The spread of Knopper gall wasps (Andricus quercuscalicis) into the Clyde area
Norman & Pearl Tait
11 Rosshall Place, Renfrew, PA4 0BA.
The Knopper gall wasp (Andricus quercuscalicis) Burgsdorf 1783 (Hymenoptera: Cynipidae) has invaded western and northern Europe from southern and eastern Europe over the last 400 years. In the late 1950s the insect arrived in the south of England, presumably having crossed the English Channel on high altitude air currents, and has slowly spread northwards reaching southern Scotland in 1995. This species of gall wasp has an obligate sexual generation in springtime on Turkey oak (Quercus cerris) and an agamic generation in autumn on Q. robur. The random distribution of Turkey oaks in Scotland will limit further northward expansion. This paper describes the Knopper gall wasp’s advance into suitable sites in west and central Scotland.
The complex lifecycle of the Knopper gall wasp Andricus quercuscalicis Burgsdorf 1783 (Hymenoptera: Cynipidae) involves an annual obligate alternation between two different oak tree hosts, one introduced to Britain and the other a native species.
Fig 1 The lifecycle of the Knopper gall wasp Andricus quercuscalisis.
In spring asexual females oviposit into the male flowers of the introduced Turkey oak Quercus cerris (L.). The hatching larvae induce tiny, thin-walled, flask-shaped galls to develop on the catkins in April and May giving rise to a sexual generation. Males and females of the sexual generation emerge in late May to early June of the same year. The sexual females then oviposit into the female flowers of the native Pedunculate oak Quercus robur (L.) or occasionally Sessile oak Q. petraea, (L.) where an asexual or agamic generation develops, producing galls on the acorns in the summer and autumn (Stone, 1995). These large, 2cm galls are very conspicuous and consist of a mass of pyramidal-shaped, ridged tissue that breaks out between the cup and the acorn. Occasionally there are two, three or more growths on an individual acorn which completely cover it. The distinctive shape of these galls gave rise to the common name of ‘knopper’ from the German word knoppe meaning a kind of felt cap or helmet worn during the seventeenth century which the galls are said to resemble (Stone, pers. comm.). Russet-green, glabrous and sticky at first, the structures later become reddish coloured. After hardening, the galls turn brown and drop to the ground in late autumn. Inside the gall is a single, large chamber with a small, hard, thin-walled, spherical inner structure at the base containing one larva. The asexual generation overwinters in the hard knopper galls. At the beginning of February the adult asexual female gall wasp emerges through a vent at the top of the gall. These newly emerged insects later disperse to Turkey oak where the cycle is repeated.
The obligate generation on Turkey oak restricts the range of this invading species of Andricus in Britain to locations where both species of oak are to be found.
Fig 2 Fallen Knopper galls on the ground in autumn.
Fig 3 Newly emerged Knopper gall wasp on an overwintered gall
Fig 4 Turkey oak catkins where the Knopper gall wasp lays eggs in the flowers