Plant gall

Knopper gall

Knopper galls develop as a chemically induced distortion of growing acorns on Pedunculate Oak (Quercus robur) trees, caused by gall wasps which lay eggs within buds using their ovipositor. The gall thus produced can greatly reduce the fecundity of the oak host, making the gall a potentially more serious threat than those which develop upon leaves, buds, stems, etc. The Turkey Oak (Quercus cerris) introduced into Britain in 1735 is required for the completion of the life cycle of the gall. The Knopper is a recent introduction to the British Isles, first arriving in the 1960s and now found throughout England, Wales and as far north as Scotland; first occurring for example in 2007 at Eglinton Country Park in North Ayrshire.

The physical appearance of the gall

The gall growth appears as a mass of green to yellowish-green, ridged, plant tissue on the bud of the oak. If only a few grubs are developing within, then it may appear only as a group of bland folds. Where several grubs are competing for space the shape may become much more contorted, with several tightly bunched galls.

The word knopper derives from 'knop'; a small rounded protuberance, often decorative, such as a stud, a tassel or a knob.

Although normally distinctive the knopper gall can, under some growth conditions, be mistaken for the acorn cup gall, caused by the gall wasp Andricus grossulariae.

Life-cycle and arrival in Britain

Andricus quercuscalicis (Burgsdorf, 1783) is a small gall wasp which has a two phase life-cycle that requires both Pedunculate Oak and Turkey Oak (Quercus cerris). Therefore, as with most oak gall wasps, this species has alternate sexual and asexual (all female) generations. The sexual generation develops in spring in small conical galls that form on the male catkins of the Turkey Oak.

Woodway House gardens in Devon have both the required host species and indeed Woodway House was one of the first places in Devon to record and send off for research purposes specimens of both life-cycle stages of this invasive species. Long known in Europe, A. quercuscalicis came from the continent to Devon via the Channel Islands, the first recorded sighting being in 1962. It appears to have arrived naturally from the continent. In 1979, A. quercuscalicis underwent a population explosion in England and for a time there was concern that it would seriously affect acorn fertility and thus the future of England's most iconic tree; subsequently this has not proved to be the case and control is regarded to be unnecessary.

The abnormal acorns develop during summer and the acorn is either wholly or partially replaced by the gall growth. As previously stated, the knoppers become woody and brown in early autumn, after which they fall from the tree. The level of attack by the insect varies greatly from year to year.

Gall forming insects

Some herbivorous insects create their own microhabitats by forming usually highly distinctive plant structures called galls, composed of plant tissue but controlled by the insect. Galls act as both the habitat, and food sources for the maker of the gall. The interior of a gall, formed from the acorn, is composed of edible nutritious starch and other tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect with physical protection from predators.

Inquilines and parasitoids

A number insect inquilines live harmlessly within the knopper gall and some of these, as well as A. quercuscalicis itself, are parasitised by insects referred to as parasitoids.

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