PORTLAND – While Maine officials have been focusing on milfoil as Public Enemy No. 1 for lakes and ponds, another invasive aquatic plant has slipped into the state for the first time, officials said Tuesday.

The discovery of hydrilla in Limerick’s 46-acre Pickerel Pond this fall came as a surprise because the only other New England states where the plant’s existence has been confirmed are Massachusetts and Connecticut.

“This wasn’t the next invasive plant we expected to see,” said Karen Hahnel of the Maine Department of Environmental Protection.

Hydrilla is one of 11 species on the state’s list of invasive aquatic plants. It is far more aggressive than variable milfoil, the only other one found in Maine, Hahnel said from Augusta.

The aquatic plant first was noticed a year ago on Pickerel Pond, and it became bad enough last summer that one of the waterfront owners sent a sample to the Volunteer Lake Monitoring Program in Turner.

Scott Williams, executive director of the monitoring program, said he and a colleague traveled to York County to check out the situation firsthand since hydrilla can be mistaken for common aquatic plants.

“There was no trouble finding it. It was everywhere,” he said. “It took us only a few minutes to conclude, in our minds, that it was hydrilla.”

Williams took images of the plant and shipped them to an expert in Massachusetts to confirm the finding within 24 hours.

As of now, the plant covers two-thirds of the shorefront to a depth of about 6 feet deep, with dense growth in some areas, Hahnel said.

The Maine Department of Environmental Protection has contacted all 69 shorefront owners laying out options for controlling the infestation and for preventing further infestation of neighboring waters.

Aquatic plants typically are spread by plant fragments that are attached to boat propellers, trailers and fishing tackle, and officials are considering a temporary closure of the pond’s public boat ramp.

Hydrilla is a stubborn and fast-growing plant that is difficult to control once it is introduced.

Its roots burrow under the bottom of lakes and ponds, so it is able to withstand the cold winters, and its roots spread with multiple stems, allowing it to spread quickly, Hahnel said.

Once it takes root, hydrilla can foul up boat propellers and make it difficult to swim, she said.

For now, the DEP’s Aquatic Invasive Species Program is investigating a range of methods for controlling the plants, from instructing people how to pull plants by hand to teaching residents how to install underwater barriers.

Herbicides will be considered only as a last resort and after discussions with residents, officials said.

The most important thing is for boat owners to remove plant fragments from their boats, even in ponds and lakes where aquatic plants have not become established, Hahnel said.

Before the hydrilla infestation, state officials were most worried about variable milfoil, which has been found in Maine, and its more aggressive cousin, Eurasian milfoil.

Officials have plenty of cause for worry because invasive aquatic plants quickly can overtake lakes and ponds. Some fear the unsightly plants could reduce shorefront property values and hurt tourism.