October 2014

Wasp damage on Cosmos

A neighbour of mine recently showed me some Cosmos stems that had linear scars along them.  When she told me that it was wasps that had done the damage, I wasn’t particularly surprised being familiar with their habit of chewing up any suitably soft or weathered wood.  I’ve often seen them on fence posts where they chew little grooves along the grain of the wood.  As far as I know, wasps are not into wood carving or marquetry, so what is going on here?  The simple explanation is, of course, that they are making papier maché.  This is not the papier maché that we made out of torn up newspaper and flour paste in school handicraft classes, but a much finer version made out of wood pulp and wasp spit.  Nor are they making model animals or lumpy boxes, but rather the beautiful, intricate and delicate structure of their nest, which includes both the many layered (and therefore insulated) outer shell and the honeycombs inside where the wasp grubs will incubate.

I don’t know what attracts them to Cosmos in particular, but I suspect that it not only provides the wood pulp for their nest, but also because the sap is not far beneath the surface, it also satisfies their sweet tooth.  It is their sweet tooth that also makes them such a nuisance at picnics, and it’s their nuisance value that makes many people want to destroy their nests at every opportunity, whether they are interfering with your gardening or not.  For good reasons why you shouldn’t do that, I can recommend an article written by Steve Backshall in the Daily Mail on that very subject.  And if you have an aversion to reading the Daily Mail, then it is on-line at http://www.dailymail.co.uk/sciencetech/article-2748653/Why-learn-love-WASPS-They-ve-vanished-summer-But-s-shouldn-t-celebrate-writes-STEVE-BACKSHALL.html (or better still just Google Steve Backshall Daily Mail Wasps.)

German Wasp chewing a garden chair

I mentioned the honeycomb structure above, but it has nothing to do with honey, as there is only one British native (introduced) species that stores honey – the honey bee (Apis mellifera).  I was recently asked by a friend how honey bees manage to build such a regular hexagonal structure.  I didn’t know, of course, but I did vaguely remember some research published in the last year or so that provided an explanation.  I found the article which was published as a news item in Nature magazine, as well as the full text of the research paper.  (See http://www.nature.com/news/how-honeycombs-can-build-themselves-1.13398 )  Basically, what happens is that each bee tries to build a circular cell of a particular diameter (6mm) as close as possible to the next bee doing the same thing.  As the cell grows they turn into a series of adjacent cylinders which are heated by the bodies of the wasps which partially melts the wax.  This allows surface tension to pull the wax into a basically hexagonal shape.  (You can show this by creating identically sized bubbles in soapy water, which then form perfectly regular hexagon-shaped bubbles on the surface.  The research team showed it by melting bunches of plastic drinking straws.)  The bees also knead and reinforce the wax to make sure that the walls don’t get too thin.  The result is the perfect hexagon pattern of the honeycomb which has rounded internal corners, no doubt for the comfort of the grubs.

This does leave some questions unanswered though – like how do the bees measure the circle in the first place?  I suspect that this has to do with the fact that all worker honey bees are roughly the same size and they will measure things like we did before we invented rulers and inches.  We had to use measures like cubits (the length of your forearm) and spans (the width of your outstretched hand).  It’s probably got something to do with the size of the bees head.

The next question is – how do wasps make their hexagons with papier maché which doesn’t melt like wax does?  Maybe it’s something to do with surface tension as the paper dries, but the truth is – I haven’t a clue.  It’s still marvellous though.