Biocontrol of wavyleaf thistle being studied in Texas

Dr. Jerry Michels, Experiment Station entomologist at Bushland, along with Nagendra Earle, a West Texas A&M University graduate student, began looking at controlling the intruding noxious weed with natural controls about two years ago.

Michels' entomology team travels the highways to Colorado frequently each year to monitor biocontrol work with the U.S. Fish and Wildlife Service. At first, they noticed wavyleaf thistle growing in small clumps along roadsides, Michels said, but in the past few years it seemed to be spreading.

Deciding they wanted to look at possible control measures, he submitted a proposal for a grant to the Joe Skeen Institute for Rangeland Restoration. His team received funding for two years.

In spring 2005, Earle began mapping the infestations across the Panhandle. The highest concentrations were found in the northern Panhandle down to U.S. Interstate 40, he said. Wavyleaf thistle has been found along I-40 from New Mexico to Colorado, but not much to the south.

“Our idea was that it was coming in through vehicle traffic, because it is common in Colorado, Nebraska and Wyoming,” Michels said. “Also, thistles love disturbed areas, so any road work could have increased the infestation.”

Once it was all mapped, Earle looked for naturally occurring insects that fed off the plant. He found one large beetle that fed on the seed heads, but that beetle was considered an economic pest of sunflowers, so it was undesirable for use as a biocontrol agent, Michels said.

This spring, Earle resurveyed to measure the plant's spread and density, Michels said.

“We're finding it is spreading into pastures and becoming a problem,” he said. “We need to continue to monitor it for several years to get a good line on it, but we wanted to start treatments also.”

A large patch covering several acres in Hutchinson County was selected as a treatment site, Michels said. Cages were set up with four treatments: no action, mowing, chemical control and biocontrol agents.

The thistle seedhead weevils used for the biocontrol study were collected around Kerrville, where the beetle has been released on wavyleaf thistle with good results, he said. This beetle feeds on the seed heads.

“This preliminary year, the biocontrol seems to be working better than the mowing or chemical control,” Michels said.

However, he is going to study a musk thistle rosette weevil used in Colorado to help control musk thistle. This beetle feeds on the root of the plant.

Michels hopes the combination of beetles can control the perennial plant and keep it from resprouting each year.

The musk thistle rosette weevil is found primarily north of Texas where the growing season of thistle is different, Michels said. The problem is trying to gather enough of the beetles in Colorado and bring them to Texas while the plants are still growing.

While the musk thistle rosette weevil hasn't been established in Texas, it has been approved by the U.S. Department of Agriculture for use as a biocontrol. Neither beetle is considered a threat to crops.

“Whenever we do biocontrol work, that is our first concern,” Michels said.

The ultimate goal is to get both beetles established in cages on roadside wavyleaf thistle plants at high enough numbers so they can be released around the region to naturally spread, he said.

Another part of the study will be to look at a combination of chemical or mechanical methods with the biocontrol using the beetles, Michels said. The hope is a combination will either kill out the plant or weaken it enough to lessen reproduction.