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 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.
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.
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 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.
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 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.
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.