Ermine Moths

Several of us have noted unusual defoliation, especially of willow trees, in the Severn Vale recently. The trees look as though they have almost entirely lost their leaves, as if winter is approaching. We have never noticed such extreme loss of leaves in previous years, so it seems to be an exceptional phenomenon this year. We first noticed this at the Gloucestershire Wildlife Trust’s Ashleworth Ham reserve, where Richard Humphreys has taken a series of photographs. It is clear that the outbreak is caused by the Willow Ermine Moth, which leaves webs containing caterpillars on the trees.

Rob Homan, the East Gloucestershire Moth Recorder for Vice-County 33 (East Gloucestershire) comments: “It really is an extraordinary phenomenon”. He confirms that the occurrence seems to be much heavier than usual, and notes that the same phenomenon has been noted on spindle trees.  He has provided a map showing the distribution of adult Willow Ermine moths in Gloucestershire (green dots) and the distribution of larval webs (red dots). All records up to 2023 are included. The contrast in the two patterns is marked. Note the rogue web in Cheltenham, seen last year, but not seen at that location this month.

Map produced by Robert Homan (Contains OS data © Crown copyright and database right 2024.)

He adds: “I have been in contact with the Worcs moth recorder – plenty of webs as is normal along the Severn and Teme valleys, but no records for 2024 for defoliation. Nothing of note has been added to iRecord for the species.”

Further searches have recorded intensive occurrences of moth webs in a variety of sites across the county, in particular at Coombe Hill, along the Leadon, and on the Avon north of Tewkesbury, mainly on willows but also on spindle trees (as far south as Frampton).    

This is a preliminary note to make GNS members aware of the numbers of Ermine Moths this year, and to appeal for further information. We may be able to present a more complete account of this year’s observations later in the year.

Meanwhile, the following note from May 2014 on Butterfly Conservation’s website may provide some useful background:

“At this time of year we often receive reports of ghostly silken webbing covering sections of hedgerows and, on occasions, individual trees. Although it can look rather sinister, don’t be alarmed. The most likely culprit is a harmless caterpillar.

“Webs have already been seen in parts of Dorset in the last week or so. These striking and obvious webs hide hundreds and sometimes tens of thousands of caterpillars of a group of moths called the Small Ermine moths. There are eight species in this group, although only the Orchard Ermine Yponomeuta padella, Spindle Ermine Y. cagnagella and Bird-cherry Ermine Y. evonymella tend to produce such extensive webbing, the former mainly on blackthorn and hawthorn, the others on spindle and bird-cherry respectively. The Bird-cherry Ermine tends to have a more northern distribution compared to the other two and occasionally whole trees can be covered by their webs, the leaves stripped bare giving the tree an eerie appearance. Sometimes these webs are so extensive that they can cover nearby objects such as benches, bicycles and gravestones.

“Why do these species spin these webs and live together in such large numbers? It’s a successful evolutionary strategy, providing protection from predators through safety in numbers. However, numbers are hard to hide and hence the production of the silken webbing.

“These webs and caterpillars are harmless and usually last from May to June. The webs slowly disappear over the summer and typically the hedgerow shrubs/trees recover. The adult moths fly later in summer and all look superficially similar, being white or greyish with many small black dots, hence the ermine name.

“Ermine moth webs should not be confused with other web-forming larvae, which can be found around the same time, although these nests tend not to be so extensive and the caterpillars of most are hairy. Nests could belong to the nationally scarce Small Eggar Eriogaster lanestris, whose webs can reach the size of a small football; the declining Lackey Malacosoma neustria, with their striking stripy caterpillars; the Brown-tail Euproctis chrysorrhoea, which is expanding its range; and the introduced Oak Processionary Thaumetopoea processionea. The caterpillars of the latter two have urticating hairs, i.e. these can cause rashes, and because of this we advise that all hairy caterpillars and webs should be avoided and not handled.”

Mark Parsons
Head of Moth Conservation

Stephen French adds…

“Yes, I’ve witnessed the evidence around Ashleworth and Tirley, where I often walk. Both Spindle and Willow Ermine are having a “good” year – if that’s the right term!

It’s certainly the most noticeable outbreak of Willow I have ever seen. Even some of the tree trunks are totally clad in the webs.

There is normally no lasting damage to the trees – we will have to wait and see whether the sheer scale of things this year changes that. As to why they are so numerous, I have no idea (Climate? Lack of predators?).

As Robert says, the adult moths will be on the wing soon, so we can monitor how the trees recover (although we may have to wait until next year now).

I’m doing some targeted moth recording/trapping at the nearby Hasfield Estate. It will be interesting to see if numbers of the adult moths reflect the dramatic increase in the larval stage.”

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