Beetles Take a Bite Out of Saltcedar
Ecologist Ray Carruthers examines saltcedar (Tamarix
parviflora) along Cache Creek in California.
Can a tiny beetle spell doom for a rugged, aggressive weed that's already invaded
streambanks and river channels throughout the American West?
That's what ARS scientists and their
colleagues have found in their investigations of a leafbeetle, Diorhabda
elongata, that they've brought to sites infested with saltcedar, also known
This salt-tolerant, fire-resistant, drought-hardy water guzzler was imported
in the 1800s to help hold stream and river banks in place.
Today, saltcedar's range extends from the Great Plains to the Pacific and from
Canada to northern Mexico. It has crowded out native plants such as cottonwoods
and willows, disrupting the natural surroundings needed by these plants as well
as by birds, fish, and other forms of life.
Saltcedar resprouts vigorously even after the hottest blazes. It blocks river
and stream accessby river rafters or thirsty wildlife and livestockby
forming dense, often impenetrable thickets, first as bushy shrubs and, later,
as pink-blossomed trees that grow to 30 feet.
In addition, saltcedar plays havoc with watercourses. When rivers and streams
overflow their banks, saltcedar bushes can trap natural flood debris, blocking
waterflow and causing new, erosive channels to form. These channels sometimes
undercut farm roads and fields, causing them to collapse.
But the jaunty beetle, imported from China, Kazakhstan, other parts of Asia,
and the Mediterranean regionall lands where the troublesome tree is nativehas
taken an impressive bite out of saltcedar in several study sites in the West.
To this insect, America's vast stands of saltcedar are just one enormous, seemingly
endless banquet. The plant's scale-like leaves offer a nutritious treat for
the insects, whether they're in their caterpillar-like larval stage or have
matured into quarter-inch-long adult beetles.
Besides thriving on emerging leaves and shoots, the beetles also nibble on
the bark of small twigs, all the while posing no hazard to people, pets, or
crops. That's according to ARS entomologist C. Jack DeLoach and ARS ecologist
Raymond I. Carruthers. DeLoach is based at the Grassland Protection Research
Unit in Temple, Texas. Carruthers works in the San Francisco Bay area at the
ARS Exotic and Invasive Weeds Research Unit, part of the Western Regional Research
Center at Albany, California.
Scoring Big in Lovelock, Nevada
Diorhabda elongata larva (typically 7 to 12 mm
long) feeding on saltcedar.
The beetles' greatest success since first being turned loose in saltcedar
stands in 2001 has been along the Humboldt River in Lovelock, Nevada, about
80 miles northeast of Reno. Here, an original colony of some 1,400 laboratory-reared
beetles has "expanded exponentially and has now defoliated about 5,000
acres of saltcedar," says Carruthers.
Today, the helpful insects number in the millions. What's more, they've extended
their range, spreading at least 100 miles along the Humboldt Riverdefoliating
and stunting saltcedar. Some of those trees are beginning to die.
"We've seen a similar impact in some other study sites in Colorado, Nevada,
Utah, and Wyoming, with hundreds of acres of trees hit hard by the beetles,"
notes Carruthers. He and DeLoach expect that figure to climb to thousands of
acres by summer's end.
Experts like Carruthers and DeLoach view biological control agents such as
the D. elongata leafbeetle as perhaps the best long-term solution to
getting tough with tamarisk.
Of course, the insects represent a considerable investment. Initially, they
have to be collected abroad. Then they have to be extensively studied to make
sure they don't inadvertently damage other plants. Finally, they have to be
put to work outdoors and carefully monitored. Ideally, the beetles will provide
an impressive return on these investments.
Technician Julie Keller and entomologist John Herr assess
results of a host-specificity experiment measuring feeding damage by the
biological control agent Diorhabda elongata on saltcedar (Tamarix
spp.) and on nontarget native plants in the genus Frankenia.
A Little History
The beetle saga began in the 1970s when ARS entomologist Lloyd A. Andres at
Albany, now retired, became concerned about saltcedar's unchecked incursion
into agricultural lands in western states. He enlisted the help of colleagues
in Asia to search for insects to control the plant.
Entomologist Robert Pembertonthen at an ARS laboratory in South Korea
and now with ARS in Fort Lauderdale, Floridacollected D. elongata
in China, making him the first to identify this insect's promise as a biological
control agent for the United States.
Then followed nearly two decades of pioneering investigations of the beetle
and the weed by collaborators Bao Ping Li in China, Roman Jashenko and Ivan
Mityaev in Kazakhstan, and DeLoach.
In ARS-led outdoor tests, begun in 1998, beetles were confined with saltcedar
and other plants in carport-sized screened cages. "The beetles proved effective
in controlling saltcedar and at the same time safe for the environment,"
Carruthers says. Because the beetles controlled saltcedar but didn't feed on
other plants, the scientists received federal and state approvals to turn the
beetles loose at test sites in the West.
These studies represent the first timein the more than 170 years during
which tamarisk has run rampant in the United Statesthat any natural organism
had been lined up to challenge this pest.
But much more remains to be done. Though the beetles have established many
flourishing colonies, they originally failed to reproduce at southerly test
sites, such as those in Texas.
Deceptive Daylengths Confuse Beetles
Browning, defoliating saltcedar trees in Big Spring, Texas,
in July 2004, 3 months after Diorhabda beetles from Crete, Greece,
Entomologists Phil A. Lewis, formerly at DeLoach's lab, and Daniel Bean of
the University of California at Davis, working at Carruthers's lab, discovered
the reason for this failure to survive: Mistaking the region's shorter days
as a signal of winter's approach, the beetles went into hibernation, called
diapause, in July2 months too early. The mistake was fatal. The following
year, they never woke up to complete their life cycle. Without enough fat reserves
in their bodies to tide them over until saltcedar leafed out in spring, they
starved to death.
To help solve this problem, ARS scientists and collaborators collected D.
elongata from Uzbekistan through Greece to Tunisia, where daylengths better
align with those of the American Southwest.
"These southern-adapted beetles," says DeLoach, "are now defoliating
saltcedars in California, Texas, and New Mexico," where ARS and university
scientists are monitorning their progress.
Adult Diorhabda elongata leafbeetle (about 5 mm
long) on saltcedar flower buds.
Spying From the Sky
Now, the researchers are streamlining tactics to track the spread and success
of the hungry insects. For example, ARS remote-sensing specialists Jim Everitt
of Weslaco, Texas, and Gerald Anderson of Sidney, Montana, are using a trio
of technologies to spy on the beetle and the plant.
This technological mix includes color aerial photography and two types of
scanner-derived aerial imagery, hyperspectral and multispectral.
All have helped the saltcedar scientists assess the beetle's work. Monitoring
the beetle's progress also entails scoutingon foot. That job may be less
of a hassle in the near future, thanks to a new, ARS-developed lure that beetles
can't seem to resist. (See "Tracking
an Attackerof Saltcedar" in this issue.)
The intensive studies of the diminutive beetle have revealed other secrets
about it. For example, what's been regarded as merely one species of beetlewith
biotypes adapted to different areasmay actually be several different species.
"Saltcedar Scientists Trounce Troublesome Tree" in this issue.)
Too, the work has underscored the need for other biocontrol agents to complement
the beetle. Those studies are being carried out at the ARS European Biological
Control Laboratory, in Montpellier, France.
Setting New Records
ARS's tamarisk-taming studies link with efforts of others in the national Saltcedar
Biological Control Consortium. Specialists from federal, tribal, state, regional,
and environmental organizations are involved in tackling this weed. That makes
this consortium likely the largest of its kind ever established in this country.
Also unparalleled in the history of U.S. biological control of weeds are the
international scope of the research, the variety of ecosystems ARS scientists
have scrutinized in their studies of the beetle, and the number of states encompassed
in the outdoor tests.
Perhaps in another few years, this carefully orchestrated campaign to stop
saltcedar's spread may become one of America's most remarkable biological control
Wood and Don
Comis, Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at www.nps.ars.usda.gov.
C. Jack DeLoach is in the
Grassland Protection Research Unit, 808 E. Blackland Rd., Temple, TX 76502;
phone (254) 770-6531, fax (254) 770-6561.
Raymond I. Carruthers is in the USDA-ARS
and Invasive Weeds Research Unit, Western Regional Research Center, 800
Buchanan St., Albany, CA 94710; phone (510) 559-6127, fax (510) 559-5737.
"Beneficial Beetles Take a Bite Out of Saltcedar" was published
in the April 2005
issue of Agricultural Research magazine.