I have been inspecting my boxwood hedge every 2-3 days for any signs of the Boxwood caterpillar. Since BTK is not a preventative treatment, I am prepared to start spraying if I find any signs of the caterpillars. When will the caterpillar/ moth cycle come to an end this year ? Was is the likelihood that they could still appear this season ?
Good question! Vigilance is key to staying on top of box tree moth infestations.
Toronto Master Gardeners have a Box Tree Moth Gardening Guide that you may find helpful. The guide mentions the following with regard to BTK:
“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.
Frequent reinfestation is likely given the mobility of the adult moth. Treatment is not a “one and done”. You need to spray your box trees at least three times a year. Use a pressurized sprayer to spray, and focus on the bottom of the leaves. You can keep spraying every 5 – 7 days as long as you see any caterpillars or pupae on the plants. Note: only spray in the presence of larvae. Preventative spraying for long periods of time will affect good moth catepillars. OMAFRA (Ontario Ministry of Agriculture, Food and Rural Affairs) recommends that the first time to treat your plants and spray the larvae (worms) is between early May and mid-June. The second spraying is done in mid-July. The third spraying is done in mid September.“
The ONnurserycrops blog from August 2, 2024 states “Late July we start into the period of asynchronous life stages where nearly all life stages can be found at once and management can start to get tricky!” It shows 3 different larval stages found on boxwoods at the end of July.
Halton Master Gardeners have produced a detailed 2-part article on boxwood history, pests, treatments and alternatives. Part 1. and Part 2. You may find them a useful resource for boxwood care and maintenence, or replacement.
Best of luck with your boxwoods!