Oak Marble gall wasp Andricus kollari

Andricus kollari has 2 generations per year. The first of which is sexual, whereas the second is agamic (all female, and needs no male to reproduce). 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. In Spain and North Europe they develop also on Q. pyrenaica, Q. faginea, Q. fruticosa, and Q. canariensis.

The sexual generation was previously named A. circulans before it was realized to be two generations of the same species. The sexual gall is easily confused with some of the other bud galls from the Andricus family and is first visible in late January to early February on the Turkey Oak. It has the appearance of an apple green bud with a pinkish flush along one side of the gall. This rarely covers the gall and can even occasionally appear deep purple. As the gall grows and matures the flush reduces towards the tip and tends to turn black. It reduces to the point of becoming a tiny black spot at the tip or even vanishing completely when the gall is mature.

Gall maturity tends to occurr at the end of March. The mature gall tends to be about 2x1mm to 3x1.5mm in size and there are often several galls per bud. The shape is rather bud shaped for want of a better description, however when there are
several together they can be misshapen to the point of being banana shaped in some cases. Each gall has a single cell, which produces one wasp either male or female.

The colour of the mature gall itself, is yellow or yellowy brown with small tufts of hair sometimes found on the upper parts. They can however, appear varied in colour, with darker patches from the remains of the bud scales. The tip often has dark or black markings and a raised ridge runs up many of them.

The wasp stays in the gall to pupate and will exit from March to June. Parasites that may kill and feed on the wasp and may exit instead of the Andricus kollari consist of 4 Pteromalids from the Mesopolobus genus. These wasps are ; M. dubious, M. fuscipes, M. tibialis and M. xanthocerus.

The male wasp is very dark brown to black on the head and hunched thorax, with a black gaster (abdomen), The legs are yellowy brown and the antennae are a translucent yellow or pale brown colour and consist of 14 segments. The head and body length varies from 1.5 to 2mm.

The female has a dark brown head and thorax, which again is hunched, but has a dark brown gaster which does not have the typical Cynipid ploughshare. The legs are tapered brown/yellow. The dirty brown antennae comprise of 13 segments. The head and body measurements are similar to the males at 1.5-2mm.

After exiting the bud galls the adults will mate and then the female will seek out a common oak (Quercus robur) and lay her eggs in the bud where the agamic generation galls are then formed. These agamic galls can generally be seen from June onwards. The gall is smooth and round, green in colour and very soft to begin with, and will grow upto 20mm across, although some may be larger. There may be lumps or bumps, or even slightly mishapen ones which is often caused by inquilines around the edges of the centre chamber. There may also be scarring on the outside which is sometimes caused by aphids feeding off the gall as it grows. Occasionally two galls growing very close together may combine to form a double gall in effect. Occasionally they will not grow very large and this could well be a sign of paratisation although I have had several fully grown A. kollari from small galls.

The larva that lives within is white and legless and infact resembles a very typical cynipid larval form. The internal structure of this gall consists of the smooth exterior surrounding a spongelike interior, which in turn sometimes surrounds several air chambers with the larval chamber in the centre, where the larva feeds and grows. The air pockets are believed to possibly be a way of preventing parasites such as Torymidae by fooling them into thinking that when they break through into the air pocket they are in the central chamber and laying there egg in a sealed off empty space, not on the larvae itself. However the mltichambers that are in the picture below are the effects of inquilines of the Synergus genus in the centre where they have squeezed out the original occupant and created 6 segments. To the right is the exit hole and just above that is another chamber from a different Cynipidea inquiline.

As the gall matures the outer surface turns from green to brown and hardens to a very dense structure, making it very hard. This usually occurs in August and September and this is when the wasp larva inside will pupate prior to exiting. It then chews a tunnel out from the centre to the outside and after cleaning itself will then fly off in search of a Turkey oak to start the whole process again. In some cases though the wasp will not emerge in the first year, and will infact wait for the 2nd or even upto the 4th year before emerging. Old galls that have been exited can often be seen still on the tress for some years after, complete with circular exit holes. The larger ones are generally from the gall creater wereas the smaller ones tend to be from parasites or inquilines. In fact it is possible to get a rough guess of the ratios of successful A. kollari, if there is one large exit hole, or one large exit hole plus several smaller holes, then these would be inquilines. If there are smaller holes but no large hole then the gall was probably parasitised. It is very difficult however to differentiate from inquiline and parasite holes so the ratios between them would be difficult to document.

The adult is very similar to some of the other Andricus sp and coloured yellowy brown or chestnut biege with an orangey brown gaster (abdomen) and a black or dark brown triangle area at the top front of the gaster. It also has bands of hairs on the edges of the segments. The size is an average of c. 5mm and the number of antennal segments is 13.

Common inquilines species found in marble galls include;
Ceroptres arator, Saphonecrus connatus, Synergus albipes, S. gallaepomiformis, S. pallicornis, S. pallidipennis, S. reinhardi, S. umbraculus

Parasite species of marble galls include;
Eurytoma brunniventris, Sycophila biguttata, S. variegata, Megastigmus dorsalis, M. stigmatizans, Torymus auratus (=nitens), T. geranii, Ormyrus nitidulus, Caenacis lauta, Cecidostiba fungosa, C. semifascia, Hobbya stenonota, Mesopolobus amaenus, M. dubius, M. fasciiventris, M. fuscipes, M. sericeus, M. xanthotherus, Eupelmus annulatus, E. urozonus, and Aulogymnus trilineatus.

Other associated species include;
Brachymeria rugulosa, Aulogymnus gallarum, Baryscapus berhidanus, Eupelmus vesicularis, Eurytoma pistaciae, Sycophila iracemae, Sycophila submutica, Caenacis inflexa, Cecidostiba adana, Cyrtoptyx robustus, Eumacepolus obscurior, Mesopolobus tibialis, Adontomerus crassipes, Torymus angelicae, Torymus calcaratus, Torymus cingulatus, Torymus erucarum, Torymus nobilis.

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