Wildlife: Bizarre variety of ‘galls’ on oak trees that affects leaves

The particular one shown in this picture is the ‘silk button spangle gall’ and caused by a tiny wasp called a ‘gall wasp’ (scientific name Neuroterus numismalis). This is diminutive wasp forms chemically-induced leaf galls on oak trees. At present, these are super-abundant on ancient oak trees in hedgerows in the Derbyshire Moss Valley. Gall wasps are sometimes misnamed ‘gall flies’ which they are not. These insects are actually tiny members of the hymenopteran family the Cynipidae that is part of the wasp superfamily Cynipoidea. Their common name derives from the galls they induce on plants and that are vital for their developing larvae. There are over 1,300 known species worldwide and they range in size from 1 mm to a ginormous 8 mm in length. The gall is somehow induced by the egg-laying of the female wasp to form a protective and nutritional tissue around the developing larva.

The growing juvenile will feed for some months on this materials produced by the host tree. However, the situation can be quite complicated in that the gall wasp is parasitic on the oak tree; but other parasitic wasps have evolved which lay their own eggs though the tough layers of the gall.

These are thus able to parasitize the gall-former. Yet even more bizarre is that there are further wasps developed to lay their eggs on the parasite of the parasitic gall-former in a process known as hyper-parasitism. To complicate matters further, these little wasps often produce young through two processes, one a sexual generation and the other known as parthenogenesis being asexual. Effectively the female buds off her offspring without mating similar to many greenfly for example. The gall wasp pictured has both bisexual and parthenogenetic generations and these produce two distinct types of gall on oak leaves. These are called ‘silk button galls’ and ‘blister galls’ and can have over a thousand per leaf.

Professor Ian D. Rotherham, researcher, writer & broadcaster on wildlife & environmental issues, is contactable on [email protected] and @IanThewildside on Twitter.