I have a lot of boxwood hedges and they are not doing well and after a lot of research I discovered we have box tree moth and I purchased BTK the biological insecticide. I was having trouble determining when to spray. The pupa are hatching now and turning into moths how long from now should I be spraying? I read anywhere between September 1st and the 15th. My husband sprayed yesterday because I thought the pupa were caterpillars ready to hatch but in fact now I know the pupa is morphing into moths.
Unfortunately this pest has arrived in Toronto specifically in the last 18 months (it is not yet in the United States), and Landscape Ontario is trying hard to track cases and reduce it’s spread in the future. The links below may be the same ones that you found in your research. Please use the links provided in the articles to report your location and actions to them, as they may wish to follow up.
Your BTK application appears to be correct, if perhaps a little late to stop the moths this season. You can keep spraying every 5 – 7 days as long as you see any caterpillars or pupae on the plants. There may still be another cycle of the pests hatching on the plants that you can treat throughout the fall.
https://horttrades.com/box-tree-moth-a-new-plant-pest-in-etobicoke
https://horttrades.com/landscape-ontario-leads-program-to-control-the-spread-of-box-tree-moth
Fortunately, boxwood is normally a pretty tough and hardy shrub, so you can prepare to fight this battle again next year, and Landscape Ontario may have issued more instructions as they get data on the behaviour of the moth.