I followed an OMAFRA slide presentation and I’m certain I have Cydalima perspectalis infesting my boxwood hedge. Is it too late to spray BTK? I can’t see any caterpillars but there are a few hibernariums? Im not a resident of Toronto. I live in Petrolia ON. Thanks for your great website. Linda Harvey
Ah the dreaded box tree moth. I see it in many of the gardens I work in, so you are certainly not alone. What began in a few select areas of the west end of Toronto has become much more widespread.
The usual telltale signs for me, are the “window paning” of the leaves- where the interior is eaten to the point that it is almost translucent as if looking through a window. If you have not seen actual live caterpillars, I would not recommend spraying at this time. This is because for the BTK to be successful, the caterpillars must actively consume it. It is not a preventative treatment. The first time to treat your plants and spray the larvae (worms) is usually between early May and mid-June. The second spraying is done in mid-July and the third spraying is done in mid September.
Although it sounds as if you have researched the topic, I will include the Toronto Master Gardener’s Garden Guide to Box tree Moth here: https://www.torontomastergardeners.ca/gardeningguides/box-tree-moth-a-toronto-master-gardeners-garen-guide/
I would recommend inspecting your boxwood in the spring and treating the caterpillars at that time. I am also going to recommend a site with some boxwood alternatives should you choose to go this route in the future. You can find these here: https://extension.uga.edu/publications/detail.html?number=C1107&title=think-outside-the-boxwood-alternative-plants-for-gardens-and-landscapes
Lastly there is a previous answer on ailing boxwood that is worth reading as it has many useful links for further information. It speaks of boxwood blight, another disease affecting this shrub. You can find this here: https://www.torontomastergardeners.ca/askagardener/blighted-boxwoods/#:~:text=Boxwood%20blight%20is%20a%20fungal,that%20your%20tools%20are%20disinfected.
October 2, 2023