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August 5, 2010

The Tale of the Wasp and the Weevil


Every insect has a fascinating tale to tell, and some have a sting in their tail...


Some are quite simple: from the pupa the insect emerges, they fly around a bit to find a mate, some might pause to lap up some nectar along the way, the males die off whilst the female lays her eggs and then too dies or maybe she will overwinter, emerging again the following spring. And there is a larval stage in there somewhere too.


This is a familiar metamorphic pattern followed with unerring regularity (aside from stochastical effects completely out of the insects, um, hands, so to speak!).


But to call an insect’s life-history simple is to do most of these fascinating creatures a disservice. There is one insect in particular which arrived on the wings of the postal service in to our inbox that is simply amazing! Well, I say insect, rather evidence of said insect!


And now for the challenge:


Any ideas?

No? At first, neither did we!


What arrived was a clear plastic measuring container with obvious insect debris inside collected by a lovely lady convinced that this was something to do with Colletes hederae (the Ivy bee). Her hypothesis was that this debris was the cocoon of the solitary bee (found in a cliff face), but what was the other ‘stuff’ found nearby? Looks like dead bodies, but Ivy bees are not predatory!


This somewhat macabre collection is the empty abdominal carcasses of weevils (minus heads and other appendages), in fact, the debris of a fierce predator.


The deflated cocoon type debris, is, …empty cocoons.


The predatory solitary Crabronid wasp (family Crabronidae) has left incriminating evidence behind!

And so the fascinating, macabre, and deadly tale of the wasp, Cerceris arenaria, reveals itself. See this link to BWARS to see C. arenaria in action


This species of Cerceris is one of the largest found in Britain (between 10-15mm). The wasp can be seen over dry sandy areas such as cliff faces from May onwards, where this evidence was found.

The cocoons or cells in the image are constructed inside a tunnel, up to 40cm.

The female then goes out hunting for weevils. Weevils are dive-bombed, then paralyzed by venom injected into the exposed soft tissue of the beetle. The cells are then provisioned with the paralyzed weevils, sometimes as many as 12 per cell. The wasp larvae then feed at their leisure from the internal organs of the tragic weevils.
This is actually quite a common species but few would know its terrible secret!