If you’re walking through an oak woodland this autumn, you might be lucky enough to come across a fallen leaf with a fascinating decoration on its underside.

Oak apple gallThis isn’t, as it first appears, a berry that has just dropped there by accident;     it is a cherry gall, caused by a small black gall wasp that rejoices in the name of Cynips quercusfolii.

Cynips quercusfolii; photo via Wikimedia

Cynips quercusfolii, no doubt pondering her next crafty move; photo via Wikimedia.

The wasp (I hope she won’t mind the familiarity, because her name is a bit cumbersome) is a complete specialist in the exploitation of oak trees.   I didn’t realise that, in fact, there are hundreds of species of gall wasps, each with its own unique adaptation and life cycle.

What happens is that the female wasp simply lays an egg on the leaf while it is still growing on the tree.   Other species of gall wasps might lay eggs on the trunk, the twigs, or in the roots.  All she then has to do is wait, because either the egg or the larvae will exude a special cocktail of chemicals that stimulate the cells of the tree in a way that is as sinister as it is intriguing.  The host leaf is forced to grow an alien structure around the egg:   depending on the wasp species, this may be a perfect apple-like sphere, such as the cherry gall shown above;  it may be a flattened, speckled disc like a tiny fried egg;  or a fuzzy mass of tangled strands resembling cotton wool.

Cherry galls measure about half an inch in diameter, and they are succulent, because they contain all the food necessary to support the larva as it matures.     Safely tucked inside, the grub will emerge as an adult wasp during the winter;   the only danger it faces is that of a hungry pigeon pecking at the walls of its home in search of the juicy centre.    Later, the gall will harden and darken to become a tough brown ball like a marble.

A similar-looking structure, known as the oak apple gall, is caused by the wasp Biorhiza pallida, which lays its eggs in oak twigs.  But galls are by no means restricted to oak trees – in a huge variety of forms, they are found on just about every species of plant life, from nettles to juniper, and they can be caused by wasps, flies, midges or even fungi (it is the Witch’s Broom Fungus which causes dense twig-like growths resembling birds’ nests on birch trees).   If you want to find out more, the website Trees for Life has some fascinating information on their life cycles and specialities.

Ruthless exploitation… or clever management of resources?    It’s all going on above our heads and beneath our feet!

Galls on oak leafThe oak leaf on the right is spangled with tiny galls

Sources:

Main photos copyright © Colin Woolf

Oaks 1You might also like to read ‘The Wisdom of the Oak‘, another feature on The Hazel Tree…