Get to Know Galls 

Galls show staggering diversity not only in the forms the gall takes, but also the gall causing organism. You would be forgiven to think that all galls are caused by a group of very closely related organisms and there certainly groups of closely related gall causing organisms. Gall forming has actually evolved several times in a process known as convergent evolution, where distant species develop the same evolutionary trait (for instance flight) or in this case gall forming. Species from different kingdoms have independently developed this trait which means galls can be caused by fungi or wasps, both wildly different organisms.   

The relationship between gall causing organism and host is most often parasitic, though it can also be mutualistic.

What is a Gall? 

Simply put a gall is an abnormal growth that is induced by the presence of another organism. We normally think of galls on plants, in fact this is where the word gall has its roots. 'Galla' in Latin translates into 'Oak Apple' one of the commonest galls. 

What is the purpose of a Gall?

The structure of the gall provides the organism with food and shelter. The relationship between host and parasite is often highly specialised only being able to use a certain host species.

What causes the formation of a Gall?

As touched on above galls can be caused by a wide range of organisms. Most commonly galls are created by bacteria, fungi or invertebrates.

How do we identify Gall causing species?

The parasite is often extremely small, and it is far easier to find and distinguish these organisms by the galls they create rather than species themselves. 

To illustrate my point we will look at two groups of gall causing species, wasps in the Genus Andricus and fungi in the genus Taphrina 

The galls of wasps in the genus Andricus 

Wasp species in the genus Andricus are specialised to forming galls on oak trees, they are known as the Oak Gall Wasps. We have at least five species in this genus here at Warley Woods, let's take a look at four of the more recognisable galls caused by these similar wasps including the most useful and probably most recognisable gall the Oak Marble.

Andricus wasps look much like the Cola-nut Gall Wasp (Right). These wasps measure only 4-5mm long so without a magnifying glass it would be tricky to even recognise them as a wasp. Their galls on the other hand are much easier to spot, not only are they much larger, many remain on the trees for several years after the wasp has vacated. The location of the gall on the host can also be a useful determining factor, some galls appear similar and a quick and easy way to differentiate them is the location on the plant where they occur.   

Gall causing wasps (Cynipids) have fascinating lifecycles, they often have two generations a year, one sexual and one asexual, the latter being devoid of males, the female essentially cloning herself. All the galls you see above are laid by the Asexual generation of their respective wasp species. Galls from the asexual generation tend to be more substantial and woody, this generation needs to develop and overwinter in the galls before hatching out the following spring to lay the sexual generation.  Conversely the sexual generations (containing both males and females which mate) galls tend to be smaller softer and short lived. 

The staggering forms the galls take originate from the unique concoction of chemicals which are either injected by the parents at time of laying or exude from the developing larvae (sometimes both). These interfere with the plants growth hormones creating weird growths that benefit the parasite. Depending on where they insect is laid also impacts the form of the gall, take the Hop gall, it still bears some resemblance to the oak bud that it corrupted.  It is worth noting that perhaps unsurprisingly the wasps tend to lay in developing tissues, ones that will be getting pumped with nutrients and growth hormones. 

The oak marble gall (Andricus kollari) is not native to the UK, and is thought to have been purposefully introduced to the UK in the 1800's as it served a useful purpose. The high tannin levels in this gall were historically used to make iron gall ink, a natural ink and dye, this ink can be seen on numerous historical documents including the US Declaration of Independence. 

Now lets have a look at some galls caused by a vastly different group of organisms.

The galls of the fungi in the genus Taphrina  

These three galls are all caused by species of fungi in the genus Taphrina. They each have their own respective host, to avoid any confusion there species name is actually based on their respective tree hosts.  Taphrina alni the species that infects Alder (Alnus), Taphrina betulina for the species that infects Birch (Betula) and finally Taphrina pruni for the species that infects Blackthorn (Prunus). Whilst these three fungi are closely related their choice of host varies greatly, they do however, have fairly similar lifecycles. 


The pocket plum gall is called such as the fungus destroys the developing stone of the sloe leaving a cavity or 'pocket' where it once existed, these galls start off green but dull to grey/off-white as they develop, the spores are then released from the surface of the plum, infecting the stems where it remains until it is time to once more re-infect the developing sloes the following year. 

The alder tongue is an aptly named and unmistakable fungal gall caused by Taphrina pruni. In this instance the fungus infects the scaled of the alder cones, this causes these large tongue-like eruptions out of the side of the catkin, they start of pale before turning a bright red and eventually brown. Spores develop on the outside of the gall.

The final fungus in this genus we will look at is Taphrina betula which causes the witch's broom gall. This gall is more persistent than the other two and is actually somewhat easier to spot in the winter. Starting off as a cluster of buds which somewhat resemble a hedgehog, eventually the buds sprout weak shoots, and the fungus produces spores on the outside of the leaves on these shoots. These galls can grow to quite an exceptional size with ones around 1m in diameter having been reported. 

All three of these species are Ascomycetes, these fungi use specialised cells 'asci' to shoot their spores into the air, aiding distribution.

This webpage has offered you barely a glimpse into the diversity of Galls and their causing organisms. We have not even touched on mites, aphids or flies, and have hardly looked at gall causing wasps which are as numerous as they are amazing. Hopefully this have whetted your appetite and will make you look a little closer and plants, if you spot an abnormal growth it could be a gall - happy hunting!    

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