November 2014

There are 3 species of deciduous oak native to the British Isles – the English or pedunculate oak, the sessile oak, and the turkey oak.  Telling them apart from their leaf shapes alone is difficult, and the most reliable way of separating them is to look at their acorns.  The English oak and the sessile oak have similar acorns, but the English oak acorn has a stalk, whereas the sessile oak acorn doesn’t.  (The word sessile actually means stalkless.)  The turkey oak, on the other hand, has a very distinctive acorn – it has a furry or spiky cup.  If you find yourself face down on the lawn outside the Royal Oak, you may well see such spiky acorn cups.  This shows that whatever its regal connections (I think John Taylor wrote an article a couple of years ago regarding those), the tree outside the Royal Oak  is actually a turkey oak.

Turkey Oak Acorn (Quercus cerris)

Turkey oaks were introduced to the British Isles in 1735, so their ‘native’ status is questionable, but they have a connection with something introduced much more recently.  They play host to a gall wasp that introduced itself from continental Europe around 1950 via the Channel Islands.  Gall wasps, as you probably know, use parts of plants in which to lay their eggs so that when the larvae hatch out, they have a ready source of food and some protection from predators.  Often, the presence of the eggs or larvae, chemically alters the plant’s growth and a gall is formed, which is completely different to the normal form of the plant.  Oak marble galls are probably the most familiar type of gall, they are hard and woody and grow directly on the twigs of the tree.  One of the gall wasps that the turkey oak plays host to (Andricus quercuscalicis) has a relatively complex life cycle.  The female wasp lays her eggs in the male catkins of the turkey oak.  This produces a quite small conical gall that would not be noticed unless you happened to be climbing the tree and specifically looking for it.  But from this gall emerges either the male or the female wasp.  When the males and females have mated, the female wasp has no further interest in the turkey oak.  Her tree of choice is the English oak.  If she can’t find one, then she will settle for a sessile oak, but the English oak seems to be the first choice.  She will then lay her eggs in the developing acorn buds of that tree.  As the acorn grows, it becomes chemically altered by the presence of the wasp egg and larva, and it grows into the most bizarre

Knopper Gall on English Oak

Knopper Gall on English Oak (Quercus robur)

shape, looking more like a piece of popcorn than an acorn.  This type of gall is called a knopper gall – allegedly named after a type of German hat called a knoppe.  On a walk through Hastings Country park recently, we came across a tree that was so infested with knopper galls that there were no normal acorns visible.

The galls fall off the tree in autumn and the following spring, the gall wasps emerge from the knopper galls, but they are all female wasps.  Not only are they female but they are parthenogenic.  This means that they can lay fertile eggs without the help or hindrance of the male.  So they go off and lay their eggs – not on the English oak this time, but on the male catkins of the turkey oak.  And so the process continues with alternate parthenogenic and sexual generations on alternate English and turkey oaks. 

Apparently it is not uncommon for the gall wasps that parasitize oaks to have alternate sexual and parthenogenic generations.   Just what evolutionary pressure has led these gall wasps to lead this strange two-stage lifestyle is unclear, but perhaps it has something to do with other insects that fertilize the oak catkins.  When you start to look at the ‘birds and bees’ in detail, it quickly gets complicated and fascinating – but then isn’t that true of all the natural world? 

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.