Pest
Box tree caterpillar
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in March | Watch for in April | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September | Watch for in October |
What it is
Box tree caterpillar is the larva of an East Asian moth that arrived in Britain in the late 2000s and has spread quickly, particularly around London and the south-east. It feeds only on box, and a heavy attack can strip a mature hedge in weeks. The adult moth is white with a brown border, though a dark brown form also occurs.
Symptoms
Young caterpillars are yellowish green with black heads; older ones are green with black and white stripes, reaching about 4 cm. They feed inside the plant first, so damage is easy to miss until outer leaves turn pale and straw coloured. Look for fine webbing, green or black droppings caught in it, and leaves eaten down to a thin skin or the midrib. Box blight also browns plants, but it causes leaf spots and stem streaks rather than webs.
Cause
The caterpillars overwinter in small silk cocoons between leaves and resume feeding in early spring. After pupating, moths emerge and lay flat egg clusters on the undersides of leaves. In warm summers there can be two or three generations, so attacks may continue well into autumn.
Control
On small plants, part the foliage every week or two and pick caterpillars off by hand. Hose them out onto a sheet, or vacuum webbing with a handheld cleaner. A biological spray based on the bacterium Bacillus thuringiensis targets caterpillars and is most effective on young larvae; repeat according to the label. A nematode spray is another non-chemical option. Pheromone traps catch male moths and tell you when a new generation is due.
Prevention
Clip box after the first generation of caterpillars has been dealt with, since trimming opens up the plant for inspection. Keep hedges healthy with mulch and water in dry spells. Where attacks are relentless, consider replacing box with yew, Japanese holly or small-leaved hebes.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: March, April, May, June, July, August, September and October.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.