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UF Researchers Seek Bugs to Battle Aquatic Weed Plaguing Central, South Florida

Jim Cuda and Abhishek Mukherjee examine Hygrophila polysperma plants
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GAINESVILLE, Fla. — Years of aggressive hydrilla control efforts have paid off for some Florida communities – unfortunately, their success against the invasive aquatic weed has had unintended benefits for a more troublesome plant, says a University of Florida expert who’s researching insects and diseases that might help control the upstart.

For the past decade Hygrophila polysperma – a southern Asian plant known as “hygrophila” for short – has been taking over the ecological niche left when hydrilla was eradicated from waterways, said Jim Cuda, a UF associate professor of entomology. It’s now a significant problem in South and Central Florida.

Like hydrilla, hygrophila (“high-GRAW-fill-uh”) was sold as an aquarium plant, got into Florida waters decades ago and managed to survive. But the similarities end there, he said.

Hydrilla is strictly a water weed, and can be controlled with herbicides, hungry grass carp or mechanical harvesting. Hygrophila can grow fully submerged or up on river banks. Herbicides aren’t very effective against hygrophila, grass carp don’t like it, and mechanical harvesting breaks its stems into tiny pieces, each capable of spawning a new plant.

Given that grim scenario, Cuda and colleagues with UF’s Institute of Food and Agricultural Sciences are hoping to get help from nature – they’re looking for natural enemies that attack the plant on its home turf in India.

“There aren’t any good, cost-effective management options for hygrophila,” said Cuda, who leads the project. “That’s why there’s interest in biological control.”

Biological control is the practice of battling invasive organisms with their natural enemies. It’s the focus of Cuda’s research, and he specializes in using insects to curtail plants running amok in Florida.

Last fall, Cuda and entomology graduate student Abhishek Mukherjee made a collecting trip to several Indian states, described in an article published in the spring issue of Aquatics, journal of the Florida Aquatic Plant Management Society.

The researchers found evidence of at least one insect Mukherjee hopes to capture on a return trip this summer. They also collected samples of wild hygrophila that are now being genetically analyzed to determine if they’re identical to plants found in Florida.

If so, that would mean insects and diseases found in the same parts of India would have a better chance of attacking the hygrophila found in Florida. If not, the researchers may need to save time and move on. That’s because natural enemies are sometimes so highly adapted to a host plant’s genetics that they’ll ignore plants of the same species if they come from a different geographic area and, as a result, have minor genetic differences.

Saving time is a big consideration in biological control, because it takes so much laboratory research to confirm that natural enemies will attack their intended target, and even more research to show that they won’t go hog-wild over other plants such as food crops.

The UF team – which includes Cuda, Mukherjee and Bill Overholt, also a UF associate professor of entomology – recently discovered that the larvae of a native moth species will feed on hygrophila.

The moth has no value as a biological control agent because it isn’t host-specific – the larvae attack more than 60 plants – and is unlikely to put a dent in hygrophila populations. But it can be a great research tool in the laboratory, Cuda said.

“We can sue this insect experimentally, to see if an insect that defoliates the plant in India would have any value as a control method,” he said.

By putting hygrophila in enclosures with various numbers of moth larvae, researchers can learn whether loss of leaves has any long-term effect on the plant, or if it just bounces back. If it’s the latter, they’ll need to focus their efforts on natural enemies that do something more than munch leaves.

Speaking of native organisms, hygrophila closely resembles native alligatorweed, Cuda said. So residents who think they’ve found a patch in local waters should report the location to their county extension office, which can be found at solutionsforyourlife.com. Extension agents will make sure an expert checks the site to determine the plant’s identity.

And, despite its unwelcome presence, residents should not try to eradicate hygrophila on their own, Cuda said. There’s too much chance something will go awry, causing further environmental harm.

In the United States, hygrophila is currently growing wild only in Florida and Texas. It’s been officially confirmed in 10 Florida counties, though Cuda suspects it’s present in at least 20. Previous research indicates the weed can survive cold climates, and could potentially spread as far as hydrilla did – from Delaware to Florida, all along the Gulf Coast, and north to Washington state.

In Texas, hygrophila has already become established in two lakes and a river system, said Marcos De Jesus, a state fisheries biologist with the Texas Parks and Wildlife Department. But right now, the population is small enough that other invasive aquatic weeds take priority.

“The money goes into chemical control or mechanical removal of these other species and hygrophila hasn’t spread enough to warrant a lot of attention,” said De Jesus, in San Marcos, Texas.

But the situation could change, he said. One of the places hygrophila is established is in the San Marcos River, a springfed system that’s home to Texas wild rice, a plant found nowhere else on Earth.

The river is also home to numerous invasive aquatic plants, but due to concerns about the survival of Texas wild rice, no herbicides are used on the invaders.

“If hygrophila gets to be a bigger problem (in the San Marcos River), then biological control might be a good idea,” he said. “If the insects couldn’t survive on other plants, then I think it would be very beneficial in areas where chemical controls can’t be used.”

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