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 plants after being invaded by some very unique organisms. Galls appear on over half of all plant families and have a range of causers, including viruses, fungi, bacteria, insects and mites.

While these weird structures have intrigued humans for millennia, there is still much that we don’t know about them. However, we know for sure that when we take a closer look at them, a fascinating micro-world opens up before our eyes!

The gall of it

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. This triggers 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 can be so distinctive that we can often identify the causer from the growth alone.

Why create a gall in the first place? What’s in it for the causer? In summary, they may provide their inhabitants with any combination of food, shelter and protection. It is a parasitic relationship, in that the invader benefits, while the host loses out. However often 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 the work of 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

Biodiversity and galls

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 of our planet. Take for example the alien-looking galls on stinging nettle leaves caused by a rust fungus called Puccinia urticata. Or the hairy structures on germander speedwell, giving away the presence of a gall midge.

Some galls causers rely on more than one host. A fungus known as Gymnosporangium clavariiforme produces strange orange tentacle-like growths on juniper. The spores from these then infect the leaves of hawthorn. More galls appear, which are very different in their growth form, and these then re-infect juniper, and so on. This demonstrates the fact that the more plant species in an ecosystem, the more species will be supported overall.

Witches and woodland sprites

While some galls are well hidden and hard to spot, others are much more conspicuous. Have you ever looked up into a birch tree 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 the work of a fungus which stimulates the tree to produce this dense, nest-like cluster of twigs. The fungus can then feed on the shoots. Such growths have puzzled people for centuries, and it was once believed that they appeared after a witch had flown over the tree!

If you spot an odd-looking growth on a dog rose it could well be a Robin’s pincushion gall, caused by a small wasp. There was once a belief in England that these were caused by the woodland sprite, Robin Goodfellow or Puck. It’s not surprising that people ascribed supernatural causes to some galls – they look pretty strange, and their causes aren’t exactly obvious!

Oaks and wasps

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 called 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. A single oak tree may host thousands of galls. Each cynipid species creates its own unique and bizarre structure: some resemble cotton wool or marbles, pineapples or tiny UFOs! Their life histories and interactions with other species are no less intriguing than the structures themselves.

As there are so many cynipids, and they have been relatively well studied, let’s take a closer look at them. The process begins when the female wasp lays her egg in some part of the tree. She does this 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 various parts of the tree. These include the leaf bud, catkin or even the roots. Either the eggs or the larvae then exude special chemicals (with some non-cynipids, the adult does this herself while laying the eggs). The chemicals have strange effects on the tree, stimulating cell growth to create the perfect microhabitat for the grub. A chamber (or multiple chambers) develops for the larva or larvae to grow in. Remarkably, the larva can stimulate the plant to direct more nutrients to the cells immediately surrounding the 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 is a general picture of how some galls develop. Now 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 caused by two separate species. Thanks to the dedicated work of patient naturalists, rearing gall wasps through successive generations, we know more about their complex life histories.

‘Currant’ affairs

One species of wasp, called Neuroterus quercusbaccurum, develops in tiny disc-like spangle galls. You can often see a lot of spangle galls 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 plot thickens

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 benignly in the gall, with each occupant minding its own business. Others grow in the same chamber as the original occupant, outgrowing and smothering their reluctant ‘landlord’.

Under siege

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. While a parasite feeds from its host, usually without killing it, a parasitoid will occupy a host, eventually killing it. 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.

Holding the fort

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. It gets toughened by lignin, the chemical compound that also makes wood rigid. This makes it 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 complex exteriors making it more difficult for parasitoids to land and penetrate. 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. This is obviously great news for the besieged 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 many chambers within them. While some of the larvae on the periphery may be parasitised by an invading wasp, it can’t attack all of them, especially those near 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 a forest, and if they invade and decompose the gall, the cynipid larvae will not survive. This is where chemicals called tannins come in. Oaks, like many other plants, produce a lot of tannin, which help ward off fungi, parasitoids and hungry herbivores. In galls the concentrations of tannin can be much higher than they are in the surrounding plant tissue, making them extra effective. Interestingly, this concentrated source of tannin has even been used by humans. The oak marble gall was originally introduced to Britain because it yields a black dye, although it turned out that the tannin content of galls grown here is too low for this purpose.

Further up the food chain

Even the most aggressive parasitoid is vulnerable, as there are bigger, hungrier mouths about. While effective against smaller foes, the tough exterior of some mature galls is not enough to deter a great spotted woodpecker, which will peck the gall open to extract the soft and juicy prize within. Other gall predators include rodents such as wood mice and birds including great tits. When the tiny, frisbee-like discs of spangle galls drop from oak leaves onto the forest floor in the autumn, wood pigeons can be seen feasting among the leaf litter. One pigeon may eat dozens of galls in a single feeding session.

Unsolved mysteries

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. Even so it is clear that galls make a huge contribution to the diversity of life in a forest.

Sources and further reading