The boxwood borer has been spreading rapidly with us for several years. It causes enormous damage and, if left untreated, causes the book tree to die.
The boxwood is a fairly easy-care plant. However, it is susceptible to various diseases and pests. This also includes the boxwood borer, which has been spreading in Europe since the beginning of the 2000s. The larvae of the small butterfly, originally from East Asia, are very persistent and damage the boxwood so much that it would die without treatment. It is therefore imperative to take suitable measures to combat and ideally also to prevent.
Life cycle bus tree borer
Each female lays up to 150 pale yellow, lenticular eggs on the underside of the leaves, from which the larvae hatch after only 3 days at temperatures above seven degrees Celsius. At a temperature of around 15 degrees Celsius, these go through a total of seven development stages over a period of around ten weeks before they pupate.
By the way: From an ambient temperature of 20 degrees Celsius, the larval development cycle is shortened to three weeks.
After pupation, it takes about a week for the 40 to 45 millimeter moths to hatch. These have a miximal lifespan of nine days. During this time, the male and female mates mate. The females then lay their eggs and the cycle starts again.
In this way, the boxwood borer can achieve about generations per year. The larvae of the last generation of the year hibernate inside the boxwood in a cocoon of bent leaves, which they glue to their cobwebs, and begin to feed again in spring.
The larvae of the boxwood borer can grow up to five centimeters long, depending on the stage of development. They are yellow to dark green and have white and black stripes and black spots. The head is black too.
Due to their coloring, the larvae of the boxwood borer, which mainly eat inside the bush, are difficult to spot with the naked eye. However, there are other indications that the green caterpillars are infected:
- Web with droppings on leaves and shoots
- brown, eaten leaves
- Egg laying on the underside of the leaf
- Butterfly dolls on the underside of leaves
- Boxwood guilds fluttering around
Fight boxwood borer
If you have found any evidence of an infestation, you should immediately start to fight the boxwood borer. Because as already mentioned, this multiplies rapidly.
❶ Collect caterpillarsYou can try picking up the boxwood borer caterpillars. Be sure to wear gloves. Because box trees are poisonous and can trigger allergic reactions when they come into contact with the skin. Alternatively, you can try to remove the pests with a vacuum cleaner. You should use a fine nozzle for this.
Tip: In order to better recognize fallen animals, it is advisable to place a light cloth under the plant.
Never dispose of the caterpillars on the compost. It is better to dispose of them in the residual waste. But make sure that they cannot crawl out again.
❷ Cut back
In the case of massive infestation, a vigorous pruning can be useful, in which you remove the affected plant parts. Again, a bright cloth under the plant helps to better recognize fallen caterpillars.
Sometimes it is also necessary to dispose of the entire plant. To prevent the pests from spreading further, you should not dispose of the plant or the clippings on the compost. In the best case, you will burn them or dispose of them with the residual waste. For the latter option, it is advisable to carefully pack the plant or plant parts in plastic bags.
❸ Spray with a high pressure cleaner
An effective method to remove the caterpillars from the plant is the pressure washer. Because these cannot hold on and are simply washed out of the plant. Before you start, lay out a light tarp around and underneath the plant and fasten it well. You can then easily collect and dispose of the caterpillars. Note, however, that they are very agile animals and will try to crawl back into their hiding places. So hurry up with the collection and repeat the treatment with the high-pressure cleaner if necessary to catch all the animals.
❹ Bring natural enemies to the gardenSparrows decimate the boxwood borer during the breeding season - since the boxwood borer is a neozoan - as an animal that was not originally native to us - there were no natural enemies for a long time and the butterfly was able to spread freely. In the meantime, however, the tide has turned decisively and some birds can be found in nature that caterpillars like to eat. These include sparrows, great tits, red tails and chaffinches. So make sure that birds feel comfortable in your garden. For example, offer bird feeders and feeding stations to attract them.
Tip: Wasps and hornets also eat the pests. Arrange with them rather than drive them out of your garden.
However, please note that you still need to take further measures as this will not get rid of all the caterpillars.
❺ Cover with dark film
You can also take advantage of the fact that the European larvae of the boxwood borer fight against the fact that they do not tolerate high temperatures. All you need to do is pack the affected plant in a dark film on a sunny day.
The caterpillars do not survive this enormous heat. However, any eggs that are present will not be damaged. You should therefore repeat the procedure after about two weeks to also kill the newly hatched larva.
Tip: If there is a lot of sunlight, you should leave the film on the plant for a maximum of two hours. Otherwise it could be damaged.
❻ Use of insecticides
There are various preparations on the market that you can use to fight the caterpillars. You have the choice between biological and chemical insecticides. The preparations are sprayed with high pressure directly into the inside of the plant and also spread over the surface. The caterpillars absorb the active ingredients when eating.
The active ingredients thiacloprid and acetamiprid have proven to be particularly effective. Insecticides with acetamiprid should be given preference over other chemical insecticides. These have a systemic effect. This means that the insecticide is absorbed by the plant and is only released when the caterpillars tamper with the leaves and stems. These insecticides are therefore less dangerous for beneficial insects than those insecticides that remain on the plant surface and show their toxic effects even when they come into contact with them.
Those who prefer to do without chemistry. can use biological insecticides. On the one hand there are products that contain the bacterium Bacillus thuringiensis and on the other hand products with neem oil. The products are distributed on the plant in the same way as the chemical products, but are less aggressive and must therefore be used again after about ten days to achieve a better effect.
Please note that insecticides are very effective, but they can pollute the environment. For the sake of the environment, these should therefore only be used if possible, if all other measures have not led to success.
Tip: Make sure that it does not rain for up to three days after application, otherwise the preparations will be washed off.
Prevent boxwood borer
It is best if your boxwood is not affected by the caterpillar's caterpillars in the first place. Here are a few ways to reduce the risk of an infection.
It is best to place pharomone traps near boxwood in the spring. These contain fragrances that attract male boxwood borer. Once trapped, they cannot escape.
On the one hand, pharomone traps serve to contain the population because the moths caught can no longer mate. On the other hand, they are an indication of an onset of infestation, so that you can start treatment in time.
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Protect boxwood with a net
The best way to protect your plants is when the boxwood borer does not even get to them. There are also special nets with narrow meshes through which the moths cannot get through. When using such a net, make sure that you do not give the moths a chance to get under the net.
The net is only really effective if you leave it on the plant until the temperatures are below seven degrees Celsius - like from March to November.
Infestation control when buying
If you buy a new boxwood, you should examine it carefully to see if there are any pests. The pests very often get into your own garden in this way.
Use of algae lime
Even if the effectiveness has not been scientifically proven, more and more hobby gardeners are using algae lime and mussel lime to combat the pest. The caterpillars do not seem to like the pollinated foliage and therefore leave the plant alone.
Tip: If you also want to try to keep the boxwood borer with algae lime away from your plants, you should dust them completely with the powdered lime on a dry day.
Find alternatives to the boxwood
In Germany, the boxwood borer usually only attacks box trees. As a result, you can prevent the pest by relying on alternative plants. Here are a few examples for you: