This year has seen a spectacular crop of Knopper galls on my oak trees, so numerous that it was almost impossible to find a normal acorn. As the acorns started to develop during July it became clear that that something was amiss. Little green frills appeared around the edges of the acorn cup and quickly grew into the strange misshapen gall caused by a small gall wasp (Andricus quercuscalicis). There was generally one per acorn but sometimes two or three grew in the same acorn cup. The galls were green or bronze in colour and were covered in a sticky resinous substance; this dried up as they matured and became a fawn / orange colour.

Galls plucked from the tree every three or four days were cut through to follow the development of the wasp. The larva was at first embedded in green tissue at the base of the gall; as it grew the tissue around it thickened, eventually making a thick walled spherical capsule. The galls started to fall from the tree during September and had become so hard and woody that a kitchen knife and mallet were needed to cut them in half. The tissue had dried and pulled away from the capsule which lay free in the cavity. Breaking open the capsule exposed a white pupa, or more frequently the imago wasp, fully pigmented.

Photo by Michael Kirby

To estimate the number of galls on the tree, the canopy was treated as a hemisphere. Measurements of the radius of the crown and the number of galls per square metre yielded an estimate of 91,000 galls. This involved a number of assumptions, for example, that the galls are evenly distributed all over the canopy, justified as far as possible with binoculars. Even if this number is out by 100%, 40,000 galls per tree is a very large number. Many trees in Westleton parish are equally severely infected, suggesting astronomical numbers of knopper galls in the area. The gall is produced by the agamic generation of the wasp, the alternate, sexual generation of which is produced in bud galls on Turkey oak (Quercus cerris). The nearest Turkey oak trees that have been located so far are in Dunwich Forest, about 2 km away. The mind boggles at the thought of hordes of the tiny wasp migrating across the open fields in the spring.

The life cycle of the Knopper gall wasp is similar to that of the marble gall which also occurs on Quercus robur and the alternate host of which is also the Turkey oak. In some cases, where marble galls occurred there were no Turkey oaks in the vicinity and it was suggested that the life cycle had adapted so that the sexual generation was omitted and the parthenogenetic females emerging from the marble gall were able to lay their eggs on the buds of Q. robur (Redfern & Askew, 1998). Such a change in the Knopper gall wasp life cycle might explain the very high incidence of Knopper galls in trees remote from turkey oaks, but direct evidence for such a change is lacking.

Depending on one’s point of view, the Knopper gall wasp is either a threat to our oak trees or a fascinating creature worth more study.

Reference
Redfern, M. & Askew, R (1998). Plant Galls. Richmond Publishing, Slough.