TREES IN DANGER: The hemlock woolly adelgid, a tiny black insect with a white ring around its perimeter, manifests its presence this time of year when it produces egg sacs that look like a white cottony substances on the bottom of the needles. (Photo courtesy of Wikipedia)

By Donald Gilpin

It’s a diversion perhaps amidst the current pandemic, but Princeton’s trees are confronting their own crisis in the shape of at least three destructive pests: the emerald ash borer, the spotted lanternfly, and the hemlock woolly adelgid.

A concerned resident last week reported that “many of the hemlock trees in the Riverside neighborhood have woolly adelgid on them and will likely die from this pest.” She added that treatment is not expensive and would be helpful in saving the trees.

Robert Wells, Wells Tree and Landscape founder and associate director of the Morris Arboretum at the University of Pennsylvania, noted that the hemlock woolly adelgid (HWA), native to East Asia, has been around for 30-40 years. Hemlocks face many challenges, he said, but pockets of hemlocks have adapted and are doing quite well.

Princeton Arborist Taylor Sapudar commented on the HWA problem, but suggested that hemlocks, more prevalent in New England, are not currently the Princeton tree community’s greatest concern. “I don’t see hemlocks planted as much now as in the 1940s, ’50s, and ’60s,” he said. “On older properties, in the Western Section of town, on Boudinot and Library Place, you see a lot of them, many hemlock hedges planted in the 1960s, but in developments now you don’t see them.”

Sapudar pointed out that infestations of the HWA appear in the form of a white cottony substance, the insect’s egg sacs, showing up at this time of year on the underside of the leaves. “Look for a small white cotton substance,” he said, “but don’t confuse that substance with the natural underside of the hemlock leaf, which displays two white lines called lenticels that are like the lungs of the leaf.”

Sapudar recommended contacting a local arborist who has the equipment to apply the horticultural oil to suffocate the insect if signs of HWA are detected on the hemlock leaves. “Home Depot sells horticultural soap that you can spray, but a homeowner is not going to be able to apply it as efficiently and effectively as a local tree care company with the compressors and guns that can spray a big tree,” he said.

Wells also recommended a professional application of oils or soaps and warned against the use of pesticides, which can do more harm than good if applied sloppily. He stated that hemlocks can be kept healthy for many years with proper attention and careful use of horticultural oils and soaps.

The HWA, related to aphids, is only about a millimeter in size — a flat, black oval with a ring of white wax around its perimeter. The insect injures the tree by sucking up plant sap and introducing a toxin into the tree that eventually damages the vascular system. Hemlocks suffering from previous stresses, if untreated, could succumb to this damage in as little as three years.

“If people are going to plant them, they should be prepared to do a spray twice a year to keep the adelgid off,” Sapudar said.

It is the emerald ash borer (EAB), however, that poses the direst threat. Wells and Sapudar agree on the EAB’s devastating effects on Princeton’s ash trees. “We saw it coming more than 10 years ago,” said Wells. “There were predictions that it would kill every ash out there, and that’s what it’s doing.”

Wells described the EAB as a “secretive borer,” visible only when it emerges in the spring but probably present whenever woodpeckers are heard pecking away the bark to uncover them. “Almost every ash tree will be affected,” Wells said. “They’re goners.”

Wells advised that ash trees be taken care of preemptively by either being treated or taken out. “By the time you see the effects, it’s too late,” he said.

Wells noted that trees that had been treated were surviving. He cited 17 ashes in Marquand Park that have been treated proactively in three cycles, now every other year, and are currently thriving.

Sapudar pointed out that the municipality treated a number of ashes last spring, but for the most part the plan is for mass removal. “If they’re there they mostly have the EAB, and we’re removing them, but we’re also working on replacing trees as well,” he said.

Princeton Council has allocated funds for removing the larger trees and problematic trees that the Department of Public works cannot handle in-house.

Another non-native Asian species, the spotted lanternfly, has appeared locally in the last couple of years, spreading quite rapidly, Wells said. Sapudar added that they had been seen on ailanthus trees in Princeton.
They can cause significant damage to trees. When they feed they take in a lot of material and excrete it back out again as a sugary substance called honeydew. “It makes a big mess and is attractive to other small insects, including wasps and bees,” said Wells.

The biggest lanternfly threat, Wells said, is to agriculture: grapes and apples in particular. “For homeowners,” Wells said, “it’s not much of a threat at all.”

The spotted lanternfly is approximately one-inch long with gray and black spotted forewings and hindwings that are contrasting patches of red and black with a white band. The abdomen is yellow with broad black bands, and the legs and head are black.