Thank you contacting Ask Extension. Once you have a positive identification that you have box tree moth, you can spray a Bacillus thuringiensis (Bt) product once every two weeks. Bt is a bacterium that kills insects when ingested. There are subspecies of Bt that affect specific types of insects; kurstaki is the subspecies that specifically targets caterpillar larvae. Look for product labels that say Bacillus thuringiensis subsp. Kurstaki (Btk). They should start spraying once the caterpillars are present on the boxwood, usually around the beginning of May and continue through mid-September. The caterpillar of the box tree moth is mostly green, with black strips, shiny black head, and white stripes and hairs. For small infestations you can also hand-pick larvae from plants and apply Horticultural oil, insecticidal soaps, or Spinosad. Depending on what your landscape service sprays with, spraying frequency may be different. You are trying to prevent the eggs that hatch into larvae at different times through out the season from injuring your plant.

If you are spraying as a preventative measure, MSU Extension recommends homeowners spray boxwood shrubs twice during the summer (in late July and early September) with bifenthrin (several ready-to use products should be available at your local garden center) or any pyrethroid insecticide for homeowner use. Pyrethroid insecticides are those where the active ingredient ends in thrin, for example cyfluthrin, permethrin, resmethrin, cypermethrin, lamba-cyhalothrin , sumithrin. Avoid making preventative applications with broad-spectrum insecticides when box tree moth is not present. Insecticide effectiveness against eggs, pupae, and older caterpillars may be low since they tend to be on the underside of leaves and hidden deep inside the plant. Keep the area under your boxwood plants clean by removing leaf letter and debris to lower potential overwintering of the box tree moth. Double-bag any debris in plastic and place in municipal waste. The information comes from these two articles:

https://www.canr.msu.edu/news/what-growers-and-landscapers-need-to-know-about-box-tree-moth

https://www.canr.msu.edu/resources/box-tree-moth-alert

Another good site that includes links to several more articles about the box tree moth are available here:

https://www.canr.msu.edu/tag/box-tree-moth

Alternatives to the boxwood shrub

Few shrubs offer the compact size, evergreen foliage, and resistance to deer browsing that boxwood offers. However, with the amount of pest and disease pressure these shrubs are now facing, homeowners may opt for plants that will require less maintenance. Consider the following shrubs when replacing boxwoods:

• Japanese Holly (Ilex crenata)
• Inkberry Holly (Ilex glabra)
• Junipers (Juniperus spp.)
• Yew (Taxus spp.)
• Arborvitae (Thuja spp.)

These shrubs among others offer similar form and function in the garden. Be sure to consider your local climate, soil conditions, and deer pressure when finding a suitable boxwood replacement.