The same thing happened in 2010 — but this year’s setback came as the northerly weevil populations were just getting big enough again to have noticeable effects on the weeds called giant salvinia.
It takes about three years along the coast for populations to recover enough to control salvinia, and it may take a bit longer farther north, said Dearl Sanders, the LSU AgCenter scientist in charge of Louisiana’s salvinia weevil nurseries in south Louisiana, where the bugs are still flourishing.
“They have to be out in the wild and reproduce through about 15 generations to get numbers high enough to have an impact. If you wipe them out every three years or so, you have to start again,” he said.
A strand of giant salvinia — Salvinia molesta to scientists — looks like a lei made of furry, nearly nickel-sized leaves. A patch can double its size in as few as four days, creating mats up to three feet deep. Weevils kill the floating fern by eating and laying eggs in the bud at the end of a string.
In December, Sanders said, there were as many as 15 weevils per pound of salvinia in north Louisiana lakes. By mid-January, about a week after the coldest weather, “the populations were hammered pretty hard,” Sanders said. “Some places we didn’t find any weevils, some we just found one or two per two pounds of plant material.”
In a salvinia nursery, about 120 pounds of the floating fern — enough to cover 64 square-feet — holds about 3,000 weevils, each about one-tenth of an inch long. Containers full of the weevily weed get hauled statewide from four nurseries to fight salvinia.
Sanders and other scientists in Louisiana hope the few northern survivors can breed a new variety that’s more cold-tolerant than those in coastal parishes.
First the scientists must prove the four small groups really are winter-hardy and didn’t just luck into relatively warm spots.
That means breeding a thousand or so from each group for cold tests. The groups are being cultivated in greenhouses at the AgCenter’s main campus in Baton Rouge and its Red River Research Station at Bossier City.
The cold tests use chambers that can be programmed to simulate frigid nights and warmer days.
In the meantime, the state will need much less salvinia herbicide than usual, said Alexander J. Perret, the Louisiana Department of Agriculture and Forestry’s aquatic plant control coordinator.
He said the cold winter of 2010-11 killed so much salvinia the department only had to spray 57,218 acres with herbicide that fiscal year, compared to 75,000 acres after the following warm winter and 102,425 acres in the last fiscal year.
“It just kind of compounds,” Perret said.
Allen Knutson of Dallas is among Texas A&M University, AgriLife Extension Service scientists studying two sites on Cross Lake on the Texas-Louisiana state line. He said 100,000 weevils were released over the previous two years. Their descendants had controlled the weed on about 10 acres by December, but about 95 percent were wiped out during the winter.
“The big news is we are finding some. And insects have a high reproductive rate. So we’re hoping even though it’s a small population, they will increase rapidly” once it warms up for good, he said Wednesday.
The weevils used here originated in Brazil. A&M has enlisted an Argentinian national laboratory to find weevils in Argentina, which is closer to Antarctica and thus colder.
That search hasn’t gone well so far, said Guillermo Cabrera Walsh of the Fundación Para El Estudio De Especies Invasivas in Hurlingham, Buenos Aires.
He said four scientists are collecting salvinia whenever they come across infested waterways, then drying it to collect weevils.
“However, collections so far have been very poor indeed. At present we have around a dozen weevils, and we’re trying to build colonies from them, without much success up to now,” he wrote in an email Thursday.
“We are considering moving the collections further north” in hope of finding more weevils, he said.