Holm oak (Quercus ilex) is a Mediterranean tree species that was introduced to Britain in the 1500s. Unlike our native oak species, Holm oak is evergreen and holds its leaves all year round. It’s latin name, ilex, refers to the fact that the leaves can also be prickly as well as a dark, shiny green.
There are a number of Holm oak trees on campus. At this time of year, when most tree leaves are new and not yet infected by parasites and grazers, Holm oak leaves bear the signs of on-going or previous attacks from a variety of organisms.
The raised pimples on this leaf are the homes of Aceria ilicus mites. A look underneath the leaf reveals that these raised domes are full of brown plant hairs. It seems a rather clever strategy to force a plant to grow you a home where you can live and feed rather than crawling around on the exposed leaf surface!
The same leaf has also been munched by a leaf-miner. In this case the larvae of a moth: Ectoedemia heringella. This is a relative new-comer to Britain. The first adult moth was caught in London in 1996 but was not identified until 2001. The larval leaf mines can now be found throughout the south-east of England. The moth itself is a ‘micro-moth’, having a wing-span of only 5-6 mm.
A micro-moth species that has been in Britain for much longer is Stigmella suberivora. The larvae of this species produce a broader ‘gallery’ within the leaf and often mine near the leaf edge.
Not all leaf-miners produce galleries. The micro-moth Phyllonorycter messaniella produces an oval papery ‘blotch’ that is often located between two major veins. This micro-moth species can mine our native oaks as well as Holm oak.
So next time you walk past a Holm oak, don’t just think of the tree as a large plant blithely photosynthesising in the sun. Just remember these large individuals are under constant attack!