The weeds appear to be arrested at Pickerel Pond.

AUGUSTA (AP) – Samples taken from a pond near the New Hampshire border in Limerick indicate that Maine’s first-ever application of an aquatic pesticide to kill a fast-spreading invasive plant was a success.

Biologists from the Department of Environmental Protection who visited Pickerel Pond this week to survey the state’s only known hydrilla infestation reported that it had largely disappeared.

The biologists took 95 samples at 10 locations in the 46-acre pond and found only two viable hydrilla tubers, John McPhedran of the DEP’s Bureau of Land and Water Quality told members of the state’s invasive species task force Wednesday.

Before the pond was treated last summer with the herbicide fluridone, biologists turned up between 15 and 40 tubers per sample, he said.

Hydrilla has been described as the most troublesome of all the invasive aquatic plants on the state’s watch list. The infestation was spotted by a local resident, then identified as hydrilla in December 2002.

Hydrilla is a particularly effective invader because it can spread through tubers, seeds or portions of leaves. Biologists used a homemade tool to dig through several feet of lake sediment in search of tubers.

McPhedran cautioned that this week’s good news doesn’t mean that the resilient plant won’t make a comeback.

Samples were taken in areas where hydrilla was growing prior to the pesticide application last year, but it’s still possible that the relatively small samples missed some clusters of tubers.

“There’s patchy growth 1/8with hydrilla 3/8, even though it looks like it might be continual over an area,” he said.

Hydrilla hurts aquatic ecosystems by forming dense canopies that often shade out native vegetation and over time change a lake’s chemical makeup.

Once it has established itself, hydrilla is especially difficult to get rid of.

The state’s permit to apply pesticides continues through this summer, so biologists will continue to monitor Pickerel Pond through early June, when they will have to make a decision about a second round of herbicide.