I always thought these brown, woody balls often found on oak trees were oak apples but I recently acquired a book on Britain's Plant Galls (Michael Chinery - ISBN 978-1903657-43-0) from the WildGuides series and I was surprised to discover they are actually the remains of the marble gall, formed by the larva of a small gall wasp (Andricus kollari).
Whilst the biology of these gall wasps is interesting it is also complicated and, if you are interested, then look elsewhere for the details; it would take too much to write it all here and, in any event, Michael Chinery knows far more about it than I do!
Sufficient to say that these balls form to provide a home for the wasp's larva. They are usually laid in small clusters and start out green gradually turning brown as the summer progresses. They become more visible, of course, when the leaves go from the tree. This species was introduced in the 19th century as a source of tannin for dyeing and ink. I didn't know that either, thanks Michael.
Oak Apple Gall: natural amazement
I write these nature notes as a part of my learning process about the natural world and I am constantly amazed at the intricate relationships that exist between species, in the case of most galls between insects and trees or flowers. Knowing nothing about oak apples (Biorhiza pallida) I turned to my reference book, "Britain's Plant Galls" by Michael Chinnery to find out how they come about. Here is a potted version of the incredible story.
The female gall wasp lays eggs in the leaf bud of an oak tree in early spring and the oak apple develops. Inside are several chambers each housing an individual larva. The larvae pupate and then, as winged adults, they chew their way out of the 'apple' in summer. The insects from a gall will be all of the same sex (I suppose that stops inter-breeding?). After mating the females lay their eggs in the tiny rootlets that are found along the main roots of the tree. These eggs hatch and stay underground in small, individual galls attached to the roots and remain there for about eighteen months. All of larvae are wingless, fertilised females. They crawl up the tree trunk, on to branches and lay eggs in a leaf bud to start the whole process going round again. Both sexual and asexual populations can occur on the same tree at any one time.
I think that is amazing; just how does a complex life cycle like that evolve?
Cherry Gall: the rough with the smooth
The cherry gall (Cynips quercusfolii) is the product of the egg laying of a small species of gall wasp. It lays its eggs on the underside of oak (quercus) foliage (folii) and they develop there, sometimes one, sometimes a small collection. They start pale green (like the one I photographed) and become red as they mature and that is when they begin to look like cherries.
As with similar galls a single 'fruit' has several chambers in it, each with a single larva. When the leaf falls, so too does the gall and the larvae eat their way out and live over winter in the leaf litter before laying eggs in the spring.
It is interesting that galls on our common English or pedunculate oak are usually smooth but those laid on the introduced sessile oak are rough or warty; I guess that must be something different in the chemistry of the two species of oak?
Silk Button Gall: all sewn up
A substantial number of insects, over a thousand, are associated with oak trees, it is one reason why our oaks are so important. Several of these insects lay their eggs on oak trees which then produce galls or deformities. The silk button gall (Neuroterus numismalis) is one such species. Being a bright orange-brown in colour they are one of the more easily found galls as they are quite noticeable (assuming you take the trouble to look to course!). Why the silk button gall? Each gall has a raised rim and if you look really closely, and it is far from obvious in photograph, each cup appears to have been embroidered with silk!
The silk button gall is the work of a small wasp species which lays its eggs in oak leaf buds and when the leaves open and the larvae hatch each larva creates its own gall to live in. There can be hundreds on one leaf! the larvae mature on the fallen leaf and over winter in the leaf litter before emerging in spring to start the process again. It is actually more complicated than that but that is the best I can do in trying to understand the process!
Spangle Gall: straight out of the packet
The common spangle gall houses the larvae of a gall wasp (Neuroterus quercusbaccarum); each gall containing one wasp larva. It is a species linked solely to oak tress (hence the quercus in the scientific name) and occurs where the wasp has laid an egg on the underside of an oak leaf. There can be lots of galls on one leaf but not all leaves will be affected of course. Each gall is a fairly flat packet about 5mm across with a central raised pimple. The centre of the gall can appear reddish in colour thanks to the presence of many red hairs, the disc of the rest of the gall tends to be yellowish or pale green.
When the leaf falls from the tree in autumn the larvae feed in the leaf litter over winter and the adult wasps emerge in the spring to start the next cycle of the insect's development. Galls are really quite complicated and often have sexual and asexual generations. This gall is the asexual stage of the cycle where the larvae that hatch in spring are all pregnant females! Now that is something to get your head around!
Alder Gall Mite: warts and all
Alder trees can be host to the alder gall mite (Eriophyes laevis) which produces these wart-like structures on the leaves. The galls are spherical and little more than 2 mm in diameter. Each wart has a narrow opening on the underside of the leaf through which the mite will leave when fully grown in the autumn. The mite spends the winter in an alder cone or in the bark of the tree and then, when the leaves appear in spring, promptly lay their eggs on a leaf, the warts appear and provide a home for the next generation of mites.
The galls are green at first but become red with age. They can affect an entire leaf stopping its development but not every leaf on the tree will be infected thus leaving the tree undamaged and so able to host the mite over the winter and support new mites the following year. It is all in balance.
It is not uncommon and if you look at alders in leaf there is pretty good chance you will find it eventually.
Witches Brooms: a curious tale .
When out walking the other afternoon I encountered a few silver birch trees in a line with these peculiar twig clusters. Against the weak winter's afternoon sky they looked quite sinister and it was easy to see why our country ancestors called these witches brooms! Whether it was genuinely believed this was the work of witches I do not know but, as always it seems, science has come along and put paid to long standing myths. Its good to know the facts of course but a bit of a shame some of these 'old wives tales' are being lost from the countryside.
These deformities are not uncommon on silver birch trees and they occur on other species of tree as well and are generally caused by species of fungi, the one on birch being Taphrina betulina, betulina being the scientific name for birch. The galls start as hedgehog-like prickly lumps on the bark and can remain like that for many years and that is what you usually see but when the fungus is ready to 'bear fruit' they sprout these twigs and the leaves on them bear cells that disperse the spores.
So there you have it, the truth is out! Shame really, I liked the witch theory.
Bedeguar Gall: Robins pin cushion
Bedeguar gall? I always knew this as Robin's pin cushion so I was surprised to find that it has a different name in the reference books. All three books I have that include this gall use bedeguar. As a result I went looking for my dictionary but I could find no bedeguar there so off to my old friend Wikipedia and it seems to have originally been a Persian expresion that got currupted by the French and it means 'wind-brought'. The Robin in question is not our red-breasted friend but Robin Goodfellow, the woodland sprite of English Folklore.
The gall is formed by a tiny wasp, Diplolepsis rosae, laying its eggs in a bud of a wild rose, often the dog-rose or field rose, and a subsequent chemical reaction produces the gall. Inside the gall are multiple chambers each occupied by a single larva. The larvae overwinter in the gall and hatch in the spring. The hatching wasps do not mate, they are virtually all fertilised females and male wasps of this species are, apparently, very rare. The wonder of nature!
Speedwell Gall-fly; high dependencies
For years I have been seeing fluffy heads on germander speedwell plants and I just assumed they had gone to seed. Imagine my surprise when browsing through my new book about plant galls to come across a photograph like mine. This is not germander speedwell gone to seed, it is the work of a gall fly, Jaapiella veronicae. Apparently, the fly attacks the shoot tips and causes the topmost pair of leaves to cling together to form a hairy pouch that then contains numerous fly larvae.
This fly only attacks germander speedwell and germander speedwell is a pretty common flower but is there not a real risk here for the fly? It is totally dependent on germander speedwell surviving as a species. It almost certainly will but lets say it is discovered that human beings would be better off without germander speedwell for some reason and decided to eradicate it; we have eradicated species before, in fact we are doing it all the time. If germander speedwell went suddenly the gall fly would have no time to adapt and would go as well. What if there is something as dependent on the gall fly as the gall fly is on the speedwell? What if we humans are dependant on that mystery species?
An extreme example of course but that is surely why biodiversity is so vital and why we ignore it at our peril. Nature conservation is not just about conserving wildlife, it is not just about conserving pretty places to visit, it is about conserving all life on earth and that includes human beings.
Thistle Gall: the work of a picture wing fly
Creeping thistle usually occurs in large masses and so spotting something different about one of them is not always easy. Taking a close look at thistles is not everyone's idea of an afternoon either so there is a pretty good chance you have never seen this very common gall or, if you have, you probably did not give it a second thought!
This gall that occurs on creeping thistle is the work of a picture wing fly called Urophora cardui. It is an attractive fly with black and white wings, the problem is that it is very small and difficult to see let alone recognise. The eggs are laid on the shoots of young creeping thistle plants and swell up as the plant grows. Inside each gall is a number of chambers and a lava can be found in each chamber. As the plant flowers so the gall hardens and becomes woody and the larva spend the sinter safe inside. In the spring the gall disintegrates and the now adult flies escape and the the process starts all over again.
It is quite common and once you know what it is it becomes easier to find!
Gall Mite: Aceraria echii
We found a number of viper's bugloss plants with deformed stems and leaves whilst out walking on the Purbeck cliffs and were rather intrigued by them. After some research on the internet I discovered the deformities are caused by a tiny gall mite, Aceraria echii.
The mite is, as the name implies, very small. It is shaped like the horn of a cow and has two hooks by which it attaches itself to the plant to extract nutrients. There can be countless numbers on a single plant and they produce between them what I think is a rather attractive effect! As the viper's bugloss is a perennial this invasion of small mites does not really have a major impact on the plant.
I find it amazing that one species of mite is totally dependant on a single species of plant for its survival. It would not be in the mites interest to severely damage a colony of its host plant yet other species of Aceraria gall mite are used as a biological control on some agricultural weeds, especially bindweed.
Serpentine Leaf Miner: the trail of the serpent
There are some small species of moth that lay an egg in the fabric of a leaf so that when the egg hatches the larva can mine its way through the leaf as a food source. This mining process can produce a range of different effects that show on the leaf, each shape generally unique to each species.
This one looks a bit like a snake or serpent; it starts narrow but as the larva grows so the width of the stream produced grows and eventually it stops at the point the where the larva escapes and pupates. The larva actually over winters in its mine.
The particular serpent shape is made by one of the most common of the species that do this and, not surprisingly, it is therefore called the serpentine leaf miner (Stigmella aurella). This species has a preference for bramble leaves and so this is probably the mine of that species. The moth itself is very small and looks like a midge!
Gorse Spider-mite: the mighty gorse controller
Frequently one sees white webs on gorse bushes out on the Dorset heaths. I had always been led to believe that these were the work of a moth, probably the lackey moth, but one day I noticed a brown rusty colour in one of these webs and so I stopped for a closer look. The photograph is poor as it was difficult to get the image magnified enough and yet remain in focus. Still unable to work out what was happening I collected some of the 'rust' and brought it home a put it under a microscope. I was amazed to see what looked like little spiders! It took a while but I eventually tracked it down to being a colony of gorse spider-mites (Tetranychus lintearius).
As its name implies it lives entirely on gorse and can do some damage to gorse bushes to the degree that it had been introduced in some places (especially New Zealand) as a biological control to stop gorse dominating everything else as it is such a vigorous plant. Since the initial find I have encountered gorse spider-mites quite frequently but we have so much gorse here in Dorset I think the mites are struggling to take control!
Not a gall or a deformity but this fits with mites that do create such things so I am including them in my plant galls and deformities series.