Taking a stroll recently I noticed what I thought was a wildflower I had never seen before at my feet. Excited, I bent down to take a closer look.
That was when I realised the situation was even more curious than I had imagined.
The plant was germander speedwell, a very familiar species, but it was sporting an unusual deformity.
What looked like fluffy white flowers, in complete contrast to the plant’s usual delicate blue petals, turned out to be one of nature’s most intriguing biological phenomena: a plant gall.
Plant galls
Galls are abnormal growths on plants that are stimulated by other organisms. They are found in more than half of all plant families and can form on leaves, branches, roots, flowers, fruits or seeds.
One of the most common ways a gall forms is when an insect, often a tiny species of wasp, lays its eggs in or on the host plant. This triggers an unusual growth of plant tissue, creating a structure where the larvae can develop and feed. Although the exact process remains a mystery, it results in the plant reorganising its cells and producing the gall.
Galls can also be caused by mites, fungi, worms or bacteria. They are usually harmless to the host plant but highly beneficial to the organisms that induce them.
Safe haven
The primary purpose of a gall is to provide a home and a food source for the gall-forming organism. This brings me back to the unusual-looking ‘flower’ I had spotted. The deformity was the work of a tiny orange-bodied, long-legged midge known as Jaapiella veronicae. This midge lays its eggs in the terminal buds of speedwell plants, causing the young leaves to thicken, deform and grow hairs. A small pouch develops, housing one or two orange-red larvae. I did not investigate further, but I will take science at its word that there were indeed tiny larvae tucked inside the galls.
Glorious forms
Each type of gall is specific to a particular plant and, counterintuitively, can be surprisingly beautiful. These ornamental growths can take the form of rosettes, beads, spindles, pouches or starbursts, each shaped by the unique relationship between the plant and the organism that triggers it.
In addition to their curious shapes, many galls stand out for their striking colours, which range from red and green to yellow or black. A good example is the charming robin’s pin cushion. Caused by a minute species of wasp, this gall forms bright red, fluffy growths on the stems of dog rose plants. Dog rose is already a delicate and attractive wildflower, but these vibrant pom-poms make it even more eye-catching.
Fun names
Given their variety of shapes, colours and sizes, it is little surprise that galls have acquired a delightful array of descriptive names. Oak trees, in particular, are famous for supporting numerous gall-forming insects. Among the most common are the silk-button spangle, common spangle, knopper, artichoke, oak apple, oak marble, and cherry galls.
These growths are caused by insects with rather formidable Latin names, such as Neuroterus numismalis for the silk-button spangle gall and Andricus foecundatrix for the artichoke gall. Their common names, however, are far easier to remember and often wonderfully evocative. One of my favourites is the silk-button spangle gall, although I have often thought it should be renamed the ‘Cheerio gall’, as the tiny discs resemble pieces of cereal stuck to the underside of oak leaves.
Witch’s broom
One of the most dramatic examples of a gall is known as witch’s broom. These galls form high in the branches of trees and are caused by microorganisms. The result is a mass of densely branched twigs, which look more like tangled birds’ nests than a witch’s broom. The name, however, adds a touch of whimsy. These growths can persist for many years, often lasting for the entire life of the tree.
Most galls require a careful look under leaves or along stems, as they are often small and inconspicuous. Witch’s broom is an exception. Instead of tiny structures, you are looking for large, stick-like clusters high in the tree canopy. This type of gall is especially common in birch trees, where it is caused by the fungus Taphrina betulina and is often called birch besom.
One of the white hairy galls on germander speedwell caused by a midge.
Chain reaction
It is fascinating enough to think of an insect inducing a plant to grow in a completely different way to protect it, but the story does not end there. Some insects exploit galls created by others, moving in to share the space. These opportunists are known as inquilines.
There are also more sinister players in the mix. Although a gall provides significant protection, it is not impenetrable. Some parasitic wasps specialise in piercing galls, using their long egg-laying needles, or ovipositors, to inject eggs into the larvae within.
They do this with startling precision, despite lacking X-ray vision. When the parasitic wasp eggs hatch, the larvae consume the original gall-making insect. Nothing in nature goes to waste, and many galls are eventually eaten by birds or small mammals.
Human interest
Historically, plant galls have been valuable to humans. They have been used in traditional medicine, in the production of dyes and inks, and for tanning leather. The tannins within them, which are intended to deter predators with their bitter taste, are a key ingredient in ink and dye making. There is renewed interest in reviving these age-old methods and using galls as a sustainable source of ink for art and textiles.
Next time you are out walking, keep your eyes open for these unusual plant structures. Galls are just one example of the many fascinating ways insects and plants interact. Leaf miners, for instance, create delicate, tunnel-like patterns inside leaves as they feed, but that is a story for another day.
