TDA 141 Little Nibblers
Tree Damage Alert No 141
1st April 2011
Little Nibblers
It seems that trees growing in Britain, whether woodland, rural or urban, are increasingly being threatened by previously unreported pests and diseases. The commonest single topic of enquiries received by AAIS about damage to trees this late winter/early spring has nothing to do with insect pests or diseases, but ‘little nibblers’.
Previous Tree Damage Alerts have reviewed damage caused by mammals – TDA 62 Rabbits, TDA 64, Grey squirrels, TDA 72, small mammals. This winter saw some localised bark gnawing damage caused by rabbits – identified by the size of tooth marks and position of the damage on the trunk. This damage was less severe and more localised than might have been expected in view of the heavy and persistent snow covering before Christmas.
There have been several reports of the ground under specimen Yew (Taxus baccata) trees being carpeted with green foliage. Specimens sent to AAIS, and personal experience on site, show the debris consists of fine shoots still bearing their green needles. Careful examination of the trees and the debris failed to reveal any symptoms consistent with a known disease or attack by an insect pest.
Each of the pieces of debris had oblique surfaces, where they became detached from the tree. These varied from fairly smooth to being ribbed. The size of the ribs was consistent with the width of the incisors of a mammal smaller than a Grey Squirrel (Sciurus carolinensis). But, beware, there may not always be obvious teeth marks present and squirrels do remove shoots often in significantly greater quantities than they need, for drey building material, but also occasionally to eat flowers. However, all things being considered, the ’little nibbler’ is most probably the Bank vole (Clethrionomys glareolus). Thorough examination of a damaged tree may reveal vole droppings that adhere to the upper surface of shoots for a short time. (The habitat of the Field vole (Microtus agrestis) overlaps that of the Bank vole, but the Field vole does not climb trees.) Reports of similar damage with specimens and details of location would be appreciated.
That such a small mammal normally associated with hedgerows, bramble patches, coppices and woodland edges with undergrowth covering the ground could be responsible for damage not just on Yew hedges, but also high in the crowns of specimen trees, sounds farfetched. In fact Bank voles are notorious climbers, but the reason for favouring Yew trees and not other evergreen trees this year is not known. Also why the shoots are being severed and allowed to fall suggests that this behaviour may not be food related. Certainly Bank voles do not appear to be affected by the poisonous components of Yew bark, which they are known to strip from shoots.
Bank voles generally seek to stay out of sight below ground in burrows or under vegetation. Indeed where damage to Yew trees has been reported it appears that there has been plenty of cover at the base of the tree in the form of loose dead vegetation or stacked woody material. The removal of debris from around the bases of Yew trees is unlikely to reduce the potential for damage. Also, removing voles by trapping is neither practical nor desirable and poisoning is illegal. Encouraging natural mortality through predation may have a beneficial effect and is probably the best and only option available to protect Yew trees. So do not control foxes, stoats, weasels or raptors and put the family cat on reduced rations.
Harry Pepper and Derek Patch, Arboricultural Advisory and Information Service.
Thumbnail image - Bank vole Clethrionomys glareolus © Pat Morris
This is one of an occasional series of Tree Damage Alerts produced for the benefit of the arboricultural profession and issued by the Arboricultural Advisory and Information Service.
AAIS
Alice Holt Lodge
Wrecclesham
Farnham
Surrey
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