Beetles slacking off salt cedar control

Salt cedar proved all too successful when it was introduced to North America from Eurasia in the early 1800s as an ornamental plant to stabilize stream banks and to provide shade for livestock.

The invasive plant also known as the tamarisk, which does not respond well to herbicide treatments or burnings, started soaking up the groundwater and lowering the water table.

For the past several years, to help keep the plant in check, researchers have introduced exotic insects into areas where salt cedar has put down its roots. But, what does a person do if the insects stop doing their job?

A senior researcher with New Mexico State University is studying why leaf beetles released along the Pecos River in and around Artesia did well controlling the invasive plant for one season, but seemed to disappear after that.

“Is there something about these trees that they don’t like, or is it simply that they don’t like the area they are in?” asked Kevin Gardner, senior research specialist in the Department of Entomology, Plant Pathology and Weed Science. “We don’t know in Artesia whether or not it’s the trees. We don’t know if it’s the weather or the soil or the nutrients, or what it is.”

Gardner said the problem with containing exotic weeds--those not native to the area--is that the plants’ natural enemies, such as insects and diseases, are not available to help control the plants.

“When we bring plants over from an exotic location, they don’t have those natural pests, so the plants are able to escape predation,” he said. “Without anything to feed on the plants or their seeds and keep them in control, they spread. That’s what has happened with salt cedar.”

Exotic beetles effective elsewhere

The leaf beetles were brought to the United States in the 1980s and were kept in quarantine for nearly 20 years, while scientists studied the insects to ensure that they would not be predators to native plants in the region. This protects native plants and the animals that feed on them or use them for shelter.

Originally, the beetles were released in 2000 in Wyoming, California and other northern states. The beetles did very well at controlling salt cedar and successfully defoliated nearly 30,000 acres of the plant within two years.

Leaf beetles do not eradicate salt cedars immediately, but rather defoliate them. It normally takes between three and five years for leaf beetles to permanently kill salt cedar. Biological control, Gardner said, is designed to keep a particular plant under control.

When the beetles were later released in Texas, they defoliated 15,000 acres of the plant in four years. In New Mexico, however, researchers have yet to establish a beetle that will defoliate more than one tree.

“We’ll get one tree that they’ll congregate on and defoliate, then they disappear to who knows where,” Gardner said.

Researchers found that when the beetles are released in areas along the same latitude lines as in their native Eurasia, the insects perform better at defoliating salt cedar, hence the motivation to use beetles from Crete in the American Southwest.

This summer at the Agricultural Science Center at Artesia, tents were set up to house the leaf beetles and varieties of salt cedar, so researchers could study the beetles as they fed on the invasive plant.

The beetles have done well in the controlled environment, but not in the field. Artesia, Gardner said, is in the middle of a drought, which could be a factor in the insects’ not doing well. Another factor could be that there are relatively few salt cedar plants around the Pecos River, a result of herbicide treatments in the area years ago.

Researchers will study the beetles’ habits to see if they prefer one hybrid of salt cedar to another, Gardner said. The researchers also are monitoring the water level in the trees and looking into environmental factors that could be contributing to the beetles’ poor performance.

“We are hoping that by having the Crete beetles here, we can control the salt cedar enough that our native species can continue to grow with it rather than be pushed out,” Gardner said.