Anyone who has been on the a western river such as the Colorado, the San Juan, or the Green is familiar with Tamarisk. Hikers and backpackers have likely encountered the unforgiving bush along canyon bottoms drainages and around seeps and springs. This invasive species, drawn to sources of water, has taken over many western rivers and drainages and driven out native vegetation.
Now a coalition of government and private agencies including the BLM, the National Park Serivce, the Nature Conservancy, the USDA, and the University of Utah, in Utah, as well as other western states, are using a beetle native to Asia, the tamarisk leaf beetle, as a natural, biological control. In Utah the beetle has been introduced along the Colorado River near Moab and has made its way up upriver to Cololrado, downriver to lake Powell, up the Dirty Devil, and up the Green River at least as far as Mineral Bottom.
The tamarisk, Tamarix spp., came to the United States in the early 1800's as an ornamental plant, windbreak, and to stabilise riverbank erosion. The tamarisk is so effective at controlling riverbank erosion that it is credited with channelising many western rivers, including the Colorado River. In doing so it impedes the natural tendancy of the river to move through bottomlands, thus increasing the hold it has on the river channel.
The tamarisk is deciduous shrub or small tree, growing to 15 to 20 feet in height. Under ideal conditions it can reach as high as 40 feet. The tamarisk is a phreatophyte, having a deep, extensive root system, often reaching 100 feet in length. Phreatophytes use large amounts of water. The tamarisk increases soil salinity as its saline leaves drop to the ground.
The tamarisk is native to the middle east, North Africa and the Mediterranean, where it has many natural enemies to keep it in check. There are some 60 species of the plant native to these areas, with about 8 of those species now in the U.S. Tamarisk is found throughout the southwestern U.S., from California east into Texas, from Mexico north into Nebraska. Tamarisk is also called salt cedar and tamarack, although according to the Audubon Guide to Western Tress, it is not related to the Tamarack, nor the cedar.
Seeds of the tamarisk are dispersed by air, and there are thousands per plant.
The Tamarisk Leaf Beetle
The tamarisk leaf beetle, Diorhabda elongata, a leaf-feeding beetle, is one of many natural controls for tamarisk. The beetle is about 1/4 inch long, looking much like an Elm leaf beetle. None of these natural controls made it to the U.S. when the plant was imported. The beetle was "discovered" in 1986 as a potential natural, biologically based control for tamarisk. The beetle was studied for 9 years, including secure, caged field studies in 1999, before the first beetles were released in 2001. The beetle was first introduced along the Colorado River in 2004. About 300,000 beetles, imported from Kazakhstan, were released at nine locations including Dewey Bridge and downriver from Moab near the Potash boat ramp. This original release has spread hundreds of river miles up and downriver.
Tamarisk leaf beetles eat the tiny leaves of the tree, causing the tree to brown. Each year the beetles return, denuding the same stands and moving on to new ones. Studies indicate that it will take as few as three years of the beetles returning to the same tree to cause it to die.
Stands of Dead Tamarisk
The effectiveness of the tamarisk leaf beetle is very evident with a drive along the Colorado river near Moab, with miles and miles of brown tamarisk. But the dead, dry tamarisk creates a potential fire hazard. To combat the wildfire problem, the BLM and private companies have already begun clearing sections of dead and dying tamarisk along the river corridor. While this clearing is possible along sections of rivers bordering roads, it is not possible in more remote areas. The wildfire potential in these locations will be far greater. With luck the beetles will kill the trees off allowing indigenous vegetation to reclaim its place along river corridors before wildfires occur. But with recent weather patterns and declining rainfall over the past 10 years or so, the outcome remains to be seen.
Read more about tamarisk at the USDA National Agricutural Library