If you notice more shrubs and specific plants in and around Toronto turning brown throughout summer, an invasive moth species could be the culprit. 

The box tree moth (Cydalima perspectalis) is a pest native to Asia and poses a major threat to boxwood (Buxus), a genus of common horticultural shrubs in Canada known for their dense, evergreen foliage and ability to thrive in a variety of environments. 

The species was initially discovered in Toronto in August 2018, a few years before Ontario declared an infested area in 2023. 

According to the Canadian Food Inspection Agency, boxwoods are often planted as ornamentals and typically used for edgings as hedges. However, when infested, the plants are disfigured and discoloured by the loss of leaves, and webbing spun by the species' larvae. 

Eggs are typically laid on the underside of the host plant's leaves and are usually located in clusters of 1o to 20 eggs. Once hatched, the young larvae feed on the underside of the leaves, leading to the deterioration of the plant.

Keep watch for signs of the box tree moth!

This #InvasiveSpecies attacks the common ornamental boxwood shrub. If you see eggs, caterpillars, adult moths or signs of defoliation on your boxwood, report your sighting to https://t.co/9ZYChHtWuP!

Learn more: https://t.co/V7cttKsBUA pic.twitter.com/pAFne5UsxP

— Invasive Species Ctr (@InvSp) August 22, 2019

Signs a plant is infested include chewed leaves, larvae, webbing, and sawdust-like debris. 

In southern Ontario specifically, there are two periods when the species is in the caterpillar or larval stage and can be managed — specifically from early to mid-May, and early July to mid-August. 

❗BOX TREE MOTH: A GROWING CONCERN FOR ONTARIO GARDENS

👉 To learn more, identify box tree moth populations and develop best management practices to reduce impact and slow the spread of this invasive species, visit this page: https://t.co/z0pIGbaT5O#BoxTreeMoth pic.twitter.com/CpC9X0Ibjg

— Landscape Ontario Horticultural Trades Association (@green_for_life) May 15, 2024

According to the Landscape Ontario Horticultural Trades Association, the most effective way to minimize damage and protect boxwood plants is by spraying a biological product with the active ingredient Bacillus thuringiensis (commercially called Dipel or Safer's BTK) when larvae are feeding. 

From May to September, you should also regularly inspect your boxwood plants for signs of the moth. If spotted, you can remove all adults, pupae, mature larvae, and leaves with eggs and place them in soapy water, alcohol, or an air-tight sealable container.