In the netherworld of nature's parasites

Another Life: 'Tiny wasps are known," wrote Edward O Wilson in The Diversity of Life, "whose larvae parasitise the larvae of still other kinds of wasps that live inside the bodies of the caterpillars of certain species of moths that feed on certain kinds of plants that live on other plants."

The nether world of nature's parasites, when not too visceral and gruesome, can be fascinating. A friend e-mailed an image of a strikingly odd spray of bright-pink, hairy growths on a branch of wild dog rose (see photo). They were "robin's pin-cushions", caused by the tiny gall wasp Diplolepis rosae. In a chamber at each hardened centre, if he cared to operate, would be the wasp's grub, whose own genes had commanded the rose's tissues to shape themselves into this ball of fibre, quite unlike any other structure the plant would normally grow.

More familiar, perhaps, than robin's pin-cushions are the big, brown, woody marbles, a bit like outsize Maltesers, that will reveal themselves on the topmost twigs of our tallest oak when the leaves fall. On other branches of the same oak are curious growths that, from a distance, lift one's hopes of acorns. A closer look finds, instead, scaly, twisted buds with feathery tips: the "artichoke" gall. What I've yet to see, but what now occur quite commonly on oaks in the east of the country, are acorns smothered by a disfiguring growth like a woody, curling wig: this one known as the "knopper" gall. All three, and the robin's pin-cushion, and scores of other galls, are the work of one family of small, winged insects, the Cynipidae.

A huge range of insects create galls on plants - swellings of various sizes and kinds in which their young feed and grow, protected from predators. Aphids, midges, thrips and sawflies are among the 13,000 or so species that use the strategy, picking on particular kinds of plant. But the oak gall wasps - about 1,000 species globally - induce the galls that are most structurally complex and bafflingly diverse. This, and the ease with which their galls are collected, has made them a keen focus of research.

Right from hatching from the egg (or eggs) laid in the tissue of the soft oak stem through the female wasp's drill-like ovipositor, the grub is able to organise the gall that is special to its species. As a leading UK researcher in the field, Dr Graham Stone, puts it, "Understanding the molecular tools used by cynipids to manipulate plant development is the Holy Grail of current cynipid research". Whatever the process, it needs "omnipotent cells" to take over the plant's normal blueprint of growth. First, the larva creates its nesting chamber, lined with a layer of cells that concentrate plant nutrients for its food. Around the chamber grows a thin, tough, protective wall of woody cells. This is surrounded by layers of woody or spongy tissue and then surface coats which may be of sticky resins, furry fibres or spines.

Gall wasps can have a complicated reproduction cycle, but most larvae overwinter in the galls and emerge as a winged adults in the spring. The advantages of the gall "nursery" are fairly obvious: a guaranteed food supply in an environment safely sealed off from frosts, droughts or floods. But that does not explain the diversity of galls induced by members of a given "guild" of insects on the same part of the same host plant at the same time. This must have to do with protection against predators. But why have so so many different designs evolved in closely-allied species? Woodpeckers are among birds that attack the galls, which may cause more of them to last for longer in woodpecker-less Ireland. But the main threat is from other insects that invade the galls and take them over. They include other species of gall wasp (inquilines) that lay eggs in developing galls for their grubs to share the food.

But the most formidable enemies are "hyperparasites" such as the small chalcid wasps, that drill into the galls and lay eggs in both the original grubs and the inquilines.

The tougher galls take longer to drill through, keeping the chalcids exposed to their own predators. To cope with the hardest galls, a few of the wasps have toughened their drills with manganese (also used in steel alloys!).

But they may still hit air spaces that cause their drill-tips to buckle against the hard wall of the larval chamber. They may also find themselves harassed, in some cases, by ants attracted to nectar secreted from the gall - the clearest demonstration yet, say researchers, of a trait evolved for defence against the wasp's natural enemies.