Categories
Advice

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.

Topbuxus Box Tree Moth Pheromone Trap
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.

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 help and keep an eye on the temperature. If it falls below 12C then the nematodes won’t work.

Nematodes don’t store well. They can be kept 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.

Our native birds are starting to become interested in the caterpillars as a food source but not enough to make an impact. This could be because box leaves have toxins in their leaves that must 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 damage they have been causing to 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

18 replies on “Five Ways to Beat the Box Tree Caterpillar”

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?

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.

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.

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

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.

Thank you very much for this advice – my box trees were devastated in less than 2 weeks – I will follow the advice given and let you know how I get on. Ilka

Thanks Sarah. If the growth is dead (brittle twigs without any flexibility and snap easily) then no point keeping and best to remove. Other defoliated stems that are still flexible can sometimes recover and hopefully you will start to show new leaf growth soon.

This is a very useful article, Thanks. I have a question. I’ve used other nematodes successfully but as these are sensitive to temperatures below 12C how can they survive for 4 weeks when refrigerated?

Thank Roger. I’m not sure of the science behind this but I do know nematodes need to be kept cool (not frozen) to survive longer, which is why refrigeration is recommended. I’m guessing it’s a bit like refrigerating food to keep it longer. Once out in they open air the microscopic worms need to find a host quickly (i.e the caterpillar) to release their bacteria otherwise they will die. The optimum temperature for them to be active is above 12C.

Native birds do seem to enjoy these caterpillars. Our hedges have been alive with great tits and coal tits stripping these pests out of them… and it has made me hesitant to spray to be honest. Sadly I suspect I no longer have a choice and its the only chance to save the hedge.

Yes, I think native birds are finally developing a taste for the box tree caterpillar. I only use a biological insecticide to spray as it doesn’t harm the birds when they eat the dead caterpillars.

Hi, Do you have any recommendations for fertiliser or the like to encourage regrowth of leaves after the caterpillars have gone?

I have just ordered some moth traps in the hope of saving my box hedge. An individual box plant In a pot elsewhere in the garden Has been stripped. I am wondering at what height and position I should place the box traps. Please advise.

Leave a Reply