Species of the Month

Next time you are scrunching through the colourful fallen leaves beneath an oak tree, take a moment to look down and see if you can spot lots of little brown discs. These are one of the two different types of galls produced by the tiny wasp Neuropterus quercusbaccarum. 

The tiny spangle gall wasp is a Hymenopteran and belongs to the family Cynipidae. The adults themselves are very hard to see as they are so tiny, being only a few mm long, with a typical wasp waist, simple wings and short straight antennae. The female has an ovipositor which protrudes at the end of the abdomen so she can lay eggs. 

It is highly unlikely we will see the wasp itself, but we can spot the galls produced by the alternate generations in this insect’s life cycle. Read on to find out more about this fascinating insect and don’t forget to record your sightings of the galls in the interactive map below. 

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The female gall wasps insert eggs into a particular part of the oak tree, in this case the underside of leaves or catkins. On hatching, the legless larvae secrete chemicals that cause the plant to produce distortions called galls around the developing larvae. The fascinating thing about this wasp, is that it has two alternate generations, one sexual and the other asexual. Each generation produces a different type of gall. The sexual generation is produced inside currant galls while the asexual generation is produced inside the disc-shaped common spangle galls.  

In spring, small, round, hanging currant galls can be found attached to either the oak’s catkins or the underside of leaves. They look like their name suggests and change colour as they mature from yellow to red and purple. Tiny larvae develop inside these currant galls. They emerge as adults in June and can be either male or female. This is the sexual generation, so they mate soon after emerging and the female then lays eggs in the epidermis of the underside of the oak’s leaves.  

The eggs of this generation then hatch and the larvae begin to develop within the oak leaves inducing the production of the spangle gall. These are about 4mm in diameter, hairy and disc-shaped, changing colour from a bright pink to a dull brown at maturity. By late summer when they are mature, the spangle galls drop off the leaves and fall to the ground. They are covered by the autumn leaves which provide protection for the larva, which continues to develop inside the fallen spangle gall over winter until it emerges in April. This is the asexual generation so doesn’t need to mate, but the females go on to lay eggs within the oak catkins or on the leaves, which in turn produces the currant gall generation so completing the cycle. 

In Britain there are over 70 different gall wasps on oak alone, and over 900 plant gall forming animals in the UK. These include mites, beetles, flies, gall wasps, plant sucking bugs, aphids and sawflies.   

Despite being considered a parasitic relationship, the gall wasps don’t appear to cause any problems to the oak trees and are part of the amazing diversity of wildlife the tree supports! 

The wasp and galls are common and widespread. The small round spangle gall discs can be found on the back of the leaves of the native pedunculate oak (Quercus robur) and the sessile oak (Q. petraea) during the summer. There can be over 100 of these galls on the back of a single leaf, although they don’t seem to do much damage to the host. These galls drop to the ground in late summer and autumn. 

The currant galls can be seen on the oak trees in the late spring and summer.  

Find out more about galls from the British Plant Gall Society which encourages and co-ordinates the study of plant galls in the British Isles. 

If you have space, plant a tree or create a woodland edge to provide shelter and food for wildlife. 

Whatever the size of your green space, please go chemical free and help the tiny creatures that may be food for other wildlife.