Archive
HOLEY HOLLY
20/12/2017

At this season, thoughts of many turn to plants that decorate our homes such as ivy, mistletoe and holly. Holly, Ilex aquifolium, with its cheery red berries is a special favourite. There are many attractive varieties available now, too, as demonstrated by these below from my own garden.
Have you ever looked a little closer at holly trees, though? Examine its leaves and you will find some of them are marked by discoloured blotches, like these:
These blotches are the mines made by the larva of a tiny agromyzid fly feeding under the cuticle of the leaf interparenchymatously, usually outwards from the midrib. It is known as the holly leaf miner (see drawing at bottom of page) and its scientific name is Phytomyza ilicis Curtis, 1846. It only feeds on holly. Indeed, it is the only leaf miner affecting holly in Britain. If you look carefully, you can often see the large (>1mm across) exit hole that may be on the underside of the leaf, by which the fly left after it emerged from its pupa.
Careful dissection of the mines can reveal the life history of the individual fly, whether it emerged successfully or if, as is often the case, it was parasitised by a parasitoid wasp such as Chrysocharis gemma (Walker, 1839) or a braconid wasp. (Parasitoids ultimately kill their hosts rather than just feeding off them without host mortality.) A shiny black pupal case remaining in the mine rather than a lighter brown one, plus a conspicuous, neat exit hole on the upper surface of the leaf, as in the picture here (arrowed), indicates that the fly larva was parasitised.
If you look at a holly tree with many leaf miner blotches, you will probably see that some have been ripped open, often in a V-shape. This is a feeding strategy of blue tits.
All photos © Christopher Betts.