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Scientists begin efforts to kill giant salvinia

By Terri Richardson
March 19, 2011 at 4:39 p.m.

Efforts by Texas A&M AgriLIFE Research and Extension scientists to combat giant salvinia on Caddo Lake using salvinia weevils were back in full swing Friday as a new study was introduced.

"Our spring study is to see when they lay eggs," said Patrick Ireland, on-site project coordinator for Texas A&M.

As the new study was initiated Friday, Ireland was joined at Caddo Lake by professor Allen Knutson, an entomologist for the extension service.

"In a few weeks, we hope they'll start laying eggs. As the water warms up, this will increase their numbers and control the salvinia faster," said Knutson. "It's a race to see which grows faster, the salvinia or the weevils."

As they began the study, the weevils were counted and will be placed with some green giant salvinia into floating tulle cages to be left on Caddo Lake.

The cages will be sampled each week to see if larvae are found.

"The larva is the worm stage, which causes the most damage to the plant," Knutson explained. "Adults feed a little, but larvae tunnel through the main stem of the plant and kills it. That's what we want, dead salvinia."

To control giant salvinia, about 60 weevils per kilogram of plant is needed or 30 per square foot.

So far, it is still too cold for the weevils to reproduce in the  four 100-by-1- foot tanks where water and salvinia levels are about 18 inches deep.

"The water temperature needs to be about 70 degrees before eggs and larvae, but we are trying to determine when that happens," said Knutson, as he picked the tiny weevils up with tweezers and put them into small, capped vials.

"Most of the research on them has been conducted in Australia, and we need to do it here to understand it's life cycle in a cooler climate," Knutson added.

Results from over-wintering studies of the weevils on Caddo Lake have not concluded as the water continues to warm up.

Meanwhile the team conducting these studies hope to find more cold-hearty varieties of the weevil in other parts of South America before next fall.

"It's possible these could evolve, but in the fourth year of study there has not been enough time for that to happen. Anything is possible," Knutson said. "We see adaptation in insects all the time when subjected to insecticides that stop working as they develop resistance."

Over-wintering studies were conducted using the same equipment for visually checking for larvae.  

"We're interested in looking into South America's higher elevations and in southern areas which may have weevils better adapted to colder temperatures," Knutson said.

He suggested that perhaps giant salvinia weevils from Argentina could better withstand a Northeast Texas winter.

"One of the research aspects is to find cold-hearty weevils," Knutson said. "But before they can be brought in, they must be quarantined so there will not be an introduction of disease or parasites."

Texas A&M also has a facility for the quarantine as he described it.

An obvious benefit for having these other weevils would be their ability to live on the lake year-round to combat isolated patches of giant salvinia undisturbed by cold weather.

Ireland described one such area with a football field size section of salvinia that remained green all winter.

The weather, meanwhile continues to warm up and it will soon be time to give the little guys a chance to prove themselves against the plant.

"We'll be moving them out of the tanks and putting them out on the lake once it's warm enough," said Ireland. "We will mark our sites when we release them and let local agencies know where they are so they do not apply herbicide to our study sites."

The weevils have been planned for areas off the boat roads, which require frequent attention for clearing during giant salvinia's most prolific seasons.

"We will save some back to inoculate the rest of the tanks and continue to raise weevils here all summer," Knutson included.

After the weevils have been introduced to Caddo Lake, Ireland and others from the program will periodically check for damage to the plants and for generations to continue.

"There should be several generations of the larvae before fall," said Ireland.

For more information about the ongoing project visit Ireland's blog online at

The weevil project also has a Facebook page.



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