Invasive Box Tree Moth Threatens Arboretum's Boxwood Collection

by T'ai Roulston, Curator, State Arboretum of Virginia

Most people are familiar with boxwood, an evergreen shrub commonly used to demarcate areas in landscapes. As a hedge, it can form living fences on property boundaries or partition gardens, especially English formal gardens, into rooms and alleyways. It can provide green screens in the foreground of buildings or take center stage as whimsical topiary. Few neighborhoods lack it in some form, and there are many forms.

While boxwoods as a whole (the genus Buxus) are quite diverse and native to all continents except Australia and Antarctica, the ones we know in temperate gardens derive from a small number of frost-tolerant Eurasian species. And yet these few species have been developed into a vast number of cultivars that take many forms. At the State Arboretum of Virginia, we have only 6 species of boxwood but a total of 185 different kinds (mainly distinct cultivars) derived from those 6 species. That gives us one of the most diverse boxwood collections in the eastern United States.

Non-native plants like boxwoods often have a few glory years when they are introduced to a new landscape with a favorable climate but without many of the herbivores that eat them at home.  Intercontinental travel and shipping, however, eventually relocates all things everywhere. The question for a newly shipped herbivore is not whether it can move by suitcase or shipping container, but whether it finds food when it steps out. If it eats boxwood, it is likely to find a bonanza.

The newest boxwood-eating herbivore to make the journey to Boyce, Virginia is the box tree moth, Cydalima perspectalis. It was detected regionally in the summer of 2025 and at the Arboretum on October 1. So far, we've found it only on a single plant, but the moth is dormant in winter and hard to find early in an outbreak when caterpillars are sparse and small. They will likely have 3-4 generations a year in this climate, eating leaves until the leaves are gone then spilling onto neighboring boxwoods, killing them after several defoliations. The adult moths are good fliers and readily disperse to other areas, stopping to lay eggs on the next boxwood. 

For homeowners that have only one or two plants, there is a chance to intervene in the midst of a defoliation event and save the plants. There are no known natural predators or local parasites that are expected to impact the box tree moth currently so control seems to be down to removing plants, hand-picking caterpillars (which would be daunting and likely ineffective), or applying chemical control. There are various pesticides that will work, including the biopesticide btk (a protein from the bacterium Bacillus thuringiensis var. kurstaki). The challenge is in monitoring plants regularly and applying the treatment effectively.

For the Arboretum, this is a big challenge. We have 856 boxwood plants dispersed over a square mile and many of them are 15-20 feet tall with overlapping canopies. Some have colonized outlying woodlots and slowly spread over generations, making the whereabouts and the size of our boxwood population partially unknown. What happens if we do not scout the naturalized plants on the outskirts of the property? They will likely build up a pest population that will repeatedly spread to the core collection.

Our initial approach to protecting our boxwood collection will be treating infestations as well as removing many of the boxwoods that contribute little to the diversity of the collection or to the character of the landscape. Boxwoods in the woodlot will be early targets for removal as will many in the arboretum collection that have little taxonomic or provenance information. Our goal is to maintain the collection's diversity and its structural presence on the landscape while still taking care of all the other kinds of plants in the arboretum.

It may be counterintuitive to safeguard a plant collection by reducing it, but that is likely to be one of the important steps we take. Additionally, the spaces once occupied by the boxwoods of lowest priority will become available for planting with other things, including more native plants. That will increase the diversity of the arboretum as a whole and, because we seldom need to treat native plants with pesticide to keep them alive, create a better public space for wildlife, insects and people.

For more information about box tree moth, including recognizing it and controlling it, check the frequently updated extension websites of major land grant universities such as Penn State, Ohio State or Virginia Tech.