Box Tree Moth Is New Invasive Threat to Boxwoods

Over the years I’ve written all too many times about invasive species. The earliest I remember was the Gypsy moth, which is now known as the spongy moth. It literally devastated thousands of acres of oak trees, leaving the trees denuded of foliage, and at times it was said if you stood in an infested oak forest you could hear the chewing.

This was accompanied by a species of Lythrum known as the purple loosestrife. This plant came to us from Europe and Asia. It began to invade our wetlands and wastelands. It was a serious threat to wetlands because without any natural enemies the plant crowded out other wetland plants and was so tenacious that it actually filled in wetlands, pushing out other species of native plants.

This near disaster was further complicated by a nursery industry that produced several other varieties of Lythrum, which they insisted didn’t crossbreed with the invasive species — only to find years later that they did, and very well. There were two results.

The first was that the sale of Lythrum was banned in most states. The second was the introduction of an insect that actually ate the plants. In a decade the incidence of the plant was severely reduced to the point where you rarely see it in the wild anymore, and where it does occur, the insect that loves to feed on it does a very good job at management.

Then there was the giant hogweed, the lone star tick and the boxwood leafminer, followed by the boxwood blight. The leafminer seems to be well under control, but in areas where there isn’t good monitoring and chemical control, the blight still shows up. But wait, boxwood lovers — and the Hamptons is filled with them — there is a new threat, and it’s moving in from the south and the west. We know it’s coming to a boxwood near you, and now is the time to learn about it and how to control it.

This new threat is the box tree moth. It was found in Canada just a few years ago, and in 2022/23 in Ohio, then in Western New York. Earlier this month it was found in Delaware at a private residence, and New Jersey is very nervous. It most likely is being introduced through nursery stock despite quarantines and inspections intended to slow its spread. The other problem is that if you don’t know where to look for the damage and what it looks like you can easily pass it off as blight damage until the shrub is killed. Unlike the leafminer and the boxwood blight, this insect can kill a shrub in just one season.

The damage from Cydalima perspectalis isn’t from the moth stage of the insect but from the larval, or caterpillar, stage. The caterpillars are about a half to three quarters of an inch long, yellowish green in color with a row of black dots down the length of the top with black stripes from front to back down the length of the caterpillar’s sides. The moth has seven instars, or growth stages, and each instar results in a larger caterpillar with a larger appetite. To make things worse, it’s been suggested that the insect can have as many as five generations a season, depending on the location.

As the growing season draws to a close the caterpillars pull the boxwood leaves together and construct overwintering structures called hibernaculum. You’d need to pull the leaves apart to expose the structure within where silks are used to hold the structure together. At this point the insect can be somewhat immune to control and discovery.

Ohio State Extension has observed that the damage to boxwoods seems to be much more severe in full sun areas as opposed to plants growing in more shaded spots. You might also notice the black pellet droppings on the underside of the foliage that looks like lightly ground pepper.

Ohio State also notes that as the caterpillars get larger they will eat entire leaves but often leave partially chewed leaves with the midvein intact. The feeding often leads to curling of the leaves, resulting in a hair-like curlicue symptom that no other insect on this shrub produces.

When left untreated the caterpillars can leave plants that are completely defoliated, which the blight or leafminer does not produce in one season. Another clue can be silk webbing within the shrub. No other box insect produces silks though this one will leave behind some telltale silks.

In the early pupal stages of development, the pupa don’t resemble the caterpillars, but in the early green stage you might still notice black lines along the sides. The late pupal stage is brown with black lines and is about a half inch long. From this stage the pupa develops into a moth.

So, what’s a gardener to do? There are a number of effective sprays, and some are organic and fairly specific to caterpillars. In all likelihood you’ll want to use a licensed arborist to handle the control, but if you’ve got just a few plants you may be able to manage them. Remember though: They are not here yet so even prophylactic treatments will be wasted.

You do need to be observant though, and if you are on a monitoring program with your landscaper or arborist make sure they are aware of the moth and are watching for it.

Pheromone traps that contain a sex attractant have been developed for the moth, and that bodes well for monitoring the progress. It’s getting closer to us. Remember though what we’ve learned from the Japanese beetle pheromone traps. These are great tools for monitoring populations of the insects, but since they may draw them into an area they need to be used very carefully and not for insect control.

I’ve used an Ohio State Extension bulletin for much of the background information for this column. You can find the bulletin at bygl.osu.edu/node/2262, The bulletin contains much more information and dozens of pictures of the moth and the caterpillar in its various stages. Knowing what to look for is critical, and now’s the time to read up.

When will the moth or caterpillar arrive here? Probably within the next two years. Share the Ohio link with friends and other gardeners because the better informed the East End is, the easier it will be to manage this new invasive. Don’t panic, but be ready. And keep growing.