Invasive moths are damaging boxwoods in parts of WNY: what you should do

by Connie Oswald Stofko

The box tree moth, an invasive pest that can damage and even kill a boxwood shrub within weeks, has been identified in Niagara, Erie and Orleans counties.

This pest can spread; the moths are highly mobile and are good fliers.

Anyone with boxwoods in Western New York should watch now for the caterpillar stage of the box tree moth (Buxus species).

The caterpillar feeds almost exclusively on boxwood shrubs.

The caterpillars are ravenous feeders, and heavy infestations can completely defoliate the boxwoods. After the leaves are gone, the caterpillars feed on the bark, which eventually kills the plant, according to this information from the Animal and Plant Health Inspection Service (APHIS), part of the United States Department of Agriculture (USDA).

Background on box tree moth

The box tree moth is native to North China and Korea and was detected in Europe in 2006. It spread throughout Europe since then, primarily from nursery stock shipments, according to this fact sheet from New York State Integrated Pest Management (NYSIPM).

This pest was first detected in North America in 2018 in Ontario. The box tree moth appeared in New York State in 2021 in Niagara County. It is believed that the moths flew or were blown into Niagara County, according to this page from New York State Department of Agriculture and Markets.

Detection

Right now you may see the caterpillar stage of the box tree moth. The caterpillar is green and yellow with black and white markings. They start very small, less than an inch long, and are very difficult to see, said Jacob Leeser, extension aide in NYSIPM, in a talk to Master Gardeners in Chautauqua County.

Caterpillars can be detected by heavy, characteristic feeding patterns, with leaves appearing “peeled” or only the midrib remaining. Green-black balls of frass (droppings) are also indicative of an infestation. 

They grow throughout the season. They are most easily recognized by the webbing the caterpillars create as they mature, which is visible by spreading the branches and looking into the interior of the shrub. Within about six weeks, they begin to pupate, then emerge as adult box tree moths.

Severely-infested shrubs take on a brown or scorched look. 

See more images from the USDA of boxwood tree moth and the damage it can do.

If you have boxwoods, but don’t see signs of infestation yet, you can buy a trap or lure online to see if adult box tree moths are in your area. These traps are only for monitoring; they won’t prevent an infestation, Leeser said. Check the trap regularly. The trap draws in only male box tree moths, not the females who lay eggs, so the trap shouldn’t increase your chances of breeding more box tree moths.

Dealing with box tree moths in WNY

Genesee, Wyoming, Chautauqua, Cattaraugus, Allegany counties

At this moment, the box tree moth hasn’t been identified in any of these counties, but the pest may arrive. It’s important to keep track of where box tree moths are. If you live in Genesee, Wyoming, Chautauqua, Cattaraugus or Allegany counties and find any signs of infestation, please take a photo and report it online here.

Niagara, Erie, Orleans counties

If you live in Niagara, Erie or Orleans counties, you don’t need to report your findings; the box tree moth has been identified in all three of those counties. However, you are encouraged to keep checking your boxwoods regularly for this pest, according to NYS Agriculture and Markets.

Note: This next paragraph may apply to Erie and Orleans counties as well as Niagara County; I am waiting for clarification.

If you find a box tree moth infestation within Niagara County, remove infected branches by pruning them with a thinning cut, according to NYS Agriculture and Markets. (See an explanation of a thinning cut in this video from UC Marin Master Gardeners.)

To make sure the insect doesn’t spread to new locations, dispose of the infected branches by double-bagging them in plastic bags and putting the bags out with your household trash.

Dead, diseased or damaged plant parts can be pruned out at any time of the year.

Picking by hand

With small infestations, hand picking the caterpillars can be effective.

Pesticide

With larger outbreaks it may be most effective to treat with an approved insecticide. You can see a link from NYSIPM to download a list of pesticides for home use; scroll down to the bottom of that page.

If you use pesticides, make sure you do it correctly. You need to apply the pesticide regularly.

“We’ve been to some places where pesticide was applied incompletely, then it rained,” said Leeser. “We saw very high damage on parts of the plant that weren’t covered as well (with pesticide).”

When treating, thorough coverage into the interior of the boxwood shrubs is necessary. You may want to hire a professional applicator to make insecticide treatments.

You can also contact your local Cornell Cooperative Extension office for specific products.  

Replace boxwood with another shrub

Consider an alternative shrub to boxwoods. Find some shrubs that have qualities similar to boxwoods on this list from NYSIPM.