The boxwoods are dying. Found that they’re infested with inserts. How to kill the inserts and restore the plants. Thanks
Thank you for contacting the Toronto Master Gardeners, and sorry to hear that your boxwood appears to be infected with insects!
Based on the image you provided, Box Tree Moth is likely, especially if you are seeing more of the black spotted larvae as well as webbing. Box tree moth (Cydalima perspectalis) is an invasive pest in Ontario. As this is sadly a common phenomenon recently (Landscape Ontario has been monitoring for several years now), if you type ‘Boxwood Caterpillar’ or Boxwood Moth’ into the ‘Find it Here‘ text search on our Ask a Master Gardener web page, you will find several other entries about it.
The Toronto Master Gardener’s now have a Garden Guide about this topic. Please refer to this guide, as it contains identification and treatment details, links to helpful visuals, and native alternative plants to grow instead of boxwoods: https://www.torontomastergardeners.ca/gardeningguides/box-tree-moth-a-toronto-master-gardeners-garen-guide/
- You should take action immediately to prevent spread. When unmanaged, severe larval populations have consumed 100% of the plant foliage; hungry larvae may chew on branches and stems, causing plant mortality.
- Box tree moth larvae can be effectively managed with a safe biological insecticide, BTK ((Dipel 2X DF PCP#26508), that can be legally used in Toronto.
- When the box tree moth larvae consume the spray residue on the leaves, they soon stop feeding and expire within a couple of days.
- However, this bacterial spray will not kill the eggs or pupae stage, so you need to time your spraying for when the larvae have hatched (caterpillar stage). Spraying should be done at least 3 times a year (mid-June, mid-July, mid-Sept according to OMAFRA, Ontario Ministry of Agriculture, Food and Rural Affairs but these dates will vary).
- You can keep spraying every 5 – 7 days as long as you see any caterpillars or pupae on the plants.
- BUT…only spray in the presence of larvae as to not harm beneficial insects. It may also be helpful to start removing some of the larvae by hand and destroying them.
- Safe removal of infested plants and larvae is also critical to preventing spread. If you end up pruning or trimming your shrub as part of your treatment, be sure to place the infested shrub or branch clippings in a black plastic garbage bag and secure the bag to ensure it is closed tightly. Place the bag outside in direct sunlight for 48-hours. After 48 hours, larvae should be dead and the plant materials can safely be added to yard waste bins.