A “New” Bug on the Boxwood

In the province of Quebec, boxwood (Buxus spp.) has become very popular. This is partly thanks to the arrival of more cold-resistant varieties. It is also thanks to climate change, which is making our winters less hostile. Finally, the growing popularity of mid-century inspired trends, minimalism, and discrete luxury have made boxwood the ideal plant for creating small, linear hedges.

Unfortunately, with this massive arrival of beautiful evergreen green balls has also come a little rascal. Discovered in Quebec in 2023, the box tree moth (Cydalima perspectalis) quickly carved out a place for himself in the select club of villains!

A Harmless Butterfly, but a Destructive Caterpillar

Like most of our invaders, the box tree moth comes to us from Asia. It traveled to Europe around 2007, then crossed the ocean to make its North American debut in Toronto in 2018. Since then, it has crossed borders and is found in all of Canada’s eastern provinces as well as in states near Michigan.

The box tree moth is first and foremost a beautiful gray and white moth, about 4 cm long. But of course, it’s the small caterpillars laid by the hundreds that cause the damage.

A Life Cycle to Tame: The Adult Phase

Since the insect arrived relatively recently in America, we are only just beginning to understand its life cycle. In Asia, this butterfly can have up to 5 generations per year! Luckily, in Ontario and Michigan, only 2 generations are observed per year. These generations follow one another closely, so adult butterflies can be observed almost at any time.

For this reason, adult moths are present for a long period, as the two generations overlap quite regularly. Therefore, male moths can be captured using pheromone traps from mid-July until September! These moths are not easily observed, as they are active at night.

Each butterfly lays its eggs under the leaves, in small groups of 5 to 20 eggs. In total, a single butterfly can lay up to 800 eggs in its short life of only 14 days.

The Life Cycle of the Evil Caterpillars

Just 4 days later, the eggs hatch into green larvae with stripes and small black dots. These larvae eat the leaves and, in the case of heavy infestations, can completely defoliate a small boxwood bush in just a few days!

Box tree moth caterpillars have strange feeding habits. For some reason that is still unknown to us, they like to eat the inside of the leaf. They tend not to nibble the margin (the outer edge of the leaf). This creates a curious lace-like effect when you look at a nibbled boxwood branch.

The presence of caterpillars, which are roughly the same color as boxwood leaves, can also be detected by the presence of silk. They are weavers! We can also notice the presence of small droppings, caught in the webs or piled up on the ground.

The Metamorphosis Takes Place

The caterpillar will go through 6 or 7 growth spurts to reach maturity. When it is 4 cm long, it attaches a few leaves together and hides inside to become a pupa, what is commonly called a cocoon for butterflies. This pupa will turn from green to brown, and after 14 days, the beautiful adult butterfly emerges, ready to find a mate to ensure the proliferation of its species.

But How Does it Get Through Winter?

This is another interesting feature of the box tree moth. Most butterflies spend the winter as pupae buried in the ground. Some, like monarchs, fly south as adults. But our box tree moth decides to overwinter in the middle of its larval stage. In September, after the 3rd or 4th mutation, the caterpillar makes a small cocoon and wraps it in a few boxwood leaves, and presto, it’s diapause until the following spring. As soon as the temperature reaches around 10°C in the spring, the young caterpillar comes out of its torpor and continues to feed itself to complete its larval cycle. There is therefore a first generation of caterpillars that continue their life in the spring and the second generation completes its entire cycle in the summer.

Not To Be Confused With…

Of course, the box tree moth isn’t the only pest that attacks boxwood, but according to current knowledge, it’s the only insect capable of defoliating a boxwood by eating the leaves. If the leaves are being nibbled, it’s probably the culprit.

However, boxwoods can be attacked by boxwood psyllids, leaf miners, and several diseases I’ve already discussed here. Winter damage from strong winds and de-icing salt should also not be ignored.

What Can I Do?

As you can see, because of these two annual generations, the detection period is almost constant. So, it’s almost anytime between June and September that you can inspect for all three development stages. You’ll look for eggs under the leaves from July to September. In this case, you can simply tear off and discard the leaves to prevent the arrival of hundreds of caterpillars.

Also, between June and September, we will keep a close eye on the presence of larvae. We can decide to collect them by hand or to carry out a treatment with Btk.

Then, we can take advantage of the dormant period, either late September or early April, to discover the small dormant caterpillars, rolled up in leaves.

Playing With Smells

As briefly mentioned above, female butterflies release a delectable scent to attract males, called pheromones. And researchers have managed to mimic this scent. Thus, traps can be created to attract male butterflies. During the detection phase, these traps are set near boxwood trees. But if you want to confuse the males and prevent them from finding the females, simply place these traps far from the boxwood bushes! This creates confusion, and some experiments have shown that fewer eggs are laid in areas where this tactic was used!

Aromatherapy to the Rescue?

Another study I like tested whether misting other scents near boxwood could mask the scent of females and again confuse males. After trying eucalyptus, lavender, and cinnamon essential oils, cinnamon turned out to work best!

Knowing the damage caused by this insect in Europe, it was initially believed that the box tree moth would be a deadly enemy. However, these few years of observation have shown us that even a shrub completely stripped of all its leaves can survive and grow back. The box tree moth has therefore been designated a potential killer of the boxwood, because of course, after several complete defoliations, it is possible that the boxwood will eventually die. But if we know how to detect it and take action to prevent damage, the box tree moth will be added to the many harmful insects with which we can quite easily coexist.

Acknowledgments: In May 2024, the Horticultural Research Institute held a very comprehensive and up-to-date webinar on the status of box tree moth in North America. I thank researchers Joe Boggs and Alejandro Del-Poze for sharing their valuable knowledge.