Thursday, 17 October 2013

Robin's Pincushion

Most country dwellers will be familiar with this structure, found on various species of wild rose. In many books it is referred to as the rose bedeguar gall. Whatever one chooses to call it, it is a most remarkable structure and has been the subject of much research. 

Robin's Pin Cushion. Byfield Pocket Park
16 October, 2013
The gall is induced by a tiny (3-4 mm long) cynipid wasp, Diplolepis rosae, and it begins to develop when the wasp lays its eggs in the tissues of a rose leaf bud. Up to 100 eggs may be deposited and, as the eggs develop, the rose is somehow stimulated to produce the extra tissues to form the gall. Just how the rose is induced to produce these tissues remains something of a mystery. Inside the gall the wasp larvae begin to develop, taking their nourishment from the surrounding plant cells. Sometime in the autumn, usually around late October, the larvae are full-grown. They have stored up considerable reserves of fat, and these will sustain the banana-shaped grubs through the winter. They spend the winter in a form of hibernation known as a diapause. Round about the beginning of May the grub goes through a final moult and a few days later the wasp - now an adult - chews its way to the outside world. 
A closer view of the same gall.

Extraordinary though all this is, the story of the gall now becomes far more complex. As it develops more insects, mostly other species of wasp, begin to move in. Some will feed on the plant tissues but others, known as parasitoids, will begin to feed on the original wasp grubs. Eventually an exceedingly complex community develops and so far 14 different species have been identified from these galls, with even more from the galls on the continental mainland. The precise way in which all these "inquiline" species interact is still far from clear and to explain even a little of what is going on would make this a mega-blog. I would commend a fascinating article by Simon Randolph in British Wildlife Vol 24, No 1.  Members of the British Plant Gall Society are helping to take research further

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