by Dr. Dan Bean Biological Pest Control Program Manager, Palisade Insectary, Colorado Department of Agriculture
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| Defoliated tamarisk on the Colorado River |
Tamarisk (Tamarix spp.) or saltcedar, is a shrubby tree native to Central Asia, China and the Mediterranean region. The tree was first introduced into the U.S. over a hundred years ago as an ornamental plant, and subsequent introductions were brought over for use as windbreaks and stream bank stabilization. Tamarisk is often referred to as saltcedar, due to the presence of scale-like leaves that resemble cedar foliage and that exude salt brought up through an extensive root system capable of extending to depths of 30 feet or more. This highly invasive plant infests an estimated 1.5 million acres along waterways in the western U.S., often becoming the dominate riparian species, displacing native cottonwoods and willows, and in many cases forming pure or monotypic stands.
Management of tamarisk is difficult, time consuming and expensive. Recently a tamarisk bio-control agent has been approved for widespread release and may become the most important management tool available for controlling difficult stands of tamarisk (either extensive stands or stands too isolated to easily reach). The new biocontrol agent is a leaf-feeding beetle, Diorhabda elongata (diorhabda), native to areas where tamarisk originated. The beetle has been studied by federal and state agencies since the early 1990’s and was released into the field at 10 research sites in 2001. Success has been varied but in the best cases the beetles defoliated hundreds of acres of tamarisk in the third year after open releases. At the most successful research site, near Lovelock Nevada, beetles have now defoliated thousands of acres of tamarisk and have spread hundreds of miles from the original release point. At the Lovelock site beetles have killed about 75% of the monitored saltcedar plants following multiple defoliations over a 5 year span.
Success at the research sites gives us a good idea of what to expect as beetles are released on tamarisk stands around the west. First, it takes 2-3 years for the insects to become well established and begin substantial defoliation of saltcedar stands. Defoliation is followed by resprouting which may be followed by a second defoliation within the same season or the next defoliation may occur the following season. With each cycle of defoliation and resprouting the plants weaken and the total volume of foliage decreases while flowering is cut dramatically. The net result is an opening of the canopy and a decrease in seed production. In one study it was shown that evapo-transpiration decreases dramatically during a defoliation cycle, which means there is a water savings, even before the plants are dead.
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| Saltcedar beetle release- 9-03-07 |
Opening of the canopy allows sunlight to reach the ground, enabling other plants to grow. In some areas this means beneficial plants such as native grasses and willows will make a come back, while in other areas this may open the door for other noxious weeds like Russian knapweed and perennial pepperweed. In cases where there is little or no hope for a natural return of native vegetation it will be a land manager’s challenge to do revegetation work following biocontrol. Finally, there is an equilibrium reached between biocontrol agent and weed. There has never been an eradication brought about by a weed biocontrol agent; saltcedar will not be eradicated byDiorhabda. Instead saltcedar will be diminished in abundance and health, and in areas where eradication is the goal, land managers may be better able to remove saltcedar with minimal regrowth.
Diorhabda beetles, like all bio-control agents, were extensively screened for host specificity by the Agricultural Research Service (ARS) and Animal Plant Health Inspection Service (APHIS) of the USDA. Although the USDA found Diorhabda to be a very low risk biocontrol agent, it is still reassuring that after 7 years in the field in North America, there have still been no non-target impacts noted.
Saltcedar biocontrol got a big boost in Colorado when millions of beetles entered the state late in the summer of 2007. They moved in along the Colorado and Dolores Rivers from release sites in Utah. Now that we have a large beetle population within the state, the Colorado Department of Agriculture can collect and redistribute the insects to a number of sites within Colorado. The goal for 2008 is to get beetles out to all river drainages in Colorado. In 2009 beetles will then be readily available for even more widespread distribution. Very soon the saltcedar leaf beetle will become a powerful and inexpensive tool in the battle against tamarisk.
Insect bio-control agents can be obtained free of charge from Colorado Department of Agriculture’s Insectary at Palisade, CO. Call 866-324-2963 , or go online at http://www.colorado.gov/cs/Satellite/Agriculture-Main/CDAG/1251622408364