BOSTON -- After a quiet existence for the past three years, the winter moth caterpillar is back.

NewsCenter 5’s David Brown reported that the hungry caterpillars can kill thousands of local trees.

“A lot of it is a numbers game. There are so many winter moths within the canopy of the tree, and all those holes coalesce,” said Todd Caswell, of Natural Tree.

It's a numbers game that's become a losing battle for thousands of trees in eastern Massachusetts. The number of winter moth caterpillars have been down for the past three years in eastern Massachusetts, but this year, the caterpillars are defoliating and damaging many prized New England trees.

Tree experts said the bugs are eating through leaves from Wellesley to Cape Cod. Plymouth County has been especially hard hit.

“I hate to have to see a tree that is otherwise structurally sound have to die because of an invasive pest,” Caswell said.

Teams from Natural Tree are spraying spinosad, an insecticide that kills the caterpillars, on trees on the South Shore. Although this approach can be effective for individual trees, winter moth caterpillars can be a much larger problem than what one application of the pesticide can provide.

“If it has carte blanche, it can just go into the tress and eat and nothing’s going to naturally come in and parasitize it or be a predator on this insect,” said Deborah Swanson, of the Plymouth County Extension.

A state program releasing a European fly that feeds on the bugs is ongoing, but so far, the caterpillars far out-number the flies. Plus, many towns don't have the money to combat the problem or deal with the dead and dying trees.

Experts said if you can’t spray for the caterpillars this year, there are steps you can take to make sure your trees stay healthy.

“A lot of times, drought can be the tipping scale basically for a tree. So water in times of drought,” Caswell said.