Since 2007, the box tree moth, a small butterfly native to East Asia, has been spreading, especially in southern Germany and neighboring countries, and has already destroyed thousands of valuable populations. But now nature seems to be striking back - the first birds and wasps were observed eating the protein-rich caterpillars and even feeding them to their offspring.
What natural enemies does the boxwood borer have?
Natural enemies of the boxwood moth are primarily songbirds such as sparrows, great tits, chaffinches and redstarts, which eat the protein-rich caterpillars. Wasps have also developed a taste for boxwood caterpillars and contribute to pest control.
No natural predators
The voracious pest was originally introduced to Europe via imports from Asia and spread quickly. As is usual with many neozoans (i.e. invasive species), the local wildlife initially couldn't do anything with the butterflies. Hobby gardeners also reported that they had observed birds trying on the caterpillars, but that they eventually regurgitated the pest. It was therefore initially assumed that the caterpillars absorbed and stored the bitter toxins from the boxwood. This meant that they also tasted bitter to the birds and were inedible for them. In fact, laboratory tests found that the caterpillars stored the same toxins in their bodies as those found in boxwood. Some of them were simply stored, others were even digested and thus used as a source of food.
The local wildlife gets a taste for it
In the meantime, the plague seems to be slowly ebbing, although this may also be due to the fact that many gardeners have removed the box from their gardens out of desperation and the pests that specialize in Buxus can no longer find food. On the other hand, the local bird world seems to be slowly getting a taste for it: the larvae of the box tree moth are now part of the menu for many species of songbirds. Apparently there was initially a phase of getting used to it, now that even the toxins no longer seem to bother sparrows, great tits and other feathered pest controllers.
Easy prey for sparrows
Sparrows in particular have discovered the green caterpillars as easy-to-prey, extremely numerous (there are hundreds to thousands of butterfly caterpillars on some bushes) and protein-rich food for their chicks. Many a gardener has been able to observe how the small birds lie in wait in the boxwood bushes and literally search the plantings for an infestation of borer caterpillars. Not only sparrows, but also other songbirds and garden birds such as great tits, chaffinches and redstarts have discovered the boxwood bush as a worthwhile hunting ground. So make sure that the birds feel comfortable in your garden by, for example, hanging up nesting boxes near the boxwood plantings - with a bit of luck, sparrows, titmice and the like will happily accept the offer and return the favor by destroying the borer caterpillars.
Do not use chemical substances
A bird-friendly garden also means not using chemical sprays to combat the box tree borer. These toxins have the unpleasant property of poisoning not only the pest, but also the beneficial insects that are welcome in the garden. The birds that eat the caterpillars would subsequently die from secondary poisoning. However, since the sparrow population has been steadily declining for years and both the house sparrow and the tree sparrow have now landed on the warning list of threatened species, this new food source may be the chance for these cute animals to recover. So what can you do to combat the box tree borer in a non-toxic yet effective way?
Important measures against the box tree borer:
- Installing numerous nesting boxes (hang them to be cat and rat safe!)
- Bird feeding in winter (attracts birds)
- Blowing out the boxwood trees using a high-pressure cleaner in summer (to get caterpillars out of the inside of the bush)
- no use of chemical sprays (not even if it says “bee-friendly”)
- use products with Bacillus thuringiensis instead (€21.00 on Amazon)
Dusting plants with algal lime has also had good success in the past.
Tip
In addition to songbirds, wasps now also seem to have developed a taste for the fat boxwood caterpillars. So stop seeing the animals as annoying visitors at the summer coffee table on the terrace, but also as welcome pest controllers in the garden. They are then welcome to have a piece of the sweet cake as a reward.