In April, the first generation of box tree moth boxwood borers lays its eggs. Here we explain which three steps are important to save heavily infested plants.
Boxwood fans have had a new archenemy for about ten years: the boxwood borer. The small butterfly, which immigrated from East Asia, looks harmless, but its caterpillars are extremely voracious: they eat both the leaves of the boxwoods and the bark of the younger shoots. Infested plants can therefore be so badly damaged that they only bear bare, scrawny shoots on the outside.
Many hobby gardeners then make short work of this and part with their evergreen favourites. However, this does not have to be the case, because with a little patience and a few suitable measures you can get the problem under control – and without the use of aggressive chemicals. Here we explain how to proceed.
Step 1: Cut back heavily infested boxwoods.
If you discover box elder caterpillars on your box trees, the first thing you should do is check how badly infested they are. If, after a short inspection, you can see several webs, you can assume that there are a number of beetle caterpillars in your boxwood. They are difficult to detect because they mainly live inside the crown and know how to camouflage themselves well with their green-yellow colour.
If some of the shoots already have eaten or withered leaves, a severe pruning of the shrubs is unavoidable: cut back all hedges, borders and topiaries by about half of their height and width to the basic structure. The plants do not mind, because boxwood is very tolerant of pruning and sprouts again easily from older branches. Throw the cuttings straight into a garden bag. You can compost it in a remote place in the garden or burn it. After pruning and further treatment, fertilise the boxwoods with horn meal to support new sprouting.
Step 2: Spray the plants with a high-pressure cleaner
After pruning, it is important to remove as many of the remaining caterpillars from the boxwoods as possible. This can be done particularly quickly and efficiently with a high-pressure cleaner: Before you start, you should lay out a plastic fleece or foil sheet on one side of the border or hedge. To prevent this from flying up under the pressure of the water jet, weigh down the side facing the hedge with stones. Then blow through your box hedge from the other side with the high-pressure cleaner at maximum water pressure. Hold the spray nozzle calmly into the crown – the boxwood will lose some of its leaves, but you will also catch most of the borer caterpillars this way. They land on the film and must be collected there promptly so that they do not crawl back into the boxwoods. Simply release the collected caterpillars far away from your box trees in the green meadow.
Step 3: Organic insecticides against last boxwood borer caterpillars
Despite the measures mentioned above, you should finally treat your boxwoods once again with an insecticide to eliminate the last boxwood borer caterpillars. Biological preparations that are very suitable for this purpose are products with the active ingredient “Xen Tari”: This is a parasitic bacterium called Bacillus thuringiensis, which was discovered by a Japanese plant protection product manufacturer and brought to market. The bacterium enters the caterpillars through body orifices, multiplies inside and secretes a toxic metabolite that causes the insect larvae to die. The agent is applied as a watery dispersion with a conventional sprayer. Make sure that you also wet the inside of the crown of the boxwoods well from all sides. By the way, the preparations can be used against many types of pest caterpillars and are also approved for fruit and vegetable crops in the home and allotment garden.
Boxwood borers usually form two generations per year, or three if the weather in the southwest is very favourable. Experience shows that the optimal periods for the application of Bacillus thuringiensis are the end of April and the middle of July. However, depending on the weather conditions, these periods can also be shifted forward or backward. If you want to be on the safe side, you should hang up several yellow boards or special box borer traps near the box trees. When the first moths gather in the traps, the agent is applied seven days later.