Box tree moths (Cydalima perspectalis), BTM for short, are an invasive species new to Canada in 2018. The first case was found in Toronto. Since that time, they have been gradually spreading. The United States reported its first cases in 2021. Several government bodies, universities, nursery and landscape trades have banded together to try to monitor and control it’s spread. Why all the concern? Since it’s accidental introduction to the UK in 2007 they have spread like wildfire, defoliating boxwoods and causing extensive damage. They are hoping to slow it’s spread or eradicate it to prevent such damage here.

Damage

Feeding damage occurs almost exclusively to boxwood (Buxus) species but Ilex, Euonymus and Pachysandra can also be affected. Feeding damage initially appears on one side of the leaf (usually the bottom) being eaten while the other side remains intact. This type of damage is called ‘window paning’. This is followed by desiccation and browning. As the caterpillar grows entire leaves, except for the mid-rib, are consumed. The caterpillar also feeds on the stems causing girdling and branch dieback. If the populations are high entire shrubs can be completely defoliated. In addition to defoliation the caterpillar spins fine silk webbing over the leaves and branches. This webbing becomes littered in frass, shed skins and various plant debris.

Identification and Life Cycle of Box Tree Moth

• Larvae: are tiny green caterpillars with black heads, black spots and black stripes that run down their length. They measure between .5cm and 4cm in length, depending on which instar larval stage they are at (there are 7 instar stages). The caterpillars are difficult to spot, as they are small and blend into the foliage well. They are especially hard to spot when they are deep in the plant and down near the branch crotches, covered in webbing and various plant debris that clings to the webbing. They typically begin feeding on the inside of the shrub working their way out. In the Toronto area larvae are present from mid-May to mid-June; mid-July to mid-Aug and the first 2 weeks of Sept. (Ontario Invasive Species Centre). The larvae overwinter between leaves that have been sewn shut with silken thread, encapsulating them. They resume feeding on the underside of leaves, once the temperatures warm up, roughly mid-May (depending on the weather).

• Pupae: In this stage of development the larvae stop feeding and pupate on the boxwood leaves. The adult moths begin emerging from the cocoons roughly the end of June and then again, the end of August. Begin looking for pupae mid-June then again mid-August.
• Moth (adults) immerge from the pupae, mate, then lay their eggs (typically on the underside of leaves). They live about 14 days and can potentially disperse 7 to 10 km (Llewellyn L. 2020). The moths are white with a brown border around the wings. Their wingspan is about 4cm. In their native range they can be all brown in colour. In Ontario, peak flight of adult moths is about the end of June and then again in the end of August. They are nocturnal and thus fly at night and are not easily spotted during the day. In Ontario there are 2 generations a year. In Europe, they are experiencing three to five generations.

• Eggs: Female moths will lay tiny, flat yellow, gelatinous, eggs on the underside of boxwood leaves, usually in a cluster of about 10 to 20. The eggs hatch after about three days and the larvae begins feeding on the underside of leaves. Begin scouting for eggs about beginning of July and again in the end of August.
• Hibernarium: the larvae stop feeding around mid-Sept. and begin preparing to overwinter. They spin webbing over themselves and enclose themselves between boxwood leaves that have been bound together with silk thread. This protective enclosure is called a “hibernarium”. They remain in this state until warmer spring weather, about mid-May. Note: the webbing in spider nests look similar to BTM hibernariums. To distinguish between the two pry, open the leaf and take a look under the webbing. If you find eggs it is a spider nest; if you find a small green caterpillar with a black head, that’s a box tree moth larvae.

How to Control Box Tree Moth:

• Handpicking is time consuming but effective. Aggressively brushing the shrubs with your hand will cause many of them to drop to the ground for easy pick up. Drop them into a container filled with warm soapy water, insecticidal soap, then flush them down the toilet.
• Dormant oil spray can be applied in March/April (depending on the weather) to suffocate overwintering BTM.
• Bacillus thuringiensis var. kurstak (BTK) is a biological insecticide safe for use against all caterpillars. Thoroughly spray infected boxwoods when the caterpillars are actively feeding, as they must ingest the BTK in order for it to be effective. The caterpillar will stop feeding about 1 hour after ingesting it. Caterpillar season in Ontario is roughly mid-May to mid-June, mid-July to mid- Aug., and Sept. 1 to 15.
• Heavily damaged shrubs can be cut back and should put on new growth to hide the damage. Continue to monitor them for any eggs, pupae or larvae you may have missed.

Distribution of of Box Tree Moth:

BTM are native to eastern Asia (Japan, China, Taiwan, Korea, the Russian Far East and India). They were accidently introduced to UK in 2007 where they have spread rampantly. Canada saw it first infection in 2018 in Etobicoke, Ontario. USA saw its first cases in 2021 in Michigan, Connecticut and South Carolina.

Summary

Here in Canada, we are still in the infancy stage of this invasive species and there is much we can do to slow or prevent it’s rapid spread. Inspect all your boxwood carefully. Checkout online videos and photos to help you learn how to identify this pest.

References:

Bunescu, Horia & Florian, Teodora. (2016). Studies Concerning the new Invasive Species, Cydalima perspectalis Walker (Box Tree Moth) in Cluj Area (Romania). ProEnvironment. 9. 142-151.

Gov. of Canada, Date modified: 2019-06-17. Cydalima perspectalis (Walker) – (Box tree moth) – Fact sheet. https://inspection.canada.ca/plant-health/invasive-species/insects/box-tree-moth/fact-sheet/eng/1552914498593/1552914498889

Llewellyn J., 2019 (OMAFRA Nursery and Landscape Specialist). Box Tree Moth Webinar. https://horttrades.com/box-tree-moth-webinar-and-online-discussion OR https://www.youtube.com/watch?v=2Fmuk3HXF80

Llewellyn J., 2020. (OMAFRA Nursery and Landscape Specialist). Landscape Ontario, Box Tree Moth a New Pest in Etobicoke. https://landscapeontario.com/box-tree-moth-a-new-plant-pest-in-etobicoke

Marvin D. E., (2021). Box Tree Moth – MAJOR new pest threat to Boxwood. Think IPM Blog New York State Integrated Pest Management https://blogs.cornell.edu/nysipm/2021/07/14/box-tree-moth-major-new-pest-threat-to-boxwood/

Shady Lane Expert Tree Care, 2019. Box Tree Moth; The New Invasive Insect In Ontario. https://www.shadylanetreecare.com/box-tree-moth-treatment

Skvarla M.J., (updated May 2020). Box Tree Moth. https://extension.psu.edu/box-tree-moth

Wiesner A., 2021 (Master of science candidate University of Guelph). Invasive Species Webinar, Box tree moth 101: Identification, Biology and Management in Ontario. https://www.youtube.com/watch?v=bFyqWK9O1NI

Updated on June 12th, 2022

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