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Galls: what, why, and where to find them

Spangle Galls at Teifi Marshes nature reserve. © Gretchen Taylor

People and Wildlife Officer, Gretchen, tells us all about galls, how they form and where to spot them this autumn.

A fascinating thing to look out for in Autumn are galls. So, what are they and where can you find them? 

Galls are growths on plants, made of plant tissue, caused by insects, bacteria or fungus. Galls cause no harm, though have no benefit to the host plant.  

Oak trees attract many gall wasps and can develop galls on leaves, buds, twigs and acorns. Once you start looking for them, you’ll see them everywhere! 

In the case of gall wasps, the gall is used to protect the larvae. The life cycles of these fascinating insects are complex, with asexual and sexually mature generations, some requiring different tree species to complete the cycle. 

A tree branch with brown, round lumps growing out of it.

Marble Galls at Teifi Marshes nature reserve. © Gretchen Taylor

For example, Marble Galls form on oak buds in summer and contain one female wasp larvae, who emerges in September to lay eggs in the buds of Turkey Oak. This results in male and female wasps emerging in spring who mate and lay eggs in Sessile or Pedunculate Oak forming the Marble Galls. 

Oak apples form on oak twigs and contain many larvae; males and females in separate ‘apples’, which emerge in July. Knopper Galls form on acorns but alternate generations lay on the male flowers of Turkey Oak. 

One of my favourites are Spangle Galls, found on the underside of oak leaves, some look like little flying saucers, others like cheerios! Galls are often named based on how they look; Cherry Galls on oak leaves and Artichoke Galls, on oak buds being two examples. 

The underside of an oak leaf with round brown lumps covering it.

Spangle Galls at Teifi Marshes nature reserve. © Gretchen Taylor

The branch of a rose plant with a red and green, round, hairy growth.

Robin's Pincushion at Teifi Marshes nature reserve. © Gretchen Taylor

It is not just oak that host gall wasps. A common sight on Dog Rose is a Robin’s Pincushion. Birch trees often have ‘Witches Broom’ galls, caused by a fungus rather than an insect, and look like scruffy nests. 

So, keep your eyes peeled for abnormal growths on your late summer and autumn walks in nature. With some luck, you might spot marbles, apples, cheerios, flying saucers, or fluffy pin cushions in unexpected places! 

Want to read on? Learn more about galls.

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