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Like many Cynapidae species, Biorhiza pallida has 2 generations in its life cycle. One being sexual and the other is agamic (females who can reproduce without mating). Both stages of B. pallida take place on common oaks, the more commonly seen "Oak apple gall" is the sexual generation on the buds and the agamic generation root gall is found on the roots of the tree.
Oak apple galls are very noticable, large galls that can be 50mm across. They start forming on terminal and lateral buds in may and are cream with bright pink and red with a soft spongy texture. As they mature towards the end of june, they turn light brown and loose their spongy texture to become very papery. The adult wasps emerge july to september. The adult wasps may be confused with the smaller Andricus quercusramuli.
The female Biorhiza pallida at about 2.1-3.4mm in length, has a translucent dark brown to a paler translucent gold brown with pale hairs. The eyes are quite large and prominent
and dark brown in colour. The ocelli are chestnut. The antennae have 14 segments, are golden brown in colour.
The thorax is semi translucent yellow chestnut with darker panelling. The legs are translucent chestnut gold and hairy and the wings are clear with gold to dark brown veins and pale brown short hairs.
The gaster (abdomen) is golden brown to dark chestnut, swollen and smooth, not obviously segmented. The ventral spine is short with protuding hairs that go beyond the tip. The ovipositor sheath does not protrude.
The male, 2.1-3.5mm, has a dark chestnut yellow head that is slightly granulated with large pale eyes and chestnut ocelli. The 15 segmented antennae vary from mid brown to paler chestnut yellow.
The thorax is a semi translucent yellow chestnut with darker panelling as with the female. The legs are all translucent pale chestnut yellow and hairy. The wings are clear with prominent dark chestnut veins and hairs.
The gaster is narrow, deep and triangular, and a shiny chestnut yellow to rich chestnut.
Two inquilines are commonly found in oak apple galls; Synergus gallaepomiformis and S. umbraculus.
Hyperparasites, however, are more numerous and include; Aprostocetus aethiops, A. skianeuros, Baryscapus diaphantus, Cecidostiba fungosa, C. semifascia, Eupelmus urozonus, Eurytoma brunniventris, Hobbya stenonota, Megastigmus dorsalis, Mesopolobus amaenus, M. dubius, M. sericeus, M. tibialis, M. xanthocerus, Ormyrus pomaceus, Sycophila variegata, Torymus affinis, T. auratus (=nitens), T. flavipes and T. geranii.
The Root gall is found underground, on the rootlets of the oak tree. It is globular, upto 8-10mm, and has one large chamber which is inhabited by 1 gall wasp larva. The individual galls often coalesce into clusters 30mm across. The gall matures by october of the second year and the agamic wasps emerge from october to december of that year or march of the following 3rd year.
The 4.8-6.3mm long wingless adult agamic wasp has a small yellow brown head with slight sculpture. The eyes are medium sized, long and mid brown yellow, with small pale orange ocelli. The 15 segmented antennae are a bright mid brown with an orange tint.
The thorax is yellow brown to orange with the legs being all yellow brown and the wings are not present.
The gaster is large, fat, shining, segmented and yellow brown in colour.
The gall suffers from 2 hyperparasites, being; Torymus nobilis and T. roboris
More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.
My own Oak apple gall records 2008 (excel file)
Quick referance key for Pteromalids in oak apple galls (excel file). This key is only a quick referance key and should not be used alone in identifying species. Its use is just as a quick guide and other more deatiled texts, such as the one above, should be used to verify.
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