LIMERICK – The state escalated its war against invasive aquatic plants Monday, pumping several gallons of pesticide into a southern Maine pond in hopes of poisoning a particularly nasty plant called hydrilla.

“We’ve got to nip this in the bud, or we’re going to lose a whole lot of battles,” said Paul Gregory, a spokesman for the Department of Environmental Protection, watching from shore Monday morning while an airboat buzzed around, pumping pressurized liquid herbicide under the surface of Pickerel Pond.

The state is spending $36,000 from the invasive aquatic plant sticker program on this operation, betting that Maine’s sole hydrilla infestation can be stopped here before it spreads. That’s what makes it worth the risks for the DEP to take such a drastic measure in Pickerel Pond, explained Gregory.

Despite pressure from some lakes associations, environmental regulators have long opposed using pesticides to control unwanted plants, because even the gentlest herbicides kill beneficial native plants along with hydrilla or milfoil.

Monday morning, Pickerel Pond was dotted with lily pads and ringed with cattail. Within days, most plants in the pond will start to die, and they likely won’t rebound until next spring, said John McPhedran of the DEP.

But the damage that hydrilla could do in nearby Lake Arrowhead, or even in the valuable lakes region an hour or so north of here, is very real and outweighs the cost and the risks to local plants, DEP officials said.

“We want the public to know that we’re not going into the pesticide business. This is a serious problem and it needs serious medicine,” Gregory said.

Hydrilla doesn’t look like much this time of year. The spindly green plant is just beginning to send shoots up through the water. By midsummer, however, hydrilla can form thick mats of vegetation in water as deep as 20 feet, choking out other forms of life. Last year, two-thirds of Pickerel Pond’s 46 acres was clogged with the South American plant.

“It could literally form a carpet,” said Gregory.

The hydrilla infestation surprised state officials who had been concentrating on two varieties of milfoil: variable milfoil, which already is found in 14 Maine lakes, and Eurasian milfoil, which is causing problems in Vermont. Faster-growing and hardier than the milfoils, hydrilla is actually a bigger threat, but officials had believed that Maine was protected by distance. Only five ponds in New England are known to have been infested, the nearest one in Connecticut.

Hydrilla first emerged in Florida, where biologists speculate that it probably came from an aquarium. Today, Florida spends millions of dollars controlling the nuisance plant, which has been spreading up the East Coast. The state probably will never track the source of the Pickerel Pond infestation, because the plant is so efficient at traveling. A scrap of leaf only a quarter-inch square can colonize a new lake when it is spread by shoes, boats or fishing gear. Scientists believe the plant also can pass through the gut of a bird, then grow wherever it’s excreted. In California, tubers the hydrilla puts down into the sediment were shown to remain fertile after a lake was drained a dozen times.

“This critter can move,” Gregory said. “This is an exquisite plant.”

State officials don’t use the word eradicate at Pickerel Pond. Hydrilla will always come back. The DEP, however, hopes that by killing 95 percent of the vegetation this spring, it can gain control of Pickerel Pond, so that mechanical means, such as pulling plants, can keep the growth in check, McPhedran said.

The state hired a Massachusetts company to apply only a few gallons of an herbicide called fluridone over the entire pond on Monday, then two “booster applications” later this summer. A concentration of between 6 and 20 parts per billion of fluridone will remain in the water for the next 90 days, said Gene Meserve, an employee of the state Board of Pesticides Control.

The concentration does not exceed state Bureau of Health drinking water standards, and DEP scientists say the chemical won’t harm fish, amphibians or people. Dozens of cottages surround the pond, mostly summer homes used by out-of-state residents. After the initial pesticide application is completed, there will be no restrictions on swimming or eating fish from Pickerel Pond.

Local residents will be inconvenienced, however. Later this week, the town will install a gate at the public boat launch, which will be unlocked only for access during scheduled times when an inspector hired with DEP funds is on hand to search for hydrilla and other invasive plants on boat propellers.

Local people are thrilled with the DEP’s swift action, said Martha Hamilton, who serves as chairwoman of the Limerick Board of Selectmen. When the infestation was discovered last fall, people who have lived or summered in the town for decades began to worry about losing the pond that they’ve traditionally used for swimming, boating and fishing.

“We know the consequences of doing nothing,” Hamilton said.

The DEP also will continue its efforts this summer, testing the water at Pickerel Pond to gauge the success of the pesticide, as well as searching lakes and ponds within a 10-mile radius for signs of hydrilla. If any is found, the department likely will not consider pesticides as an option.

“If we find that it’s gone downstream into other lakes, the rules of the game will change,” Gregory said.