A threat to our trees? Worcester County crews to look for new invasive pest

Starting next week, crews will fan out through Worcester County and the rest of Massachusetts to look for the invasive elm zigzag sawfly that’s a threat to the survival of elm trees.

Crews will conduct visual surveys in areas where there are dense patches of elm trees, said Nicole Keleher, forest health program director at the state Department of Conservation and Recreation. Those patches are normally located in wet areas along streams and rivers.

Sawflies munch on a tree’s leaves and leave a zigzag pattern on the leaves that defoliates the tree. Young sawflies in their larva stage are small, pale green and resemble a caterpillar. They spin a netlike cocoon on leaves and eventually emerge as winged adults.

The first sawfly appearance in Massachusetts was detected in August, in Berkshire and Hampden counties. The species is native to Asia, and Keleher said experts aren’t quite sure how the insect arrived in North America. Keleher suspects the insects latched onto cargo during overseas travel. They were first reported in Canada in July 2020.

Besides Massachusetts, the sawfly has been confirmed in six states: Virginia, Pennsylvania, New York, North Carolina, Maryland and Vermont.

It only takes one sawfly to increase its numbers, Keleher explained, because all sawflies are parthenogenic. They’re female and reproduce asexually.

“One (sawfly) finds its way to a new area, lands on an elm, feeds, lays eggs and makes more flies,” she said.

There’s a potential double whammy here because some elms are already suffering from Dutch elm disease, said Keleher. The disease results from beetles that feed on elms and introduce a fungal pathogen that attacks the tree’s vascular system.

“It leads to (tree) mortality,” said Keleher.

Two common elm species in Massachusetts forests — American and slippery — could be susceptible to damage from the sawfly. However, since the insect is relatively new to the state, Keleher said, it's unclear what will happen. Sawflies feed on Siberian elms, and since some homeowners like to have them in their yards, Keleher said, those trees could be at risk.

When the insect was first spotted in Massachusetts last year, there wasn't enough time to do field studies, said Keleher, because fall was quickly approaching. Now that days are getting warmer and trees are leafing out, Keleher said it's time to do field surveys.

Contact Henry Schwan at henry.schwan@telegram.com. Follow him on X: @henrytelegram .

This article originally appeared on Telegram & Gazette: A threat to our trees? Worcester County crews to look for new invasive pest