Last year was a ‘good’ year for oak leaf galls, at
least on some trees. On one tree examined, the lower surfaces of nearly all
leaves within reach were covered, mostly with common spangle galls but
some with silk button spangle galls. 
These are caused by gall wasps Neuroterus
quercusbaccarum and N. albipes (Hymenoptera: Cynipidae) respectively.
It must be one of the most interesting problems of insect and plant physiology
to discover how the interaction between the host tissue, the oak leaf, and the
egg and/or the larva of the gall wasp can organise the morphogenesis of leaf
cells to form these beautiful and regular structures.
On close examination of the common spangle galls it became obvious that the gall wasp larvae were not alone. Many of the galls were malformed, and lifting up the flange of a gall revealed a single, small yellow maggot. On one leaf there were 35 galls of which 28 had a maggot, ranging in size from 1.5 to 2 mm. This appeared to be an inquiline, feeding on the tissue of the gall as evidenced by damage to smooth under-surface. Mr. J. P. Bowdrey, the SNS gall recorder referred me to a short paper in Cecidology (Robbins 1997), describing similar yellow larvae which were found in Warwickshire first in 1995, on both N. quercusbaccarum and N. albipes . galls and which were identified as Xenodiplosis laeviusculi (Diptera: Cecidomyiidae).
Several
gall leaves were put on a layer of damp sand in plastic pots and covered with
fine mesh. These were left out outside and looked at regularly. No wasps have
been seen but on 19 May a number of small pale coloured winged insects were
seen crawling on the underside of the mesh. Some escaped, 
but three were
trapped and examined. They were clearly Diptera, but their characteristics did
not check with those of gall midges, so they were not X. laeviusculi. In
the absence of an SNS Diptera recorder, identification awaits access to a
detailed text, which may reveal if they also were associated with the galled
leaves.
As the oak tree that had borne spangle galls in 2003 came into flower this spring it was seen have a good crop of currant galls (photo). These are also produced by the gall wasp N. quercusbaccarum, but in the sexual phase of the alternating generation. Like the spangle gall the currant gall with its perfectly spherical shape and delicate colouring arises from the interaction between the tissues of the host stamen and a putative morphogen from the egg or larva. The evolution of such a complex life cycle and finely tuned morphogenesis is something to marvel at.
The currant galls were fully developed and beginning to fall from the tree in the last two weeks of May 2004. At this stage some exit holes (c. 1 mm) were found. Other galls, picked from the tree were examined by cutting off a small slice, leaving the central chamber relatively undamaged. A proportion of the galls contained a single wasp, ranging in development stage from a pale coloured pupa to black imago, about to emerge. A large proportion, however, contained a parasitoid in addition to the wasp. When freshly exposed, the maggot-like parasitoid larva was observed feeding on the wasp, usually in its thorax region (photo.). Two chambers had no sign of a wasp larva or pupa, dead or alive - only an unidentified larva, presumably a lethal inquiline that had smothered the original owner.
This study started from casual examination of a leaf with spangle galls, but progressively became more interesting as the complexity of the relationships centred around one species of gall wasp became apparent - and this is only one of the many different gall-formers that affect the oak tree!
References
Robbins, J. (1997) Xenodiplosis laeviusculi (Rüibsaamen) (Diptera: Cecidomyiidae) on spangle galls of oaks Cecidology 12(1), 11.
A general text for galls of all kinds is:
Redfern,
Margaret & Askew R.R. (1998) Plant galls. Naturalists’ Handbooks 17 The Richmond Publishing Co. Ltd Slough
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Common spangle galls, one affected by inquiline: photo Michael Kirby |
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Larva of inquiline (possibly Xenodiplosis laeviusculi) under flange of a spangle gall. The stalk of the gall which attaches it to the leaf is just above the larva. |
© 2004 Suffolk Naturalists' Society
used by kind permision.