One of the most recognisable galls (on oak, at least) is the knopper gall, caused by the cynipid gall wasp Andricus quercuscalicis, and these galls have started to appear on campus (Figs 1-3) on acorns of Quercus robur. Similar to Cynips divisa I mentioned earlier, the knopper gall is caused by the asexual generation of the wasp. The larvae develop their knopper gall through the summer, the galled acorn falls to the ground in the autumn and the adult females emerge in the spring (in the UK about 20-30% don’t emerge during the first spring, but may emerge 1-3 years later). These seek out Turkey oaks (Q. cerris), on which they eggs on the male catkins, forming small conical galls. Males and females emerge from these galls, mate, and the females fly to Q. robur and lay eggs in the newly pollinated flowers, carefully placing each egg between the developing acorn and its cup. Each knopper gall contains one larva, but more than one gall can be formed on an acorn.
Andricus quercuscalicis is not a native species to the UK, being first recorded in southern England in 1958. It has gradually spread throughout England and in the last 30 years has become quite common in some areas. Indeed, it has spread throughout Europe during the last 300 years from its native SE Europe as Turkey oak became commonly planted as an ornamental, but it did take a long time to cross the English Channel, as Q. cerris was first planted in England in 1735. The knopper gall is less common in northern England, and has not yet been recorded from Scotland, probably due in part to Q. robur being much less common here, and the commoner Q. petraea, not being galled by this wasp. The robur x petraea hybrid is said to be sometimes galled, and thus if you see a galled petraea-like oak, it may be an indication of a hybrid, but without further info we should be careful as this sort of reasoning can become a circular argument.
Unlike many galls, each knopper gall is a different shape (Fig. 4): each galled acorn has a tiny artist-in-residence producing a unique sculpture; the ultimate ‘installation’. If we slice through a developing gall we can see some of its structure (Fig. 5). The cut surface quickly turns brown (the gall contains about 50% tannin), but before this happens we can make out the tiny hole in which the white grub lives, surrounded by white, fleshy nutritive cells, parenchyma with starch grains and vacuoles.
As the gall develops these are continually replaced as they are eaten; when the gall is fully grown the larva will eat all of these cells, out to the woody shell which will develop around the inner gall. The rest of the gall makes up the outer cortex, which contains vascular bundles linking to the plant’s vascular system. Although widely different in shape, other cynipid galls have a similar structure, and in all cases the inner gall is about 2-3mm diameter.
Quercus robur is probably host to the greatest number of galling species in the UK: galls are appearing all the time, for instance it looks like new galls are starting to appear on these oak leaves on campus (Fig. 6), I wonder what they will turn into?