Hillary Thomas, a third-year doctorate student in the Department of Entomology, University of California, Davis, recently won three scientific awards for her research on an exotic beetle that feeds on the leaves of the invasive tamarisk or saltcedar tree.

Thomas received a $1000 Western Aquatic Plant Management Society scholarship; a $500 prize for her student research paper and oral competition at the California Weed Science Society meeting in Ventura; and a $500 Tamarisk Coalition award presented at the Tamarisk Research Conference in Fort Collins, Colo.

The Tamarisk Coalition, based in Grand Junction, Colo., praised Thomas for her commitment to protecting natural resources and her contributions to the field.

The saltcedar leaf beetle, Diorhabda elongata, native to central Asia, is a natural enemy of saltcedar (Tamarix spp.), a plant imported to the United States in the early 1800s for erosion control and as an ornamental, windbreak and shade tree. It now negatively impacts several million acres of prime riparian land in the western United States.

"The beetle has shown great potential to control the weed in some release areas," said Thomas, who collaborates with the Exotic and Invasive Weeds Unit of the U.S. Department of Agriculture/Agricultural Research Service. "It causes defoliation to the extent that it appears there will be a population-level effect on Tamarix stands, but it has not performed well in California over the last few years, until recently."

"My work focuses on improving establishment success here (the Cache Creek watershed, near Rumsey, Yolo County). I have found evidence that differential host preferences of the beetle may be playing a role; the beetles seem not to prefer invasive T. parviflora, and this is the dominant invasive at release sites in Northern California."

Thomas has now imported a new beetle population collected from a site in Greece where the host plant "can be definitively identified" as T. parviflora. "I plan to do comparative host range testing alongside our original colony in quarantine in the spring," she said. "If it shows higher preference, it will be interesting to see whether these beetles will perform better in the field."

The saltcedar beetle, yellowish with dark stripes, is a biological control agent that belongs to the family Chrysomelidae, or leaf-eating beetles. D. elongata feeds on saltcedar foliage during both its adult and larval stages.

Thomas studies for her doctorate under major professor Richard Roush, director of the UC Integrated Pest Management Program and an entomology professor at UC Davis.

A native of San Francisco, Thomas grew up in the Bay Area. She received her bachelor of science degree from UC Berkeley in 2003. Her research interests, in addition to improving establishment success and efficacy in biological control, include population genetics and evolutionary biology.

Saltcedar, which grows as either a small shrub or tree, forms dense thickets along streams, rivers and lakes, pushing out willows, cottonwoods and other native plants. Chemical, mechanical and cultural control techniques are effective, but expensive and temporary, weed scientists say.

The saltcedar was first brought to North America in the 1800s from southern Europe or the eastern Mediterranean region, said UC Davis weed specialist Joseph DiTomaso of the Department of Plant Sciences and a board member of the Tamarisk Coalition. Now thoroughly established in the western United States, it is extremely aggressive and invasive, he said.

DiTomaso attributes the success of the saltcedar to "its growth habit, reproduction, water usage, ability to tolerate highly saline conditions, and redistribution of salt from deep in the soil profile to the soil surface. The leaves of saltcedar excrete salts that are deposited on the soil surface under the plant, inhibiting germination and growth of competing species."

A very thirsty tree, the saltcedar takes in massive amounts of water through its long tap roots that reach deep water tables. "It's estimated that the saltcedar in the southwestern United States consumes about twice as much water a year as all the cities in southern California combined," DiTomaso said.

He described saltcedar as a "facultative phreatophyte," meaning that it likes to have its roots below the water table but does not require it--and "it is often able to survive under conditions where groundwater is inaccessible."

Also known for its high seed production, a mature saltcedar can produce as many as 600,000 seeds annually, DiTomaso said.