WHAT A KNOPPER!

Under one of the veteran pedunculate oaks on one of our Midlands sites these curiously deformed acorns with “growths” on them were found recently. They have become common on Quercus robur in Britain and are called knopper galls (after either old English or old German words knop or knoppe), but before the 1950s they were unknown here, having arrived from continental Europe around that time.

Knopper galls are caused by a tiny cynipid gall wasp (inset – not to scale) called Andricus quercuscalicis (Burgsdorf, 1783) which lays its eggs in the oak buds in spring or early summer. This triggers chemically-induced tissue changes to the acorns so that these distinctive growths, which contain chambers in which the wasp larvae develop, are formed by the tree. Affected acorns are infertile and there were once concerns that pedunculate oak populations may be seriously reduced by these wasps, but that does not seem to have happened.

The life cycle of this gall wasp is complicated – it is in two phases with two generations, one being female only (parthenogenesis) and the other sexual. It is the parthenogenetic generation that induces the knopper galls on pedunculate oak; the sexual generation uses Turkey oak Quercus cerris catkins. Both species are therefore needed for the wasp’s life cycle. Oaks in Britain are host to dozens of species of gall wasp, the “oak apple” being one of the most familiar.

The knopper galls are bright green and sticky when they first appear but turn brown and woody, as in my photo, by the autumn. Inside the galls’ chambers, the wasp larvae feed up on the tissue and then pupate over winter to emerge the following spring. This will be the sexual generation.

Recording tiny insects with complex lifestyles is a significant challenge for Betts Ecology, not least because their entomology is such a specialised area of knowledge. Nonetheless, we don’t shy away from such work and we continue to build up records and gather information so that our specialist site knowledge increases and helps us measure biological diversity and the net biodiversity gains we are all working for on our precious greenspaces.

© Betts Ecology