First discovered in 2006 in the southwest of Baden-Württemberg, the box tree moth has spread almost everywhere in Germany. Three or four caterpillars are enough to completely bare a boxwood ball with a diameter of 25 centimeters. While garden enthusiasts had hardly any means of combating it in the early years, there are now effective measures that can save the popular tree from the hunger of the caterpillars.
How do I recognize and combat a boxwood borer infestation?
A boxwood borer infestation can be recognized by light spots on the foliage, droppings, fine webs, old pupal casings and caterpillars. Mechanical measures, covering with foil, biological insecticides and pruning can contribute to control. Possibly consider replacement plants.
What does the boxwood borer look like?
The boxwood moth is an approximately 40 millimeter large butterfly. The wings are white and have dark brown outer edges. The little butterflies hide under the leaves of the boxwood, where they also lay their eggs.
In April/May, as soon as the temperatures permanently rise above 7 degrees, and August/September, the green-colored, black-dotted caterpillars hatch from these. When fully grown, they reach a size of around four centimeters.
How do I recognize the infestation?
The damage pattern is very characteristic and easy to recognize even for laypeople:
- The first feeding activity (cockroach feeding) causes light spots to appear on the leaves.
- If you bend the shoots of the boxwood apart, you can discover droppings, fine webs and possibly old pupa casings.
- Caterpillars cannot always be found because they are well camouflaged. If there is a problem, they simply let themselves fall or abseil down on a web thread.
- In the advanced stage, only the midrib of the leaves remains.
- The varieties Buxus sempervirens and Buxus microphylla are predominantly affected.
Which regions are affected?
Due to a lack of natural enemies, the pest, which comes from Asia, can no longer be eradicated. The butterfly was first seen in April 2007 in Weil am Rhein, near the Swiss border. From here it spread rapidly throughout Germany, supported by the plant trade.
What control options are there?
First of all, it is very important to carefully check boxwoods regularly. In particular, take a close look at the inner and very densely grown areas of the trees. The use of pheromone traps (€22.00 on Amazon) has proven successful in order to be able to estimate the time and quantity of the butterflies.
Mechanical measures
To decimate the next generation, you should collect all the caterpillars and pupae you find. Use tweezers for this and be very careful, otherwise the animals will retreat to other areas or drop.
You can take advantage of this and alternatively knock down the tree. Please do not use a high-pressure cleaner to help, as this measure also kills many beneficial insects.
Animals that have fallen onto a tarpaulin or blanket placed under the box should be disposed of in a tightly sealed bag with household waste.
Covering by foil
Since the boxwood moth caterpillars are quite sensitive to heat, you can simply cover infected individual plants with black foil on a sunny day. Due to the rapidly increasing temperature, the caterpillars die within a very short time.
However, the egg clutches are not destroyed by this measure. Therefore, repeat the treatment several times every few weeks.
Biological insecticides
The BUND recommends spraying biological insecticides such as Bacillus thuringiensis as well as plant protection products based on tea tree or linseed oil. However, these funds must be applied multiple times. The reason: When the caterpillars have reached a size of around three centimeters, emergency pupation occurs. The doll's shell keeps the agent out and the agent remains ineffective.
pruning
If the borer has caused massive damage, pruning can save the boxwood.
Composting the clippings on your own compost is not recommended, as the temperature necessary to kill the caterpillars and pupae is not reached. Therefore, bring larger quantities of infected branches to the regional composting facility in tightly closed bags.
What to do if you can't get the boxwood moth under control?
In this case, you should not attempt to replant. Various small shrubs such as
- Japanese holly
- Small-leaved Rhododendron
- Dwarf Yew
- Evergreen Honeysuckle
are similarly cut-friendly and are a good replacement. Maybe you could try a completely different solution, especially with bed borders, and border vegetable beds with robust herbs such as chives or lavender.
Tip
Cases are known from China in which the boxwood borer has also attacked poppies and holly trees due to a lack of food. Although this has not yet been observed in Germany, you should also keep a careful eye on these plants.
