Plant protection
Fighting boxwood borers: This is how you deal with the voracious caterpillars
The caterpillars of the box tree borer like to eat box trees
© AlbyDeTweede/Getty Images
Once infested, they literally suck the life out of box trees: the caterpillars of the box tree moth are a horror for (hobby) gardeners. If they are discovered too late, the plants can usually no longer be saved.
As is so often the case, the pests are not a freak of nature, but were introduced by humans: “They probably came to us on a container ship because tree nurseries were keen on cheap boxwoods from Asia,” he explains Nature Conservation Association Germany eV explains the background to the spread in European areas. The boxwood moth is actually native to Japan and China. It was first spotted in the southern Upper Rhine in 2007, and since then the small butterfly has continued to spread and now lives in the Netherlands, Switzerland, Austria, France, Great Britain and Germany. However, it’s not the butterfly that causes so much damage to the plants – it’s the caterpillars.
Boxwood borer caterpillars: How to recognize the infestation
In contrast to caterpillars, the boxwood moth is usually not found on boxwood trees, but on the underside of the leaves of other plants. The females prefer box trees that have not yet been infected to lay their eggs, so that new generations of box tree moths (up to four) emerge from March until late summer. The caterpillars hatch after just three days: They can be up to five centimeters long, are yellow-green to dark green and black in color, have black dots and white bristles. Because of their preference for older leaves, they start their feeding attacks inside the plant – and work their way outwards. As a result, the beginning of the boxwood’s denudation usually goes undetected. A watchful eye is therefore important to detect the infestation.
To find out whether boxwood borer caterpillars are attacking your plants, you should pay attention to the following signs that indicate an infestation:
- Initially, the larvae attack the top of the boxwood leaves by scraping off the top layer. This causes the leaves to have bright spots, although these are not immediately noticeable. In addition, there are the first webs inside the plant, i.e. thin white threads in which the larvae and their droppings (i.e. fecal crumbs) can be found.
- As it gets older and grows, the caterpillar eats its way through entire leaves and doesn’t even stop at the branches, or more precisely the bark of the shoots. The further the pests reach the outside of the plant, the better they recognize the leaf damage and the caterpillars themselves – and their white webs, which are spreading more and more.
- At the latest when the first shoots dry up and die, the infestation is visible from afar: the yellow leaves, the baldness, the webs and the caterpillars leave no doubt that the box tree borer has struck. The good news, however, is: “Even heavily eroded boxwoods can recover and sprout again,” the Nature Conservation Association of Germany is convinced.
This is how you fight boxwood borer caterpillars
Two to four generations of boxwood moth caterpillars can attack your garden every year, so you should remain on alert from spring to late summer – even if you think you have already removed all the pests from your boxwoods. The following tips are among the most effective methods to combat caterpillars:
- Pheromone traps for infestation control
One soaked in sexual attractants Pheromone trap attracts the male boxwood moths so that they end up in the prepared container from which they can no longer escape. The trap primarily serves as an early warning system to find out if and when pests are spreading in your garden. As soon as the first butterflies are in there, you should start effective control – this is the only way you can stop the spread and reproduction of the small butterflies in a timely manner. - Biological plant protection products
Most plant protection products contain the natural active ingredient Bacillus thuringiensis or Neem oil. They are harmless to other insects such as bees or beneficial insects, but they kill boxwood moth caterpillars. Normally, the plants should be sprayed the first time in April (when the pests are still active inside the boxwood) and a second time in mid-July to increase success. To ensure that the pesticide reaches densely overgrown boxwoods and the caterpillars in their webs, you can use a Pressure sprayer take help. - Use natural enemies
According to the Nature Conservation Association of Germany eV, the boxwood moth caterpillars initially had no natural predators. The reason for this is said to be the plant’s poisonous toxins, which they absorb when they eat them – and therefore also become poisonous. However, it is now being observed more and more often that certain bird species such as house sparrows, great tits, sparrows and chaffinches attack the caterpillars. Therefore, you can attract the natural predators by using a Birdbath set up and one feeding place hang for birds. - Pick caterpillars by hand
If a boxwood tree is already covered in caterpillars, you need to act quickly. Instead of driving away the animals with a pesticide, you can simply collect them by hand. This takes some time and patience, as the caterpillars are quite agile and will hide inside the plant if they sense danger, but it is still effective. You can also use one for support leaf blower or use a strong jet of water after first placing a plastic sheet under the boxwood to catch the falling caterpillars. - Cut back boxwoods
If none of the tricks mentioned have been of any use and your boxwood trees are in danger of dying, there is only one thing left to do: you have to cut the plants back so that they can sprout again. Then burn the cuttings or enclose them in a garbage bag so that the boxwood moth cannot spread any further. Afterwards, you must absolutely avoid reinfection of your plants, as they would not survive a second attack.
This is how you prevent a boxwood borer infestation
Last but not least, you can of course also take preventative measures to ensure that the box tree borer – or more precisely its caterpillars – do not infest your plants. These include the following:
- If you want to buy a new boxwood, pay attention to possible infestation when you buy it from a plant store – the typical signs include white webs and brown piles of excrement.
- Put Algae lime preventative: The powder is intended to strengthen plants that are susceptible to pests and diseases, such as boxwood, to make them more resistant to attackers such as caterpillars. Algae lime also supports the regeneration of plants.
- Set the above Pheromone traps to recognize a borer infestation in good time and take action against it – before the caterpillars spread on your box trees.
- Check your box trees regularly, as the small butterflies appear in up to four generations per year and can therefore always return, even if you have just driven out a generation.
- Even though the boxwood borer is only active between March and October, you should keep an eye on your plants throughout the winter – as the last generation often overwinters and doesn’t emerge until the next spring.
Sources: Nabu buchsbaumzuensler.net
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