The Oak tree plays a special part in our culture and our environment. The Common Oak (Quercus robur), also known as the English, or Pedunculate Oak, supports over 2,300 species of wildlife which is far more than any other native tree. At the Rec, we have a small oak tree adjoining the play area, 1 or 2 growing at the perimeter, and 3 small oak bushes (as a consequence of regular pruning), in the garden area.

Amongst the wildlife making a home in our Oaks, are the galls of a number of different tiny wasps known, unsurprisingly, as Gall Wasps. A gall is an abnormal growth that serves to protect and nourish the larvae of the wasps, or in some cases, midges or aphids. When it comes to the wasps, many have an interesting life cycle that consists of an alternating sexual and asexual generation, each with its own type of gall. It's thought that the asexual phase (parthenogenesis) confers advantages such as being able to establish new territories more quickly, and allowing for rapid population growth, because the female is able to produce viable eggs without the need to mate. The downside is the lack of genetic diversity, which would ultimately harm the future of the species. Through adopting this two phase lifecycle, the Gall Wasps are effectively hedging their bets

According to their species, and generation (sexual or asexual), the female Gall Wasp will lay her eggs on various parts of the Oak such as twigs, leaf buds or developing acorns. Once the eggs hatch, the larvae secrete chemicals that cause the tree to respond with localised abnormal growth, resulting in the formation of galls. The galls encircle the larvae, incorporating some of the plant tissue, which provides them with protection and nutrition as they continue to develop. Eventually they will emerge as mature adults. All galls are part of the trees natural ecosystem and don't generally cause any harm

This article includes photographs of various Oak Galls, taken in the garden at the Rec over the past 3 years. The galls that are illustrated are all products of eggs laid by females that have mated early in the year, and they will all give rise to asexual female offspring. The females that emerge from these galls will lay unfertilised eggs resulting in a completely different type of gall (not illustrated), and they will contain both male and female offspring. The galls containing both sexes occur over the winter and spring, and are usually far less conspicuous than those that are represented here. They may occur on the same species of tree as their summer equivalent, or require a different species to complete the lifecycle. Once the male and female wasps have emerged in the spring, they mate, the males die, and the females go on to start the cycle over again, creating the galls you see here.

The first two pictures show an Oak Marble Gall. This structure is created to protect a single larva of a small wasp, unsurprisingly named, the Oak Marble Gall Wasp (Andricus kollari). The life cycle of this wasp is dependent on 2 species of trees; the Common Oak for the summer gall and the Turkey Oak (Quercus cerris) for the spring galls. The latter are small oval galls, that can be seen in March or April between the bud scales of the leaves. The Oak Marbles themselves are hard, up to 2cms in diameter, and turn from an initial green colour to a brown, woody texture, as they mature. The female inside creates a hole through which she's able to emerge, as shown in the second photo.

Here's an Oak Apple Gall......yes, you've guessed it! It's made by the Oak Apple Gall Wasp (Biorhiza pallida). The spring gall is larger and softer than the marble gall and has a spongy interior containing several larvae, rather than one. It is larger than the oak marble, sometimes reaching 5cms in diameter, with young galls having a pinkish tinge, again turning brown as they age.

Unlike the Oak Marble, the life cycle occurs over 2 years, but still with 2 different types of gall. In this instance, both generations of gall occur on the same tree; the spring ones being formed on the roots. The female that emerges from the spring gall is wingless and having mated, has to climb the tree to lay the next generation of eggs. These will produce the Oak Apple Galls.

Next is the Oak Knopper Gall, product of the Knopper Gall Wasp (Andricus quercuscalicis). This gall does unfortunately cause damage to the developing acorns, making them unviable as seeds.

It is a knobbly outgrowth, initially green and sticky, turning red, before becoming brown and woody. Each one contains a number of larvae, and the more contorted the gall, the greater number of larvae, vying for space.

The wasp only arrived in Britain in the 1960's but has since spread rapidly throughout England and Wales. Like the Oak Marble wasp, this species needs both Common Oaks and Turkey Oaks to complete its life cycle. The spring galls are small and inconspicuous, having been laid on the male catkins of the Turkey Oak.

The Oak Artichoke Gall is named after its appearance, which is said to resemble an artichoke.

The mated female of this wasp species, Andricus foecundatrix, lays her eggs inside a terminal leaf bud. The young galls that form, have scales, and tufts of long hairs protruding from the centre of the 'artichoke'.

As with the other species, they are initially green (left), becoming brown as they age (right). When they mature in the late summer, the galls fall from the tree and the single larva overwinters inside it. The eggs of the next generation are subsequently laid on the oak catkins, by these unmated females. They produce galls called Hairy Catkin Galls.

Finally, these are photos of 2 types of Spangle Gall that are also produced by wasps, and develop on the leaves of the Oak. On the left is the Common Spangle Gall created by the Common Spangle Wasp (Neuroterus quercusbaccarum); a very long name for a very tiny wasp!

The 'spangles' contain the generation of asexual females and will fall to the ground and overwinter amongst the leaf litter. The wasps that emerge in spring, will mate, then lay eggs in Oak catkins or leaves, giving rise to the generation that create Currant Galls, containing both males and females.

The Spangle Galls occur on the underside of Oak leaves and there can be as many as 100 on a single leaf, each containing just one larva. They are flat discs with a raised centre and up to 6mm in diameter, starting out greeny-yellow and becoming red with age. The dots you can see on the 'spangles' are probably tufts of red hair that are a characteristic of this species

The photo to the right is of the Smooth Spangle Oak Gall which is created by Neuroterus albipes. I'll leave you to guess the common name of this wasp!! This saucer shaped gall is up to 5mm in diameter, and is formed by the larvae of mated females, producing only asexual female offspring. It is pale green and sometimes has a pinky-red rim, or blotches. The generation that emerges, one wasp to each gall, goes on to lay their unfertilised eggs in the margin of the leaf blades. The Schenck's Galls that result protect the offspring of both sexes. As with the Common Spangle Gall, the Smooth Spangle Gall occurs on the underside of Oak leaves, but not in such great numbers.

Whilst it may be that these organisms don't confer any benefit to the tree, those that have evolved alongside the Oak don't often cause lasting damage, and are part of a healthy functioning ecosystem. But unfortunately, we are seeing an increasing number of invasive species which our native trees are not equipped to deal with. Oak powdery mildew is thought to have originated in tropical Asia, and other diseases which include things such as Sudden Oak Death, are also caused by invasive pathogens. But we can help to protect the future of our precious trees by practising good biosecurity measures when visiting woods or parks, buying plants from trusted sources, and reporting any concerns to projects such as Observatree or TreeAlert.

Denise Long, September 2025

Friends of Portswood Rec, Southampton, UK