They appear as if from nowhere. A previously healthy looking plant suddenly has an abnormal growth protruding from it. Some are hideous and some strangely beautiful but they leave the plant looking as if it has been invaded by miniature aliens. And in a way that’s exactly what has happened. Take a look at the weirdness of the plant gall.
Galls are caused by a variety of parasitic organisms, which can range from bacteria and fungi to insects and mites. They look as if they are grown from the plant itself and are often structures with an extremely high level of organisation. Such is the structure of many galls, the parasite can be quite easily identified. These are the galls of the Silk-button Spangle Gall Wasp. They take advantage of oak trees in two ways, First of all they create galls on the underside of leaves which overwinter on the ground attached to fallen leaves. This produces a single gender hatching – all female. So far, so Alien!
When the larvae pupate and emerge from the galls in March they lay their eggs in the buds of the oak tree and this will produce a second generation. This generation of wasps will go on again to lay their eggs on the underside of leaves which will fall and overwinter – and so the cycle continues. The galls form around the eggs. Galls do not only have to be formed by insect or mite activity, however. The example below is growing on a hemlock tree in Oregon. It is an example of Phellinus punctatusa nasty looking fungus if ever there was one. These lumps on the side of the tree are the size of cats.
The artichoke gall, (below) on the other hand is caused by an insect. When an egg is laid the gall surrounds it – it is a chemically induced distortion of the buds on, in this case, the Pendunculate Oak. A single eggs is laid within a leaf bud by the gall wasp. The artichoke itself looks quite fragile – as if a squirell could destroy it if it wished. Of course – it is not to be confused with the vegetable that bears the same name – they share the title simply because of the similarity in shape not because they are related in any way.
However, there is a hard case inside the artichoke and this is where the larva lives until it is ready to emerge in the Autumn. Strangely enough this generation of the wasp (Andricus fecundatrix) is asexual. It lays its own eggs in oak catkins and it is this offspring that produce the sexual generation of wasps which will ensure the genetic diversity of the species. This is one of the few insects in the world that will produce two generations in a year – one that is asexual and one that is sexual. Are you ready to see what’s inside?
This looks a little scary. It is a knopper gall – so called because the word derives from German and was a type of helmet worn in the seventeenth century that had small protuberances, often studs or tassels. This knopper gall contains several grubs of the gall wasp Adricus grossulariae. The nodules that appear on it are because the grubs inside are competing for space and pushing each other away to the extremities of the gall. Altogether a little disturbing if you are of a delicate disposition.
It is not only wasps who induce galls on plants. Some species of aphids do this too. Below you can see the offspring of the Poplar petiole gall aphid. Each July they will drop green cottonwood leaves to the ground and emerge – a ravenous family looking for their first meal.
Some galls can look quite pretty – they are not all the stuff of nightmares at all. Or are they? Below is a gall is known as the Rose gall or the Robin’s pincushion gall (quite a charming name). It is caused by the Diplolepis rosae gall wasp and can contain up to sixty eggs. In medieval times it was dried out and then powdered – to be used as a toothache remedy. It was even thought to cure baldness when rubbed on the scalp and was placed below pillows to help insomniacs get some sleep.
However, as lovely as it looks you have to remember that inside it are up to sixty larvae, growing and maturing inside it. It may have been used to send people to sleep but the sight one the interior may give some people nightmares. Take a look. You can see the fronds of the gall which give it its name at the edges of the picture – and the larvae within!
The gall below is that induced by Disholcaspis coralline- the Coral Wasp and it is on a Blue Oak in North California.
Some galls looks like eggs but they are not – that has hatched within it. The gall is the protective layer around the egg which is forced by the parasite. Although many look attractive, such as this striped pea gall of the wasp Cynips longiventris they generally reduce the fertility of the host plant quite greatly.
Galls are sometimes even prickly, as can be seen above. Back to fungus for a second here. This is a sloe berry plant and the fungus is known as Taphrina pruni. The common name is pocket plums – they are not to be eaten.
The gall below is on the leaf of the Arroyo Willow. There is a certain almost florescent beauty to it – remember though that the ‘spikes’ appearing on the gall is the action of the larva within – jostling for growing space.
The gall that most people will recognize is known as the Oak Apple (again, not to be confused with the real thing). The gall can be up to five centimeters in diameter and it can look like an apple, certainly in its early days. They are common both in the US and the UK and share the same name – but are caused by different wasps. They have their uses for us, however. If you fish then you can use the larvae inside as excellent bait. However unlikely, If you find yourself in an oak forest with nothing to eat then the larva are full of protein. You do not need any further explanation there, perhaps.
Sometimes, they can look like bits of human brain hanging from a tree – to be frank.
The lime tree too has its own gall, caused by a mite. Here, though the protuberance takes the form of an erect and curved pouch and will cover the upper surface of the leaf of the lime tree. They are extremely visible because of their red color and can be spotted in the late spring.
The mites, Eriophyes tiliae tiliae, spend the winter in crevices in the bark. The gall inducer used is only about two millimeters in length but release chemicals powerful enough to have this dramatic effect on the leaf. The grown mites will emerge in the autumn, pass the winter in the bark of the tree and then in the spring they will start the cycle all over again.
Some galls really do look as if they are the eggs of an alien species, ready to hatch and wreak terror on an unsuspecting humanity.
Although the gall has a detrimental effect on its host – we have something that we must thank it for. When mixed with Iron Sulphate the oak gall creates a permanent ink of a purple black color. It was the standard writing ink in Europe from the twelfth to the nineteenth century. Without the ink created from the oak gall many of our precious medieval manuscripts would have faded centuries ago.