What's in a gall? Part 2: home-makers

Regular readers may have seen last month's musings on the contents of a knopper gall of Andricus quercuscalicis. The focus was on the structure of the gall and the presence of the small parasitic wasp Cecidostiba fungosa - this time, a relatively brief look at the gall-causer itself.

A quick pictorial reminder of C. fungosa - a little parasitic green jewel.

I kept the gall to see if any other species emerged, and after a couple more weeks, two individuals of the galling species itself, A. quercuscalicis, emerged - adding to the seven C. fungosa already seen.

A. quercuscalicis

A. quercuscalcicis isn't the metallic green of its parasite - more a dark brown, but interesting nonetheless. The photo above shows the overall form with paler yellow-brown legs and antennae - the latter clearly showing their simple filamentous shape and curvature. The abdomen is clearly rounded and shiny and even in this picture some hints of sculpturing and bristles can be seen.

A. quercuscalicis - close-up of the front half.

Zooming in, the thorax and head clearly bear small pale bristles as well as sculpturing in the form of pits and wrinkles, and the tiny bulbuos base of the forewing is also visible. Just behind the wings (the bottom centre of the photo), there is another clear patch of pale bristles. As ever, such seemingly unassuming invertebrates provide a wealth of visual detail.

However, already knowing the identity of this species as it emerged from a gall in a sealed container, I do not wish to dwell on the finer points of diagnostic morphology (not this time anyway...); instead I'd like to look at the biology of the Knopper Gall. Knoppers are asexual galls and (despite only arriving here in the 1950s) are common in Britain, mainly on pedunculate oak Quercus robur and rarely on sessile oak Q. petraea. Fresh knoppers are found on acorns in summer and autumn, starting green and sticky but becoming brown and woody (like the one illustrated below). The thick wall contains an air space with an ovoid inner gall inside.

An old, woody knopper gall, around 2cm across

The sexual galls of this specis are very different and are found in spring on the male catkins of Turkey oak Q. cerris. Unlike the knoppers, these are tiny (1-2.5mm long) and thin-walled; each one replaces a small number of anthers in the catkin, and a catkin may bear up to around 20 such galls. The galls prolong the life of the catkin slightly as it usually lasts around 12 days while the gall requires 14 days to develop (other gallers may extent catkin life considerably more, by up to three months).

In May the sexual males emerge and wait for females to chew their way out - the males fight to secure the best mating locations. Mated females then fly to Q. robur trees and lay eggs (in late May) into newly pollinated female flowers; each egg is positioned precisely between the developing acorn and cup. In September, the galls drop to the ground (before ungalled acorns) and overwinter, along with the larvae inside, within the leaf-litter. Most (in Britain, 70-80%) of asexual females emerge the following February or March, the remainder staying in the knopper for up to three years and joining a different cohort. In either case, they then fly to Q. cerris trees to lay eggs in male flower buds and begin the life cycle once more.


To finish (for now), I have retained the knoppers and will be interested to see if anything else emerges although it seems unlikely unless further adults remain. So, there may be a third instalment covering internal knopper structure and possibly more inhabitants. I would also like to recommend the following book which has recently been published - it is excellent and was the source for the life cycle summary given above:

Redfern, M. (2011). Plant Galls. Collins, London. [vol. 117 in the New Naturalists series]