AUGUSTA – State biologists surveyed the state’s only known hydrilla infestation this week and reported that last summer’s pesticide application seems to have been effective.

Pickerel Pond, located near the New Hampshire border in Limerick, harbors Maine’s only known population of hydrilla, often described as the most troublesome of the 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. State Department of Environmental Protection officials responded last June with the state’s first-ever application of aquatic pesticide to kill the invasive plant.

For the season’s first sampling Tuesday, biologists used a homemade tool – essentially a post-hole digger with a long extension – to dig through several feet of lake sediment in search of tubers, fleshy underground stems from which new plants can grow.

Hydrilla is a particularly effective invader because it can spread via tubers, seeds or portions of leaves.

Biologists took 95 samples at 10 different locations Tuesday 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 morning.

Before the herbicide treatment last summer, biologists turned up between 15 and 40 tubers per sample, he said.

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

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 [with hydrilla], even though it looks like it might be continual over an area,” he said.

In Massachusetts, a pond was similarly infested and treated with herbicides, and showed a drastic decrease in tubers. Yet a second round of treatment was necessary because the hydrilla plants appeared once the water warmed up the following spring, he said.

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.

Biologists will visit the pond every other week to observe plant growth and take water samples to determine how much herbicide remains in the water from last year.

Nearby ponds are also being monitored, and thus far, no new hydrilla infestations have been discovered.

Though it’s too early in the season for proof, McPhedran expects that the native plants in Pickerel Pond will come back this spring. The herbicide used to kill the hydrilla also knocked back many native populations, but seeds remain in the soil, so biologists predict a full recovery.

There is no evidence that the herbicide harmed wildlife in the area, as the pesticide works by interfering with plants’ ability to produce food through photosynthesis.