The strange world of knopper galls

Wenlock Edge, Shropshire: Once thought of as a threat to the UK's native oaks, these horny growths have joined the rich community that inhabits the trees

On one of the finest autumn days for years, sharply lit under a Wedgwood sky, the oak bears strange fruit. Little green and brown apples covered in horny, foliate protuberances grow where acorns should be. They formed last month when a tiny gall wasp, Andricus quercuscalicis, inoculated embryonic acorn buds with her eggs. The oak responded in an entirely specific way to this wasp by producing growths known as knopper galls, from a German name for a type of helmet.

They weren't seen in Britain until the 1960s, but there was a spike in the knopper population in 1979 and people were worried that the iconic British oaks were in danger of not producing viable acorns. Although the wasps have spread as far north as Scotland, this has not been the case, and knopper galls have joined the rich community of life that inhabits the oaks. The galls are communities themselves, too, containing microhabitants such as inquilines (cynipid wasps lay eggs in the gall and their larvae feed on the oak tissue) and parasitoids (chalcid and ichneumon wasps inject their eggs into the gall wasp larvae to feed on them).

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