Posted by: scrubmuncher | June 1, 2010

A bit galling

What often bamboozles me about the natural world is the bizarre goings on just beneath the surface of something that otherwise looks rather normal. Take for example the gall in the photo below. Galls are formed by various organisms in the tissues of plants and they serve as a refuge as well as a ready supply of food. The gall below is formed by a small gall wasp – Andricus quercustozae, a European insect that stimulates these peculiar swellings on Pyrenean oak (Quercus pyrenaica).

The gall in question. These unusual growths, stimulated by the eggs and larvae of a gall wasp are common on Pyrenean oak (Ross Piper)

The hole in the gall is where the adult wasp emerged from its nursery to go and cause more unsightly growths on other trees; however, look a bit closer and this not just a vacant gall. Fortunately the gall’s structural integrity was not what it once was and it broke in two under the blade of a penknife. Inside was a bizarre scene frozen in time. Following the exit of its maker the gall had become a nest for a small solitary bee, which had enlarged the gall wasp’s exit hole and hollowed out much of the interior to form a cosy brood chamber for its young. You’ll know by now there are few happy endings in nature and the bee’s nesting activities had attracted the attention of a cuckoo wasp – small, beautifully coloured insects – the real cads of the hymenopteran world. Many solitary bees and wasps work their little tails off to make nests and stock them with food to give their offspring a fighting chance in the world. Cuckoo wasps make a living by taking advantage of these hard workers. They come along, bold as you like, and lay their own eggs in the nest of the diligent bee or wasp. The larvae of the interloper feed on the eggs and larvae of the nest maker before making short work of the provisions. To us, with our sense of right and wrong, this is heartbreaking and in this particular situation the cuckoo wasp may have received its comeuppance.

Here's the bee that made a nest in the gall following the departure of the gall-maker. It's dead, not just posing for the camera. At the centre of the gall is what remains of the bee's brood. The small cavities at the edge of the gall are pupal chambers of parasitoids, two of which can be seen below (Ross Piper)

Here's the dastardly cuckoo bee that punished the bee that was nesting in the gall. Again, it has long since stopped being extant (Ross Piper)

It seems both the bee and cuckoo wasp reached the end of their natural lives or the nest was infested by a fungus and the whole microcosm with the bee nestled just inside the entrance to its nursery and the cuckoo wasp on the far side of the brood chamber has been frozen in time. I kept the gall in the hope that some other things would emerge – creatures that had played a part in this miniature drama and had survived the winter. After a few days some things did emerge – these incredible torymid wasps (a male and female of the same species – see below). As a group, these handsome, yet tiny insects (3-4mm long) are parasitoids of many other insects, but it seems the species here was not attacking the gall former. For the sake of justice and decency I hope it was giving it to the cuckoo wasp, but it’s just as likely the law-abiding bee was receiving a double-helping of punishment. This I have yet to find out.

The female torymid. Still not sure was this was doing in the gall. I hope it was punishing the cuckoo bee. The long curved thing is her ovipositor and it is this she uses to drill through woody tissue to deposit her eggs on her quarry (Ross Piper)

The male torymid wasp. Same as the previous photo, but lacking the ovipositor, naturally (Ross Piper)

All of this – the bee frantically building and provisioning a nest, the depredations of the cuckoo wasp and the mysterious menace of the torymid wasps – took place in a woody orb no more than 3cms across that had grown on a Pyrenean oak on a hillside in northern Spain.  If that’s not the magic of nature right there I don’t know what is.


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