NCDA&CS Spotlight: The Box Tree Moth

By Amy Michael, NCDA&CS

Boxwoods (Buxus spp.) are one of the most popular garden plants in the world, with records of ornamental boxwood plantings as early as 4000 BC (NCCE). Planted as hedgerows and in parks and gardens practically everywhere, with species and varieties ranging in tolerance from tropical climes down to USDA Zone 4, the boxwood is a very lucrative nursery plant. In the United States alone, an estimated 3.6 million boxwoods are produced each year, with North Carolina ranking No. 7 for production (Frank, 2021). However, every plant has its share of vulnerabilities, and the ubiquity of boxwoods in the landscape has turned the box tree moth into a wellfed, prolific pest capable of destroying whole stands of boxwoods in areas where the insects have been introduced. For that reason, it is critical for North Carolina landscapers, nursery resellers and boxwood producers to understand the background of this pest and prepare to prevent it from becoming a major problem in our state.

The box tree moth (Cydalima perspectalis, BTM) is a defoliating pest of boxwoods, though it will also eat new bark growth once it has depleted the leaves. BTM can produce up to five generations per year in favorable field conditions, meaning populations can grow to explosive numbers in just one season. Heavy infestations can lead to complete destruction of established boxwood stands if left untreated (Figure 1). And while Buxus is by far the most preferred host of BTM, other popular landscape shrubs are also at risk.

According to a 2023 U.S. Department of Agriculture review of BTM literature, they “consider it likely that Murraya paniculata [orange jasmine] is a host of the BTM, and Euonymus alatus and E. japonicus are possible hosts. Ilex purpurea [oriental holly] is unlikely to be a host, and many other plant species mentioned in the literature seem to only have served as refugia after all boxwood leaves and bark in the vicinity had been consumed.” (USDA-APHIS-PPQ: Plant Pest Risk Analysis, 2023). This point has further implications for nursery dealers:

While BTM has only been observed to complete its life cycle on Buxus and the three other species mentioned above, there is still the risk of caterpillars crawling to adjacent plants to pupate — meaning that they could hitchhike in otherwise innocuous plant shipments coming into our state.

Originally native to eastern Asia, BTM was first found in Germany and the Netherlands in 2007 and may have been introduced through infested plant shipments. Europe has several naturally occurring Buxus spp. in addition to ornamental plantings, and the moth quickly spread throughout the European Union, Britain and Ireland. To date, BTM has spread to an estimated 30 European countries (Canadian Food Inspection Agency, 2019).

BTM eventually made a transatlantic voyage and was discovered in a Toronto neighborhood in 2018. The Canadian Food Inspection Agency (CFIA) has surveyed for this moth ever since, and soon found populations in Windsor (near the border with Detroit, MI) and in St. Catherine’s (near the border with Niagara Falls, NY). Unfortunately, Niagara Falls poses no challenge to a winged organism, and the first U.S. detection of BTM was made in Niagara County, NY, in 2021, likely arriving by natural spread. BTM has now spread further south by hitchhiking along with humans, and was found in southeastern Michigan in 2022 and in southwestern Ohio and the Cape Cod region of Massachusetts in 2023. Canada has also continued to survey, finding BTM at several retail locations in Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland and Labrador in 2023 — the first instances in which the pest has been found in Canada outside of Ontario. The CFIA has enacted a quarantine on host plants within infested areas (Canadian Food Inspection Agency, 2019).

The NCDA&CS – Plant Industry Division has surveyed for this pest since 2021 using pheromone lures and traps that trap male moths, focusing on wholesale production locations to determine whether North Carolina has the pest. To date, NCDA&CS has not detected any BTM through its survey. We plan to survey retail locations in 2024. You can find updates about this program online at https://ncagr.gov/ plantindustry/Plant/entomology/CAPS.htm.

While North Carolina does not currently have a known infestation of BTM, we have intercepted plants associated with infested shipments before. Fortunately, we received notification quickly, and these interceptions were managed efficiently. One infested shipment to our state could easily generate an established population of BTM.

BTM is the only pest that defoliates boxwood in North America. Feeding by the caterpillar begins in March, and several generations are possible through late October. The caterpillars overwinter by wrapping themselves in silk chambers between leaves (Figure 3, inset).

BTM caterpillars chew on the leaves of boxwoods. Early feeding may look like dry/ discolored leaves before chewing proceeds to the midrib of each leaf and eventually the bark, once the leaves are gone (Figure 2). Webbing will begin to appear as caterpillars feed, and you may be able to see roundish frass stuck in the webbing, which can help to rule out spider mite feeding (Figure 3). The caterpillars themselves are green with black heads, and they will develop black stripes with white spots as they age, reaching up to 1.5 inches in length before pupating (Figure 3).

The nonlarval stages of BTM are much more cryptic. Pale yellow eggs are laid in clusters on the undersides of leaves with a fish-scale appearance (Figure 4). Pupae will generally surround themselves with silk and pupate in the host leaves (Figure 5).

Adult BTM have a wingspan of up to 1.5 inches and do not fold their wings much when resting. Most adults have white bodies and wings with a brown margin, though dark forms that are mostly brown also may be found. Coloration does not seem to be related to the sex of the moth. Both color morphs can be distinguished from melonworm moths (a native lookalike) by the white crescent mark on their forewing margin (Figure 6). The trap and lure combinations used by NCDA&CS attract male BTM that are already present within two miles, killing the moth once captured.

Reporting suspect BTM and any other invasive plant pests to NCDA&CS is essential to preventing their establishment and spread. If you see any suspect BTM of any life stage, or BTM damage, please take a picture and submit it to newpest@ncagr. gov. If you are able to capture the specimen, keep it in an airtight container until NCDA&CS can retrieve it. All life stages should be kept in the freezer. If you receive a shipment from out of the state or country, avoid commingling them with the plants you already have. If you’d like to be especially proactive, please contact us in February 2024 to volunteer as a participant in the BTM detection survey. Employees of the NCDA&CS Cooperative Agricultural Pest Survey program will then set, service and remove the traps throughout the season.

Frank, S. (2021, July 19). Urgent New Pest Alert: Box Tree Moth Found. NC Cooperative Extension.

NC Cooperative Extension. (n.d.). North Carolina Extension Gardener Plant Toolbox: Buxus. Retrieved September 27, 2023, from https:// plants.ces.ncsu.edu/plants/buxus/.

USDA-APHIS-PPQ: Plant Pest Risk Analysis. (2023, February 8). Evaluation of non-Buxus plants as hosts of the box tree moth, Cydalima perspectalis Walker (Lepidoptera: Crambidae).