Galling things

Some nerdy nature-table entomological material now.  

Galls are a fascinating subject (to me anyway). They are the kind of growths and excrescences on plants, especially trees and other woody ones usually caused by parasitical insects, though they can also come about through fungi or bacteria or occasionally other plants like mistletoe. Wiki describes how 

they are plant tissue which is controlled by the insect. Galls act as both the habitat and food source for the maker of the gall

Perhaps one of the best known is the oak apple. For a long time I only remember seeing these as the near-perfect woody spheres the size of large marbles with the tiny hole in one end, like the ones above - these are quite old, this tree had a number of them on its lower twigs at one time, now there are fewer. Calling them apples seemed rather strange, but later I discovered them as they are in spring, larger, spongy textured greenish and rosy, and they really do look quite like apples. They are caused by chemicals injected by the larva of certain kinds of wasp in the family Cynipidae, it seems.

Lately though I've noticed what I thought at first were some odd-shaped acorns on the ground where Molly and I walk on the avenue approaching the Château de Bogard.  Then there were some rather unusual large acorns in shaggy cups.

The oaks with the shaggy-cupped acorns turned out to be turkey oaks.  They're called this, it seems, not because they originated in Turkey but because turkeys liked to eat the acorns.  The weirdly shaped things were galls on regular, quercus robur pedunculate  'English' oak acorns.  They turned out to be the delightfully named Knopper galls; Wiki says

The word 'knopper' derives from the German word 'knoppe' meaning a kind of felt cap or helmet worn during the 17th-century; also a small rounded protuberance, often decorative, such as a stud, a tassel or a knob

These are an extraordinary phenomenon, the product of a small wasp, andricus quercuscalicis, which needs two generations to breed, during one of which it is all female and parthenogenetic, and during the other two-sexed.  But it also needs two breeds of oak, the turkey and the robur one, to complete this cycle.  It only parasitises the turkey oak catkins lightly in the first stage, but by attacking the acorns of the robur in the second it can threaten the fertility of that tree, so in some parts of the UK it's been mooted that the turkey oaks (introduced in the 18th century, the gall wasps appeared in the last 50 years) should be eradicated to benefit the indigenous species.  I don't know what the policy on them is here, this is the only place I've noticed turkey oaks growing in any numbers, or seen the galls, and the regular robur-type oaks around here don't seem to have any problems setting seedlings, we find them growing like weeds all over the garden, planted by field mice and voles, I assume.

The wasp lives the season inside the gall, then makes its way out through a hole it makes at the end, as you can see in the one above.  Apparently, the galls can also be a home to other fellow-traveller insects which also benefit from the micro-habitat they create, these kind of housemate creatures are known as inquilines, another new word I've learned.

The other gall-producing insect I've learned about I noticed the evidence of while foraging for beech mast, these tiny, pip-shaped growths on the fallen beech leaves:

They are made by a gall midge, mikiola fagi. This leaf had an unusually large collection of four galls, mostly there was just one, on very few leaves.  I thought they'd they'd be juicy and quite tender, and was slightly squeamish about squeezing one, in case the insect should pop out!  But in fact they were very hard, as hard as wood, which shows how successfully the insect is at converting and controlling the plant tissue for its own use.

Surprising what you can find kicking through the leaf litter, and kicking about on the internet too.

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