Ecology :: Biocontrol of wavyleaf thistle being studied in Texas

Wavy leaf thistle was difficult to find along Panhandle highways five years ago. But now the noxious weed can be found moving into pastures, said a Texas Agricultural Experiment Station researcher. 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 frequentlyeach year to monitor biocontrol work with the U.S. Fish and WildlifeService. At first, they noticed wavyleaf thistle growing in small clumpsalong roadsides, Michels said, but in the past few years it seemed to bespreading.

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

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

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

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

This spring, Earle resurveyed to measure the plant’s spread anddensity, 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 agood line on it, but we wanted to start treatments also.”

A large patch covering several acres in Hutchinson County was selectedas 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 werecollected around Kerrville, where the beetle has been released on wavyleafthistle with good results, he said. This beetle feeds on the seed heads.

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

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

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

The musk thistle rosette weevil is found primarily north of Texas wherethe growing season of thistle is different, Michels said. The problem istrying to gather enough of the beetles in Colorado and bring them to Texaswhile 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 abiocontrol. Neither beetle is considered a threat to crops.

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

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

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

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