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?