The natural world can be both strange and mysterious, and this is certainly the case with plant galls. Galls are the bizarre lumps, bumps and growths that develop on different parts of plants after being invaded by some very unique organisms. Galls have a range of causers, including viruses, fungi, bacteria, insects and mites, and they appear on more than half of all plant families. The study of plant galls is called cecidology, and yet while these weird structures have intrigued humans for millennia, there is still much that we don't know about them. However, we know for certain that when we take a closer look at them, a fascinating and remarkable micro-world opens up before our eyes!
There is a huge variety of galls, and the way they are induced and develop also varies. Usually the gall causer in some way attacks or penetrates the plant's growing tissues and causes the host to reorganise its cells and to develop an abnormal growth. The chemistry behind this is not fully understood, although it is thought to be due to complex interactions between hormones and other chemicals. Galls have such recognisable forms that the causer can often be easily identified from the growth alone.
What is there to be gained by creating a gall? In summary, they may provide their inhabitants with any combination of food, shelter and protection from predators. It is a parasitic relationship, in that the invader benefits, while the host may be harmed (although in many cases, no obvious harm is apparent, and the plant continues to thrive). Some galls are open - the gall-inducer causes the leaf to roll and then breeds within the shelter of this 'tent'. Open galls are typically caused by invertebrates with piercing mouthparts, such as aphids and mites. Other galls are closed, that is, the larva of the creature, often a wasp or beetle, develops within a fully enclosed structure.
One of the most striking things about galls is their astonishing variety; there are myriad causers and hosts, shapes and sizes. Galls are just one illustration of the incredible biodiversity (i.e. the variety of life) in our forests, and on the planet as a whole.
Take for example the alien-looking galls on stinging nettle leaves (Urtica dioica) caused by a rust fungus (Puccinia urticata); or the galls on bracken (Pteridium aquilinum) resulting from the activities of a tiny fly (Chirosia grossicauda), which causes the leaves to distort and curl over; or the hairy structures on germander speedwell (Veronica chamaedrys), giving away the presence of a gall midge (Jaapiella veronicae).
Some galls causers rely on more than one host. A fungus known as Gymnosporangium clavariiforme produces strange orange tentacle-like growths on juniper (Juniperus communis). The spores from these then infect the leaves of hawthorn (Crataegus monogyna), resulting in more galls, which are very different in their growth form, and these then re-infect juniper, and so on. This clearly demonstrates the fact that, the greater the plant diversity there is in an ecosystem, the more species will be supported overall.
While some galls are well hidden and hard to spot, others are much more conspicuous. Have you ever looked up into a birch tree (Betula spp.) and noticed what looked like large, dense birds' nests? In some cases these may well be nests, but very often they are actually galls called witches' brooms. These are caused by a fungus (Taphrina betulina), which stimulates the tree to produce numerous extra shoots, resulting in a dense nest-like cluster. The fungus can then feed on the shoots. Such growths have puzzled people for centuries, and it was once believed that they were caused by witches flying over the tree!
If you spot an odd-looking growth on a dog rose (Rosa canina) it could well be a Robin's pincushion gall, caused by a wasp (Diplolepis rosae). There was once a belief in England that these were caused by the woodland sprite, Robin Goodfellow or Puck. It is hardly surprising that people ascribed supernatural causes to some galls - they look pretty strange, and their causes aren't exactly obvious!
The real gall specialists include gall midges, gall flies and gall wasps. Perhaps one of the most familiar galls is the oak apple, caused by a tiny wasp (Biorhiza pallida). There are actually hundreds of species of oak gall wasps - or cynipids as they are known - and they cause a fantastic variety of galls on oaks (Quercus spp.). A single oak tree may support many thousands of galls. Each cynipid species creates its own unique and outlandish structure: some resemble cotton wool or marbles, pineapples or tiny UFOs! Their life histories and interactions with other species are no less fascinating than the structures themselves.
As there are so many cynipids, and they have been relatively well studied, it is worth paying them some closer attention. The process begins when the female wasp lays her egg in some part of the tree using a special egg-laying device called an ovipositor. Depending on the species of wasp and the stage in its life-cycle, the egg may be laid in any number of parts of the tree, for example the leaf bud, catkin or even the roots. Either the eggs or the larvae themselves then exude special chemicals (with some non-cynipids, the adult does this herself while laying the eggs), which begin to have strange effects on the tree, deforming and stimulating cell growth to create the perfect microhabitat for the wasp grub. A chamber (or multiple chambers) develops for the larva or larvae to grow in. Remarkably, the larva is able to stimulate the plant to direct more nutrients, such as proteins and sugars, to the cells immediately surrounding the moist chamber. The grub thus has a ready supply of food to speed it towards maturity. The outer layer of the gall has particularly high concentrations of tannins for reasons we'll explore below. Through most of its development the mid- and hindgut of the larva are sealed so that it doesn't foul its chamber. They only open just before the adult wasp emerges. This a general picture of how some galls develop.
Let's look at some specific examples of a full cynipid life cycle. Many of the gall wasps have two distinct generations, each one galling a different part of the tree. It would be easy to assume that these very different galls were instigated by two separate species, and it is thanks to the dedicated work of patient naturalists, rearing gall wasps through successive generations, that we know more about their complex life histories.
One species of wasp (Neuroterus quercusbaccurum) develops in tiny disc-like spangle galls, which are abundant on the undersides of oak leaves in the autumn. The galls drop to the forest floor, where the grubs develop over winter under the cover of fallen oak leaves. In the spring an all-female generation emerges. These are 'agamic', meaning that they are able to reproduce without mating. They lay their eggs in oak buds, producing currant galls on the catkins and leaves. The sexual generation of male and female wasps emerge from the currant galls in June, mate, and then lay their eggs on the undersides of the leaves. Spangle galls develop, and so the cycle continues.
The development and the lives of the gall inducers are intriguing enough, but the story doesn't end there. There is often a whole community - a mini ecosystem - that develops within and around the gall. This is where some other fascinating players enter the stage. Many galls will host lodgers, which zoologists refer to as 'inquilines'. The term can be applied to many different members of the animal kingdom and comes from the Latin inquilinus, which means 'lodger' or 'tenant'.
The inquiline wasps are closely related to the true gall wasps, but unlike their cousins they cannot create galls. So they do the sensible thing and occupy an existing gall, rent-free! Some inquilines dwell fairly benignly in the tissues of the gall, only modifying their immediate surroundings, and with each occupant minding its own business. Others however, grow in the same chamber as the original occupant, outgrowing and smothering their reluctant 'landlord'.
Again, each kind of gall varies and some of them may have numerous original occupants, and many inquilines. However, before long, both the cynipid larvae and inquilines will need to watch out. Enter the parasitoid wasps. These may sound like something out of a science fiction film and frankly that's just what they're like! Parasitoids are different to true parasites in that whereas a parasite feeds from its host, usually without killing it, a parasitoid will occupy a host, eventually leading to the victim's death. In the case of the parasitoid wasps, they lay their eggs within the larvae of gall inducers or inquilines. As the invader's egg hatches, the larva develops inside the host grub, devouring it from within.
Naturally, the besieged occupants of the gall have had to evolve to resist such intrusions. In the later stages of the life of a gall, it will often develop a hard exterior, through a process known as lignification (lignin is the chemical compound that gives rigidity to wood). This makes it much harder for parasitoid wasps to penetrate the gall with their ovipositors.
The diverse structures of the galls themselves are largely a result of the need to ward off invaders. Many galls, not only those caused by cynipids, have very complex exteriors making it much more difficult for parasitoids to land and effectively penetrate all the way in towards the grub. Some galls even have a sticky surface. This slows down the invader's efforts, and the more time it spends in the open air, trying to lay its egg, the more vulnerable it is to passing predators such as birds, which is great news for the cynipid. The parasitoids have therefore adapted by laying their eggs in the earlier stages of gall formation, when their prey's defences are not fully developed.
In some galls, the chamber is deep enough within the structure that it is just out of reach of the parasitoid. Others have an air space between the outer tissues and the larval chamber. This frustrates the efforts of the invading wasp, as its ovipositor can only penetrate the grub if it has structural support from the surrounding gall tissue. Where these hollows are present, the ovipositor bends and the eggs remain unlaid. One-nil to the cynipid!
Some gall wasps invest in numbers to ensure at least some of their offspring avoid being parasitised. Galls such as the oak apple have numerous chambers within them. While some of the larvae on the periphery may be found and parasitised by an invading wasp, it can't attack all of them, especially those right in the centre. The invader leaves contented and many of the gall wasps still hatch.
It's not just the parasitoids that cynipids have to be aware of. Fungi are ever-present in the forest, and if they invade and decompose the gall, the cynipid larvae will not survive. This is where the tannins come in. Oaks, like many other plants, produce high levels of tannins. These chemicals protect the tree against decay, and also against browsing herbivores, since tannins inhibit the absorption of proteins by animals. In galls, however, the concentrations of tannins can be many times higher than they are in the surrounding plant tissue, which helps to prevent fungal attack, and in some cases wards off parasitoids and herbivores. Interestingly, this concentrated source of tannin has even been used by humans. The oak marble gall (Andricus kollari) was originally introduced to Britain because it yields a black dye, although it was found that the tannin content of galls grown here is actually too low for this purpose.
Even the most aggressive parasitoid is vulnerable, as there are bigger, hungrier mouths about. While effective against smaller foes, the tough lignin exterior of some mature galls is not enough to deter a great spotted woodpecker (Dendrocopus major), which will peck the gall open to extract the soft and juicy prize within. Other gall predators include rodents such as wood mice (Apodemus sylvaticus) and birds including great tits (Parus major). When the tiny, frisbee-like discs of spangle galls drop from oak leaves onto the forest floor in the autumn, wood pigeons (Columba palumbus) can be seen feasting among the leaf litter, and one pigeon may eat dozens of galls in a single feeding session.
Although some specialists (called cecidologists, in case you were wondering!) have spent a lot of time studying galls, there is still a huge amount we don't know about these strange growths and their causers. Perhaps they have been overlooked as they are so challenging to understand, or easy to pass by. Much about their chemistry remains to be discovered and many of the life cycles of the organisms that cause them are completely unknown. Nevertheless it is clear that galls make a huge contribution to the diversity of life in the forest.
With thanks to Rex Hancy for his comments on the text.
Sources and further reading