Species of the Month: Silk Button Gall

Silk Button Gall Neuroterus numismalis

Early autumn is a great time to look for galls. Galls take many different forms, forming on leaves, stalks and seeds and can be caused by mites, wasps or fungi. Oak trees are a good place to start, and a single tree might harbour several different species with closer examination.

Silk-button Galls Neuroterus numismalis are caused by a tiny wasp, and result in thick golden brown discs with a clear central depression. Each one is about 3mm in diameter.

The wasp has a fascinating life-cycle. The silk buttons are home to a tiny female wasp, which will remain inside the gall amongst the leaf litter until the following spring. She emerges and reproduces asexually (without mating), laying eggs in oak leaf buds, the start of a new spring generation. These larvae in turn creating blister galls on oak leaves. From these galls emerge the sexual generation of males and female wasps, which go on to reproduce and complete the annual cycle.

There are numerous other galls on oak, some small, some large, but several are easily recognised.

On the underside of oak leaves, Common Spangle Galls Neuroterus quercusbaccarum f. agamic can occur in their thousands, forming small discs with tufts of reddish hairs. Artichoke Galls Andricus foecundatrix f. agamic form on oak buds, creating hard galls with overlapping scales up to 30mm long.  Knopper Galls Andricus quercuscalicis form irregular ridged growths on acorns.

For anyone interested in learning more about galls and how to and identify them, Michael Chinery’s Britain’s Plant Galls: A Photographic Guide in the WildGuides series is a useful starting point. The most accessible comprehensive guide is British Plant Galls by Redfern, Shirley & Bloxham published in the AIDGAP series by Field Studies Council.

Mark Pollitt

SWSEIC Manager