A Galling Mystery

Silk Button galls caused by Neuroterus numismalis
Silk button galls caused by Neuroterus numismalis, spangle galls caused by N.quercus-baccarum and a spherical gall caused by Cynips divisa

An artichoke gall caused by the wasp Andricus fecundator

Plant galls are breeding chambers produced by insects – often gall wasps – that lay their eggs in the plant’s buds and leaves and induce the plant to change its pattern of growth. Typically, galls develop a hard protective casing lined with feeding tissues for the developing grubs inside. Pictured here are four different galls, all on the same oak twig, produced by four different but closely-related gall wasps. The large, scaly flask-like object is an artichoke gall, produced by the gall wasp Andricus fecundator. The middle image shows a leaf with three different gall types: disc-shaped spangle galls, cause by the wasp Neuroterus quercus-baccarum; a spherical gall caused by the wasp Cynips divisa; and the exquisite silk-button gall caused by Neuroterus numismalis, also shown in the top photo. The growth of each was triggered by a substance that was injected into the leaf tissue at the same time that the eggs were laid, redirecting the normal pattern of cell growth to produce a species-specific gall. So exactly how does each different gall wasp manage to modify its host’s growth to produce its own, unique gall type? Your guess is as good as anyone else's – so far, no one has fully explained the detailed processes involved. For more about Britain's plant galls, visit the British Plant Gall Society web site at http://www.british-galls.org.uk/