Invasion of the box tree caterpillar

Finally I have found a web site with some real details on the the box tree caterpillar/pyrale du buis www.conseils-coaching-jardinage.fr This last few months we have really struggled to deal with the problem and now we have an invasion of caterpillars from all around us. It is a real catastrophe for the local environment. If you can read the French go to this site, if not, here is something in plain English.

At the moment we are at the caterpillar stage and if you haven’t already treated you need to spray the whole bush with Bacillus thuringiensis. This is the same stuff which you can spray on cabbages to treat the caterpillars and both caterpillars look similar. It is a biological control and only harmful to the the caterpillars that eat the leaves. Garden centres normally stock this product, but you can also find it on the internet. Obviously if it rains you will have to spray again and repeat again after 8 days if you still find live caterpillars. Our caterpillars have been around awhile so they must be ready to pupate. At this stage it is a good idea to use a pressure washer to remove all the debris, the caterpillar shit and hopefully any cocoons. Trim the hedge, or even cut it back to base, but give it a high nitrogen fertiliser, something suitable for evergreen hedges. It is a risk cutting it back really hard, but most of ours have come back. The next generation of moths will be with us, ready to lay eggs after 3 weeks and the whole cycle begins again.

Ideally you have a trap (piege) for the male moths. It won’t make much impact on the numbers, but it will help you to judge the most effective time to spray again. The moth lays 200- 300 eggs in the middle of the bush and they hatch in 2 days. It is suggested that you spray 8 days after you find the first moth and again 8 days later. They overwinter as caterpillars, so it is really important that this last treatment is done. The moth is around from April to November, so there are going to be several generations.

For some box bushes and hedges it is already too late, but I urge you to at least try and save a few bushes. I have found caterpillars eating other plants when they have exhausted the box, but it seems the holly and Euonymus are also vulnerable. If the moth cannot find any box it is likely to turn to these shrubs.

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