Best Biological Insecticides to Beat the Box Tree Caterpillar

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The box tree caterpillar has been causing havoc since arriving here in the UK around 2007. Box tree moths lay their eggs on Buxus and the resulting caterpillars will feast voraciously on the leaves (and sometimes the stems) until the plant is stripped bare.

box tree caterpillar damage

Look out for the caterpillar between March and October. The box tree caterpillar can now be found all over the UK, but London and the South East continue to be the hot spots for this pest. Nibbled leaves are the first indication of caterpillar activity. On closer inspection webbing and small balls of frass (caterpillar poo) can be seen. The box tree caterpillar is often well hidden within the plant, safe from natural predators like birds and protected from spraying with a contact insecticide such as Provanto Ultimate Bug Killer

Box Tree Caterpillar

Personally I am not a fan of contact insecticides. Yes, they will kill the caterpillars but also all the other beneficial insects in the area such as bees and ladybirds. This is why I always use a biological insecticide to treat the box tree caterpillar.

Biological insecticides are based on the microorganism Bacillus thuringiensis (Bt). The bacteria gets into the caterpillar’s gut after it eats a treated Buxus leaf. Toxins are produced that stops the box tree caterpillar eating within the hour. The caterpillar will then die within 1-2 days.

The biological insecticide is harmless to people, pets and beneficial insects as the toxins can only activated when a caterpillar ingests the Bacillus thuringiensis. Even birds can eat the dead caterpillars without any ill effect.

Here are three biological insecticides that I have used for the box tree caterpillar and all are very effective. I should add that currently Bt based products are not registered for domestic use in the UK but can be used by professionals. However they are widely available to buy online.

Topbuxus Xentari

XenTari® is the world’s only biological insecticide containing a natural, potent strain of the microorganism Bacillus thuringiensis subspecies aizawai (Bta). This has proved very effective at controlling the box tree caterpillar without a negative impact on beneficial insects, birds, humans, wildlife and the surrounding environment. Topbuxus have packaged Xentari in handy 1g sachets (15 in a box). Mix the powder in the sachet with a litre of water and spray on to the Buxus plants. This should be enough to treat 10 square metres of box.

Topbuxus Xentari

Agrinova Xentari

Agrinova also use XenTari®. Each pack contains a 25g sachet and a 1 gram dosing spoon. Mix 1g of the Xentari powder with 1 litre of water and spray directly on to the box. This will be enough to treat 10 square metres of Buxus.

Agrinova Xentari


DiPel contains another subspecies of Bacillus thuringiensis, Bt kurstaki (Btk), which is also very effective against the box tree caterpillar. This product is suitable for professional gardeners, nursery owners or anyone treating large areas of Buxus. The tub contains 500g of water dispersible granules. The recommended mix is 75g with 100 litres of water.


How to use a Biological Insecticide to treat Box Tree Caterpillar

The product I use the most is Topbuxus Xentari. At the first sign of box tree caterpillar activity, such as nibbled leaves, I spray all the box in the garden. Make sure it is a dry day, with no rain forecast for at least 6 hours.

Wait 10 days. If there continues to be evidence of active box tree caterpillars then apply another treatment to all the box.

Hopefully two or three treatments will be enough to remove all the box tree caterpillars. But it is very important to keep regularly checking the box between April and October as the box tree moth can have three life cycles in a year. If I do see any small patches of nibbled leaves or suspect webbing then I will spray that area with some more Xentari.

The box tree caterpillar is manageable. The key to success is regular monitoring of the box hedging and topiary. A swift response at any signs of box tree caterpillar will help keep this pest under control

By James Todman

Topiary Specialist based in The Cotswolds, UK.

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