Box Tree Caterpillars Invasion: How Does It Look and How to Stop It.

The box tree caterpillar is an insect native to eastern Asia. However, it’s an invasive species and can even be spotted around Europe. As a matter of fact, these green and black caterpillars are newcomers to the UK and arrived with imported Buxus plants. These moth caterpillars were first spotted in the country back in 2008. In the beginning, their presence was limited to the London area, but with time they’ve spread throughout the whole country. Today, they’re regarded as some of the most harmful and unwanted insects in the UK. And although pretty to look at, these species of caterpillars are a huge threat to box trees and good-looking topiaries alike.

So if you:

• Are a proud owner of a garden full of with box plants.
• Have box trees, but they look faded and desiccated lately.
• Have seen box tree moths flying around your garden.
• Are concerned about the health of your box hedges

Then you are in the right place so keep reading.

How can you tell if caterpillars are eating your box tree?

Box Tree caterpillars are hard to mistake for another species. They are bright green, with a black head and have lines on their body. These insects mask themselves in a cover of webbing in order to hide from predators. The damage caused by box hedge caterpillars can often be mistaken for box blight. However, if you look close enough and notice eaten leaves coated with webs, you have to act quickly.

The box tree caterpillar’s life-cycle begins with larvae, so tiny, they cannot be seen with the naked eye. Newly hatched, the Buxus caterpillars are greenish-yellow, with black heads. In four weeks they reach a length of 3-4 cm, shortly before becoming an adult moth. In the UK, these black and green striped caterpillars can produce up to 3 generations of moths in just a year. These moths are very active from March all the way throughout October and lay clusters of eggs under the box tree leaves. 

Тhe worst part is that these ruthless bugs have an impressive way of concealing their presence. They disguise themselves amongst the foliage, wrapped in webbing. This method protects their babies and provides them with a safe environment. Their box-green colour also plays a major role in their camouflage game. By glueing themselves to the branches, they become almost invisible.

What does box tree caterpillar damage look like?

The young caterpillars eat only a small part of the plant, while the older ones cause most of the damage by completely defoliating the box tree. Adult caterpillars can ruin the plant beyond recognition, leaving it with skeleton-like leaves in a short amount of time. Box tree caterpillar damage can be easily recognized by the spider mites left in their wake. Although the plant may look bare and dead, the box tree will grow green once again, thanks to its excellent healing ability.

However, it can be fatal for the box tree if the bark is attacked or if the plant has been eaten repeatedly. These occurrences will cause it to become dry and it will die eventually. These occurrences will cause it to become dry and it will die eventually. All we know that Buxus grows slowly and replacing all the mature topiaries and hedges in our garden can be really expensive. Anyway, if you need some new hedges or other greenery replanted, you can always count on our professional garden planting services.

Do box tree caterpillars eat other plants?

Box tree caterpillars and moths mostly prefer Buxus, so the probability of them invading other plants is reduced to a minimal. However, we should be careful with Japanese spurge which is used and sold as a box tree replacement. Since it’s a part of the Buxaceae family, it’s very likely to be eaten by box tree caterpillars. However, it has to be nearby an infected Buxus shrub to be affected by the caterpillars.

Why are box tree caterpillars expanding so quickly?

Box hedge caterpillars are one of the most invasive species in eastern Asia. However, with the help of predacious wasps, their population is limited and kept under control. Unfortunately, we don’t have such wasps in the UK and our natural and native hedge protectors – the birds, are not interested in a box caterpillar lunch. Due to the toxins in the box tree leaves, caterpillars taste terrible and are not a food source option for them. Without any box caterpillar-loving birds, it’s up to us to keep this pest under control. Unless we take things into our own hands, we can expect things to get worse.

 How to get rid of a box tree caterpillar infestation?

• Hand Picking

Hand-picking is probably the most time-consuming yet the most effective box tree caterpillar treatment. The best way to prevent the population from increasing is directly picking off the caterpillars by hand. If you are up for this method, be warned that there is a risk of an allergic reaction. As they are not native to the UK, make sure to wear gloves and a protective breathing mask. The best way to remove box tree caterpillars is to pick them up thoroughly and rub the underneath of leaves to remove any eggs of theirs.

• Using box tree pheromone traps

Another form of biological control for box tree caterpillars is using pheromone traps. They contain a specific pheromone that attracts the male moths and traps them. After that, the moths are sent to their watery grave. This reduces the number of male moths, which leads to less female eggs. Remember that this method is used only for monitoring. These traps can’t help you get rid of the caterpillars but will reduce them.

• Organic insecticides

If caterpillars appear on a box tree, you can try to handle them with a natural pyrethrum spray. It’s an organic box tree caterpillar insecticide made from the chrysanthemum flower. The bad news is that this kills only the caterpillars that were in direct contact with the spray. It cannot be absorbed by the plant as it can kill other insects as well.

• Nematodes

Nematodes are tiny worms that kill the box tree caterpillars by entering their bodies through natural openings and producing bacteria which disrupts their digestive system and prevents them from feeding on plants. The nematodes use the moth caterpillar’s body to reproduce until there is none left of them.

Nematodes are supplied as a powder, which when mixed with water is applied to the box plant, using a watering can. They should be applied correctly to be effective, as they are UV and drought-sensitive. To get the best results, apply them in the evening when the temperatures are above 12C, and the conditions are moist.

• Chemical insecticides

We do not recommend the use of chemical sprays as they can harm the pollinators and other beneficial insects and animals. However, if you still choose to use them, make sure to spray only the young caterpillars. Thanks to their shroud of webbing, box tree caterpillars are very good at protecting themselves. So a lot of spraying is necessary in order to penetrate the webbing. You might be better off removing them manually.

There are a lot of answers to the question “How to kill box tree caterpillars?”, still, the best prevention method is to regularly check every plant you are bringing home.

Takeaways

• Box tree caterpillars are very greedy creatures and can completely defoliate a hedge within days.
• Box trees can survive the invasion as long as the caterpillars don’t eat the bark of the main stems.
• The pupae can easily survive up to -30 degrees in cold seasons.

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Image source: depositphotos / phillyo77

• Last update: March 19, 2020

Posted in Garden Advice, Insect Infestations, Plants in the UK