Five Ways to Beat the Box Tree Caterpillar

The box tree caterpillar is back and its numbers are increasing. If you’ve got the caterpillar in your garden then you will know the havoc it can cause, stripping box plants of their leaves in a very short period of time.

box tree caterpillar damage
Box tree caterpillar damage

Box tree caterpillars are relatively easy to spot when you know what to look out for. The obvious signs are small areas of densely woven webbing and below this small grit-like black balls. This is caterpillar poo! Closer investigation will reveal the box tree caterpillar, protecting itself within the webbing and box leaves.

box tree caterpillar
Box tree caterpillar in webbing. Photo Credit: Victoria Bailey

This behaviour makes it very difficult to treat the caterpillar with an insecticide such as Bayer’s Provanto Ultimate Bug Killer as the spray struggles to penetrate the protective webbing and leaves. I’m also not a fan of these contact insecticides. They are fairly indiscriminate about which bugs they target so you risk killing a lot of beneficial pollinating insects in the spraying process.

Here are four other ways to beat the box tree caterpillar that I prefer to recommend:

Box Tree Moth Pheromone Traps

Targeting the source can prove effective at reducing caterpillar numbers. The box tree moth trap works by attracting the male moth with a female box tree moth pheromone. When trapped the male moths can be disposed off before they have a chance to breed. Half filling the trap with water and a dash of washing up liquid is an effective way to ensure the male moths won’t be flying again.

Bayer Box Tree Moth Pheromone Trap
Bayer box tree moth pheromone trap

Other moths and butterflies are not affected by the traps as the pheromone only attracts male box tree moths. And you won’t be encouraging box tree moths into your garden by using the traps as they will already be attracted by the box plants. The pheromone traps act as an early warning system indicating when the male box tree moths are active and another breeding cycle is about to start. This is a good time to spray your box topiary and hedging with my next recommendation.

Biological Insecticide

Biological insectides based on the Bacillus thuringiensis bacterium are the most effective at the targeting the box tree caterpillars. Topbuxus Xentari is the one most talked about but still doesn’t have a license for amateur use in UK gardens. However it is widely available to buy online. Have a look at the Topbuxus website to find out more about it.

Topbuxus Xentari
Topbuxus Xentari

A caterpillar will soon stop feeding when it eats a box leaf treated with biological insecticide, and will be no more within a couple of days. Other insects are not affected by the biological insecticide as it only targets the pests eating the treated leaves. I’ve also been told birds are not harmed when eating the dead caterpillars.

Apply the biological insecticide as soon as you see caterpillars or your pheromone traps start to fill up with male box tree moths. The treatment is only effective for about 10 days as it is broken down by UV light so you may need to reapply more than once when the caterpillars are active.

Nematodes

Box tree caterpillar nematodes
Box tree caterpillar nematodes

Nematodes are tiny worms that are mixed with water and applied to the box plants using a watering can, hopefully making contact with the box tree caterpillars in the process. The nematodes maybe small but they soon make short work of the caterpillars and gruesomely use their bodies to reproduce until there are none left.

Nematodes must be applied in the right way to be effective. They are UV sensitive so application in the evening is best. Avoid dry periods as moist conditions are important and keep an eye on the temperature. If it falls below 12C then the nematodes will die.

Nematodes don’t store well. They can be stored in a refrigerator for 4 weeks but best to order them only when you know you have time to treat the box.

Hand Picking

The most time consuming approach but the most effective. Picking off the caterpillars by hand and ‘disposing’ of them is the best way to control their numbers. Dropping the caterpillars in a bucket of water is a good way to say goodbye to them if the thought of squashing caterpillars is not appealing.

Sadly our native birds are not interested in the caterpillars as a food source. Box leaves have toxins in their leaves that make the caterpillars taste pretty awful.

I am hoping that now more people are aware of box tree caterpillar and are taking action, we may be able to reduce their population in future years and reduce the devastating impact they have been having on our box topiary and hedging.

Using box tree pheromone traps will have some effect. But combine this with hand picking or spaying with a biological insecticide and we may have a real impact on the box tree moth and its caterpillars.

Image Credits: EBTS and Victoria Bailey

5 Comments

  1. Great advice. I’ve been using Xentari and it worked very well however a week or so later I spotted more alive caterpillars so further treatment every week or so if required.

    Once a bush is consumed by the caterpillars do we know they will will come back to life?

    1. Thanks Sunny. Yes, you will need to spray a few times to ensure all the box leaves are treated. A box plant will recover if the caterpillars have only stripped its leaves. But if they have continued to feed on the bark of the plant then unfortunately the chances of recovery are reduced.

  2. James Alexander

    The ordinary topbuxus copper sulphate (I think) works well too, as a feed it’s advertised as, and as something the caterpillars die eating once the leaves have absorbed it.

  3. Jennifer Tucker

    Hi James, I bought Nematodes but then got scared of using them as I read on a USA site that they actually caused Buxus to die. Can you clarify? Thank you,
    Jennifer

    1. I’ve not heard of that before Jennifer. I would have thought it was unlikely as the nematodes seek out the caterpillars and should have no interaction with the buxus to cause it to die. If applied correctly according to the instructions that come with the Nematodes then I can see them doing any harm.

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