686-705. doi:10.1080/09583157.2019.1587739.

Siemion, G.M. and L.E. Stevens. 2015. Interactions among Tamarix (Tamaricaceae), Opsius stactogalus (Cicadellidae), and litter fungi limit riparian plant establishment. Advances in Entomology 3: 65-81. doi:10.4236/ae.2015.32008

Silva, S.O., M.S. Hernández, I.V.L. Sánchez, L.D.C. Peña, J.L.D Ángeles, and J.C.C. Hernández. 2018. First record of Coniatus splendidulus (Fabricius) (Coleoptera: Curculionidae) in Baja California, Mexico, collected from Tamarix hohenackeri Bunge (Tamaricaceae). The Coleopterists Bulletin 72: 388-389.
doi:10.1649/0010-065X-72.2.388.

Tracy, J.L. and T.O. Robbins. 2009. Taxonomic revision and biogeography of the Tamarix-feeding Diorhabda elongata (Brullé, 1832) species group (Coleoptera: Chrysomelidae: Galerucinae: Galerucini) and analysis of their potential in biological control of Tamarisk. Zootaxa 2101: 1-152. doi:10.11646/zootaxa.2101.1.1

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Patrick J. Moran, Research Entomologist, USDA;Dr. Lincoln Smith, Research Entomologist, USDA