Andricus quercuscalicis

Andricus quercuscalicis has 2 generations per year. The first of which is sexual, whereas the second is agamic (all female, and needs no male to reproduce). It is the second agamic generation that gives the wasp its common english name as the causer of the Knopper gall. This wasp also needs two species of Oak in which to breed. The sexual Gall is found on the buds of the Turkey Oak (Quercus cerris), whereas the agamic galls are found in West and Central Europe at the buds of various species of Quercus including the Common oak (Quercus robur), as well as Q. petraea, Q. pubescens, Q. frainetto, Q. virgiliana. The sexual generation was previously named A. cerri before it was realized to be two generations of the same species.

The mature sexual gall is common and can be found in the buds or catkins of the Turkey oak (Quercus cerris). It is bud shaped and easily missed or can be confused for other bud galls on the tree. In general it is pale, bright gold in colour, with a finely reticulated surface, slender and pointed in shape, and more or less covered by a golden brown slightly hairy husk. It measures from 1.7x1mm to 2.5x1.4mm and contains a single wasp, but there may be several galls per bud. It matures in early april and the adults fly april to june when they mate and lay there eggs in the common oak acorn buds.

The female Andricus quercuscalicis has a black, heavily reticulated head with dark brown medium sized eyes and chestnut ocelli. Its antennae have 13 segments and are a translucent gold colour except for the last 4 tip segments which are darker.
The hunched thorax is black and glossy with yellow brown tegulae. The wings are clear with yellow brown hairs and veins and the radial cell is open. The legs are translucent apart from darker tarsi and coxae tips.
The segmented gaster (abdomen) is round and fat with out a ploughshare, black and glossy with a faint yellow surround to the ventral spine. The whole wasp measures 1.2-1.5mm head and body length.

The male Andricus quercuscalicis has a black heavily reticulated head, like the female however its eyes are black and the ocelli are straw coloured. The 14 segmented antennae are translucent gold with the last 3/4 segments a darker neutral brown and a dark scape.
The thorax is black and reticulated with dirty gold coloured tegulae, with dirty yellow legs and darker coxae. The wings, as with the female, are clear with yellow brown hairs and veins and the radial cell is open.
The black, glossy gaster is small, pointed and hairless, with a single main segment. The head and body measurements are from 1-1.5mm.

Hyper parasites of the sexual galls are; Aprostocetus aethiops, Mesopolobus dubious, M. fuscipes, M. tibialis, M. xanthocerus and Pediobus clita

Knopper galls are very common in localised areas where turkey oaks are present along side common oaks. They grow on average to about 20mm across, although occasionally some very large examples can be found. They are seen on the trees from May to October and are either green or red in colour. They are of irregular shape (as can be seen on the accompanying photos), and are covered by a sticky resin. The galls themselves contain a large inner cavity with a smaller inner gall containing one wasp. When they mature in september they start to brown, lose the sticky substance from around it and will fall off the tree just before or along with the acorns. The wasp will sometimes emerge in october to december but more usually in the following year from january to april. If conditions are not correct then they may remain in the gall for another 12 months or so.

The agamic generation of Andricus quercuscalicis is considerably larger at 4.5-5.4mm in length. The head is orange with black vertex and back as well as around the mouth and with masses of pale hairs. The mandibles are also shiny black with touches of orange at the base and orange appendages. The eyes are small and neutral brown with chestnut ocelli. The 14 antennae segments are black with a swollen scape and orange base.
The thorax is striped black and orange with a dense covering of hairs. The wings are clear except for a bleeding of the viens around the areolet, and with dark brown veins and hairs. The radial cell is open. The coxae are dark, fading to orange and a distinctively striped femora, dark on top and orange below.
The bulky, segmented gaster is black with some orange at the bottom of segments and masses of pale hairs in bands at the edge of the segments. The hypogium is long and slender with sparse short hairs.

More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.

top of page