Are Beetles Imported into the U.S. to Kill Invasive Trees Doing Too Good a Job?

Strategy that unleashed cedar leaf beetles on Tamarisk trees may have to be revised as the chompers spread to threaten endangered birds

Anne Minard, Scientific American 19 Mar 09;

A foreign beetle imported to attack invasive trees in the U.S. Southwest may have brought its own culinary agenda. Researchers in Utah and Arizona are sounding the alarm about salt cedar leaf beetles, which were imported from Kazakhstan several years ago to control invasive tamarisk trees.

"Now that the beetle is spreading to large areas, we need to start looking for unexpected consequences of defoliation and death of the tamarisk," says Philip Dennison, a geographer at The University of Utah and lead author of a study warning of the unintended risks published this month in the online edition of the journal Remote Sensing of Environment.

Tamarisk trees, native to Europe and Asia, were first planted in the U.S. in the early 1800s as ornamentals and to stabilize soil, especially on riverbanks. The trees took off, and now dominate 1.6 million acres (650,000 hectares) of mostly riverside habitat throughout the Southwest. Dense tamarisk stands have crowded out native trees like cottonwoods and willows. And tamarisk gets a bad rap for being thirsty enough to drop water tables and dewater small streams—although the new research says the rep may be undeserved. Tamarisk was first identified as a pest around 1900, and biologists since the 1940s have implemented various control strategies, including herbicides, manual removal, and defoliation by goats and beetles. Total cost estimates approach $100 million for the decades-long efforts.

Beginning in 2001 biologists in nearly all southwestern states—with input and funding from the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS)—released the beetles in tamarisk thickets.

The program has been "spectacularly successful, one of the most successful biological control projects ever in the U.S.," says Jack Deloach, an entomologist with the U.S. Department of Agriculture's (USDA) research arm, the Agricultural Research Service.

But Dennison and his co-authors are now proposing that officials put on the brakes, based on satellite monitoring in 2006 and 2007 of 56 beetle-treated areas in southern Utah. The researchers report that beetles have been dutifully stripping the tamarisk of their needles but that some of the immediate effects are worrisome.

As it turns out, tamarisk trees have a silver lining, Dennison says. Their sprawling branches, which are covered with long, pliable needlelike leaves, provide coveted cover for native birds. Among them the endangered willow flycatcher, which routinely nests in the tamarisk thickets that replaced the willow trees there. Salt cedar beetles were originally kept out of Arizona and New Mexico to protect the flycatcher, but researchers report that the beetles are now creeping from Utah's Virgin River into flycatcher habitat in southern Utah and northern Arizona. The Center for Biological Diversity (CBD), a Tucson, Ariz.–based conservation group, has filed a notice of intent to sue the USDA and APHIS to halt the beetle program, charging that the bugs have gotten out of hand and are threatening the endangered birds.

Nate Ament, a restoration ecologist with the nonprofit Tamarisk Coalition in Grand Junction, Colo., says the greatest risk to streamside ecosystems comes in tamarisk's wake. The new satellite data show tamarisk-related water loss is lower than previously believed. If tamarisk trees are killed off by the beetles, newer weedy arrivals—like Russian knapweed, Russian olive and pepperweed—could hammer the water supply even more.

"We really emphasize revegetation with native trees as a crucial component of the restoration process," Ament says.

On that point, all sides agree. The CBD's Robin Silver says one of the aims of the suit is to prompt revegetation with native trees, like willows, in places where the tamarisk might die off.