I was out running near Glasgow in Scotland when I noticed two trees that seemed to be completely covered in spiderwebs. No other trees were affected. What was going on?
It鈥檚 likely that these trees were bird cherries that were suffering from a serious infestation of small ermine moth caterpillars, although similar moths also attack hawthorn and blackthorn bushes.
The caterpillars can quite quickly spin a silken tent over the twigs and branches where they can feed on the leaves in safety, protected from birds and parasitic ichneumon wasps by their surrounding shield.
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鈥淭he ermine moth caterpillars spin a silken tent over twigs where they can feed in safety, protected from birds and parasitic wasps by their shield鈥�
During a major infestation of ermine moth caterpillars, whole bird cherry trees can be completely enveloped in this silken stocking and become totally defoliated. These outbreaks are sporadic, however, and while the trees may appear leafless and lifeless immediately afterwards, they tend to recover in subsequent years.
Bird cherry is found mainly in the north of the UK, where it can be a common hedgerow tree. Near my home in Durham, north-east England, I鈥檝e occasionally seen long sections of hedgerow that are festooned in small ermine moth silk, shimmering in the sunshine.
Phil Gates, School of Biological and Biomedical Science, Durham University, UK
Spiders were not responsible for the damaged trees; the silken tents were produced by the communal larvae of a small ermine moth.
The silk protects the larvae from various predators and parasites. It also shields them from environmental extremes, which is necessary because the nocturnal feeding of the caterpillars can lead to total defoliation (see Photo).
There are eight British species, each feeding on a small number of tree species. Most likely, given the locality in the north of Britain, this ermine moth is Yponomeuta evonymella. If so, the affected trees will be bird cherries.
Chris Gibson, Dovercourt, Essex, UK
Your correspondent does not specify the tree species in his question, but it is quite likely that these are bird cherry trees, which have become host to species of ermine moth caterpillars.
In some seasons these trees can be totally covered in the webs that are produced by these caterpillars, which defoliate the trees, although the leaves will most likely grow again later in the season. In other years the caterpillars and webs do not appear at all.
鈥淚n some seasons these trees can be totally covered in webs and become totally defoliated, although the leaves will grow again later in the season鈥�
Ian Alcock, Aboyne, Aberdeenshire, UK