The gall of the wasps

STRANGE fruit adorn the topmost twigs of our tallest oak saplings: clusters of large green marbles, turning brown. Not "oak apples"(which are bigger and somewhat spongy), but the hard, perfectly spherical galls - "marble galls", indeed - induced from the oak's tissue by the presence of an insect's larvae. Cut open a marble. and there, at its mathematical centre, is this squidgy little grub.

On other branches of the same oak are curious growths which, from a distance, lift one's hopes of acorns. A closer look finds, instead, scaly, twisted buds with feathery tips, oddly reminiscent of something a lot larger the giant terminal buds of the globe artichoke.

Inside each "artichoke gall", again, is a single chamber with its little blob of a larva. Later in the autumn, this inner gall will drop out and a female gall wasp will emerge from it in spring to lay more eggs in oak buds.

The odd thing is that both kinds of gall, so utterly different in form, are provoked by closely-related species - Andricus killari in the marble, Andricus fecundator in the artichoke bud. Both of them are small, almost ant-like, winged insects in the Cynipidae family, quite unlike the big social wasps we wave away from the jam.

How galls of any sort are "switched on" is still only partly understood. But each species of gall wasp has its own special effect on plant tissue, the mere presence of its larvae triggering a distinctive structure of cells.

One of the wasps, a really tiny creature called Diplolepis rasae, lays its eggs in the young leaf-buds of wild roses. These provoke the growth of the "robin's pincushion", a ball of reddish, mossy fronds an inch or more across. A similar structure on another scale is the twiggy "witches' broom" of birch trees. This is also a gall formation, but one usually prompted by parasitic mites.

There are scores of different kinds of galls caused by a wide range of organisms, and the oak tree puts up with the biggest variety, sprouting anywhere from its crown to its roots. Most of them do the tree no harm, but Ireland has recently been invaded by a newcomer among the gall wasps, Andricus quercuscalcis, which changes the structure of acorns. Sometimes its hard gall smothers them completely, a sight to fill any oak-lover with dismay.

The "knopper gall" wasp, as it's known, reached Britain from the Continent around the middle of the century and attacked the acorns of the pendunculate oak, Quercus robur. Actually, with the complicated reproduction cycle so typical of gall wasps, it needs a second oak species - the Turkey oak, Quercus cerris - to carry alternate generations in smaller galls on its male catkins in spring.

The wasp was probably limited originally to eastern Europe. where the two oaks overlap naturally. Turkey oak was brought into Britain as an ornamental tree in 1735 and now grows in parks and big gardens in both islands. It took the little wasp some time to follow, but, tree by tree, it did so.

Entomologists at the Imperial College, London, are studying the ecology and population genetics of insects invading Britain. Five years ago, one team was focused especially on Andricus quercuscalcis, tracking its spread through Europe and identifying the natural enemies - mostly parasites - that are attacking the wasp in Britain. They wrote to the National Botanic Gardens at Glasnevin: have you seen these knopper galls in Ireland?

Dr Charles Nelson, then the gardens' taxonomist, went out to inspect Glasnevin's closely-planted oaks and came back to fax London: "Alas, the wasp is here." Later, he confirmed a massive infestation of the Quercus robur oaks in the Phoenix Park, with more in Dublin's Marley Park and in gardens in Dunboyne and elsewhere.

The extra vigilance was almost bound to have other results, and two more new cyniped wasps have been discovered on Irish oaks, Andricus quercusrasmuli and Andricus lignicola. Each emerges from its own, strikingly different form of gall - the first, from a tuft of white "cotton wool", the second from a shiny green gall like a tropical cola nut. A lignicola, which was first found in these islands in 1968, has already established itself in 14 Irish counties, and is positively common in the oakwoods of Killarney.

TO rear a grove of oaks - even a little group like ours, crouched together in the tee of the house - is soon to realise what teeming towers of life they are, and how they make their own generous rules for tenants. Not all of them live in a sightly way: many of the oak leaves end, each summer dusted whitely with powdery mildew, a parasitic fungus. We regretted this to a visiting professor of biology and he ticked us off: "Would you grudge it living space?"

Oaks have an age-old association with other species in these islands, almost as rich as willows in the range of plant-eating insects. mites and caterpillars they support. But biodiversity can't be laundered: song thrushes, yes, and powdery mildew as well.

As an enthusiast for native trees, especially the modest ones like alder and birch, I can become a bit stuffy about some of the exotic species carried home fashionably from garden centres to plant beside country roads. Last spring I grew especially offended by a flamboyant little tree that was waving white and pink football flags from every other garden, it seemed, between Westport and Galway.

This, if I have it right, is Populus candicans Aurora, balm of Gilead, Ontario poplar, a hybrid clone with North American sap in it. The young leaves are almost wholly white and pink-tinted, gradually turning green. Will anything Irish eat it? live in it, lay eggs on it? Perhaps a moth or two, in time. But I still can't approve it as a tree for rural Connacht the gall of me.