Tiny webs like spider silk spun through the foliage, and inside them a green caterpillar with a black head, dark green stripes and black dots: this is the box tree moth (Cydalima perspectalis), notorious for eating entire boxwood shrubs to death. Treat early and treat again, and new leaves will soon sprout back.
It is native to Asia, but has now invaded Europe. It has been spreading there since 2008.
It is a caterpillar that only eats boxwood tree leaves for food, and completely consumes entire swaths of the tree, leading to its demise.
The green caterpillar has a black head and dark green stripes. There are also black dots.
Boxwood tree moth caterpillars do not cause itches or rashes. They are absolutely harmless to humans, and can be handled without any special gear.
You will find distinctive tiny webs, similar to spider webs, which are typical of boxwood tree moths.
These caterpillars then turn into brown and white-colored moths that fly around boxwood trees at dusk and at night.
If you have detected green caterpillars on your boxwood, warn any neighbors who might have boxwood trees, because they will certainly be contaminated soon. The earlier this problem is dealt with, the higher the chances of eradicating this parasite.
The first thing to do when you spot these green caterpillars in your boxwood trees is to manually pick them off.
Removing great numbers of caterpillars early makes fighting this boxwood tree moth invasion easier.
This is often not enough, and organic treatments are available to fight these insects, too.
Take note that after the first spraying, you will already be rewarded with a better situation and new leaves sprouting, but the problem is not solved yet.
These don’t boast any advantage over the Bacillus thuringiensis treatment that we signaled above, which has the advantage of being qualified for use in organic gardens.
These chemical agents actually have the disadvantage of striking down other insect species, many of which participate in local garden or terrace ecosystems.
Chemical products generally have a negative impact on bees and other key beneficial insects in nature.
A French company (Biotop) identified Trichotop buxus, a microscopic wasp that preys on boxwood tree moths. This predator must be introduced on the invading boxwood moth at its larval stage, during the moth’s reproductive stage.
This solution has proven effective on small areas, and work continues to prepare large-scale use in the near future.
This microscopic wasp cannot sting and is harmless to people and pets.
Hang a pheromone trap for the adult box tree moths, so you know a flight is under way and the wasps are released while eggs are still fresh.
Because the moth lays eggs several times over a season, one release rarely covers a whole invasion and further batches are usually needed.
Keep chemical insecticides off any boxwood where the wasps have been released, as they wipe out the predator along with the caterpillars.
Four approaches are described here, from picking caterpillars off by hand to releasing a predatory wasp. This is what each one does, and what it costs the rest of the garden.
| Method | What it does | When to use it | Drawback |
|---|---|---|---|
| Hand-picking | Physically removes caterpillars from the shrub | As soon as green caterpillars are spotted | Rarely enough on its own |
| Bacillus thuringiensis spray | Organic treatment that kills the caterpillars | Sprayed over the whole shrub, every month until winter | Must be repeated or new invasions follow |
| Chemical insecticides | Kill caterpillars, but also other insects | No advantage over the organic option | Harms bees and other beneficial insects |
| Trichotop buxus wasp | A microscopic wasp that preys on the moth | Introduced at the moth’s larval and reproductive stage | Only proven on small areas so far |
Whichever route you take, treating early and treating again is what decides whether the boxwood survives.
Renewing the treatment at the end of the winter or at the beginning of spring in the following year will ensure that any remaining larvae that have overwintered in your boxwood trees will be dealt with.
