How to Interrupt the Life Cycle of the Box Tree Moth

Green Velvet Boxwood with Irish Moss

If you haven't abandoned your boxwood plants yet and are unsure about how to shield them from another box tree moth infestation, you're in the right place!

Last year, I delved deeply into learning about the Box Tree Moth after I observed damage on my boxwood hedge. Initially, I was inundated with information, so I concentrated on four essential aspects: life cycle, signs, prevention, and treatment. Equipped with this knowledge, I devised a strategy to disrupt the moth's life cycle and provide crucial relief to my boxwood hedge.

Life Cycle of the Box Tree Moth

*timing is temperature dependant. I live in zone 5b (Elora, Ontario).

Generation 1:

Early September - eggs are laid on the underside of boxwood leaves, by the adult female Box Tree Moth. Once hatched, these young caterpillars (larvae) will overwinter in the boxwood plant, inside a web of protection.

*Early May/June - once temperatures rise to 15°C, these young caterpillars will awaken and begin to feed on the boxwood plant.

Late June - the caterpillars will stop feeding and develop into pupae, hidden inside the boxwood plant.

Early July - the Box Tree Moth emerges and eggs are laid on the underside of boxwood leaves.

Generation 2:

*Mid July/August - once these eggs hatch, the caterpillars begin to feed on the boxwood plant.

Late August - the caterpillars will stop feeding and develop into pupae, hidden inside the boxwood plant.

Early September - the Box Tree Moth emerges and eggs are laid on the underside of boxwood leaves. Caterpillars will emerge from the eggs, however they will remain small because their primary goal is to overwinter.  They will overwinter in the boxwood plant inside a web of protection, to begin the cycle again in early Spring.

Signs of the Box Tree Moth

Frass, otherwise know as caterpillar poop.

Skeletonization - evidence of Box Tree Caterpillar feeding, leaving the outer rim of the Boxwood leaf

Moth eggs are laid on the underside of boxwood leaves in early July and September.

Webbing - the caterpillars spin fine white webbing around the branches and leaves, creating a protective "tent". They can survive here over winter in temperatures of -30°C.

Larvae - look for lime green caterpillars with black stripes, white spots, and shiny black heads.

Pupae - caterpillars will develop into pupae and emerge as an adult Box Tree Moth, in roughly 14 days.

Adult Box Tree Moths are active between dusk and dawn, thus not commonly seen on the plant.

Prevention

Maintaining the health of our boxwood plants won't stop the Box Tree Moth from attacking them, but it will lessen the stress on the plants and help deter other pests that target weakened plants. Regularly inspecting and fertilizing both the root system and the leaves will keep the plant robust. Here’s my approach to supporting my boxwood hedge and the schedule I follow to stay ahead of the Box Tree Moths.

1. 
   

Regular Inspection - Evidence of the Box Tree Moth can be found at every stage of its life cycle. This includes frass, webbing, leaves with only their edges left, pupae, or moths flying around the plant during dusk and dawn. Fortunately, none of these stages pose a threat to the plant.

The only threat to the boxwood plant occurs during the caterpillar (larval) stage from early May to early June and again from mid-July to mid-August.

2. 
   

Pheromone Trap - I consider this my alarm system. Once a male moth is caught, it signals the start of mating season, suggesting that my boxwood has likely been egged. I remain alert and monitor for caterpillars to appear within the next 3 days, reaching full size in 14 days. When they are small, they cause minimal harm to the plant, but as they grow, the risk of defoliation increases.

3. 
   

Pheromone Refills - A single box lasts the entire growing season, covering both generations of Box Tree Moths. In the third week of May, I place the trap within 6 feet of my plant using the first pheromone capsule. I keep the second capsule in the refrigerator until the fourth week of July, when it can replace the first capsule.

4. 
   

Root Fertilizer - designed specifically for Boxwood, this quick-acting fertilizer provides balanced nutrition to all types of boxwood.

5. 
   

Foliar fertilizer - is a foliar fertilizer specifically formulated to promote healthy growth in Boxwood and enhance the overall vitality of the plant. It is the sole foliar fertilizer available that is exclusively made for Boxwood.

Treatment

BTK - Bacillus thuringiensis kurstaki is a biological insecticide. Despite its intimidating name, it is not considered harmful to humans or pets, as it targets only caterpillars. It starts breaking down within 24 hours and must be consumed by the larvae (caterpillars) to work. Therefore, timing is crucial. I only spray when I notice caterpillars actively feeding on my plant. After ingesting the insecticide, the caterpillars will cease feeding within hours. They will die and fall to the ground within 2-4 days, depending on their size.

I hope this information gives you hope and perhaps some confidence in rescuing your boxwoods. After I figured out the Box Tree Moth issue, I was able to restore my hedge to its natural, fluffy condition within a single season. If I can achieve this, so can you. Wishing you the best of luck!

xx

Sophia