Species of the Month: October 2025

Oak Knopper Gall Wasp Andricus quercuscalicis f. agamic

Knopper Galls are ridged outgrowths which form on acorns, caused by the tiny wasp Andricus quercuscalicis. The distortion is induced by chemicals as wasp eggs develop into grubs. The larger the number of grubs, the more contorted the gall becomes as they compete for space.

This species was first found in Britain during the 1960s, and is now widespread throughout the country. The wasp has a complicated life cycle, alternating between asexual and sexual generations which require Turkey Oak plus Penduculate or Sessile Oak to complete a cycle. The generation which form Knopper Galls are form agamic, so these galls should be recorded with the full name Andricus quercuscalicis f. agamic.

The gall begins as a sticky bright green mass resembling a walnut (shown right), eventually drying to a hard brown (shown in the photo below). The galls can be seen both on acorns in the tree canopy and on the ground when these drop in later autumn. The galls are distinctive, though there is a small possibility of confusion with acorn cup galls caused by Andricus grossulariae.