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Biorhiza pallida root galls

The root gall cluster shown below was found inside a stag beetle nest box, an extraordinary coincidence.
These root galls are incredibly difficult to find and many people have failed entirely. If you come across any, then Hege Vårdal would like to hear from you.

Photo taken on 22 March 2006, by Maria Fremlin.

Photo taken in Highwoods Park, Colchester, 22 March 2006, by Maria Fremlin.

Initially we were very puzzled by this unexpected find; we simply didn't know what it could be. Finally this was solved when Dr Jerry Bowdrey, the curator of Colchester Natural History Museum, a gall expert, identified them.
Note how the root isn't growing past the galls. Interesting, isn't it?
Since then I've learned that the oak apple gall wasp Biorhiza pallida has a very interesting life cycle. Male and female wasps emerge from the galls on oak branches in June-July, they mate, and afterwords the females burrow into the ground and lay their eggs on fine oak roots. The galls that develop in these roots are of females only, which upon emergence in the spring will climb up the tree trunk and lay eggs parthenogenically on the fresh oak twigs; these females are wingless. Each of these twig galls have inside many male and female larvae, which will pupate inside it prior to emerging.

Questions:
1 - In the photo the root doesn't seem to be growing past the galls. Why?
2 - What is the evolutionary advantage for the wasps of having alternate generations?

As a result I've become fascinated by galls and again I've managed to come across another extraordinary thing: I have found Urophora cardui on goldenrod in north America. This thistle fly has been introduced in to control the thistles. If anybody wants to look deeper into this, do get in touch with me, please.

Links:
Biorhiza pallida (Olivier 1791), Encyclopedia of Life.
An illustrated description of Biorhiza pallida life cycle
Hainault Forest Website - Illustrations of the many galls hosted by the Pedunculate Oak Quercus robur.

Further reading:
Plant Galls - by Margaret Redfern and RR Askew, 1998. Series: Naturalists' Handbooks 17. Richmond Publishing.

Last modified: Thu Mar 10 12:02:43 GMT 2011

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