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Galls galore in the Great North Wood

Posted: Tuesday 28th May 2019 by Sam-Bentley-Toon

Knopper gallKnopper gall

Oak trees, of which there are many in the Great North Wood, are host to several species of a fascinating group of plant parasites called gall wasps. These tiny, stingless wasps produce bizarre swellings and protuberances on the buds, leaves, flowers and fruit of oaks called galls. Though you might easily overlook them, these outlandish formations once held a surprising degree of cultural and economic significance.

Virgin births

The life cycles of the gall wasps are as strange as the structures they produce. Many species have a two-stage life cycle with alternating sexual and asexual generations.

For example, the wasp known only by its scientific name Andricus kollari has an all-female generation that can produce eggs without the need to mate through a process known as parthenogenesis. This asexual generation lays its eggs in the buds of the Turkey oak, a species introduced to the UK from south-eastern Europe and Asia Minor in the 18th century. As the wasp larva develops, it releases chemicals that modify the growth of the bud that houses it to produce a gall. The gall concentrates the plant’s resources, creating an ideal microhabitat where the larvae can feast on plant tissue and remain relatively safe from predation.

A sexual generation that includes both males and females emerges from the galls of the Turkey oak. These wasps mate and the females lay their eggs on the buds of our native oak species: the English oak or the sessile oak. The resulting larvae each form a smooth round marble gall that starts green and matures to a rich brown in late summer. Once they have completed their metamorphosis the all-female wasps emerge through a neat hole to begin the cycle again.

As an aside, some animals, including certain species of whiptail lizards, have entirely dispensed with the need for sex (and males!) and reproduce solely through parthenogenesis.

Inquilines and parasitoids

Gall wasps, on having made their homes of tasty plant tissue, may find themselves far from alone. They are often joined in their feasting by a variety of inquilines. This is the name for animals, in this case insects, that take-up residence in the homes that other animals have made for themselves. In a gruesome turn these inquilines, and the gall wasp larvae themselves, may become host to parasitic wasps that lay their eggs inside the insect’s bodies. The wasp’s larvae then slowly consume their hosts from the inside out before emerging as adults. These kinds of parasites are known as parasitoids because they ultimately kill their host.

Nuisance knopper galls

Most oak galls have no measurable effect on the health of their hosts. This cannot be said for the knopper gall, which turns acorns into sticky brain-like excrescences. Once affected, acorns are no longer viable and will not germinate. This could be bad news for oak trees, which are already struggling to reproduce; light-hungry oak saplings are not doing well in our increasingly shady and under-managed woods. This problem is exacerbated by oak mildew, a fungal disease that arrived at the turn of the previous century and causes a pallid bloom on young leaves, which is thought to further reduce oak’s capacity to tolerate shade.

Like the marble gall, knopper galls require the Turkey oak to complete their life cycle. For this reason we have been clearing Turkey oak from the Great North Wood in an attempt to reduce the effect of the gall, and also allow more light to reach the woodland floor.

“In black ink my love shall still shine bright”

Shakespeare would have penned these indelible words using ink made from oak galls. Galls contain high concentrations of tannic acid that when combined with water and green vitriol (AKA Iron(II) sulphate) yields a pale grey solution that darkens to an intense purplish black on contact with air. Many ancient texts were written using iron gall ink including the Magna Carta and the Domesday Book, which incidentally contains an early reference to the Great North Wood.

Unfortunately iron gall ink pales with age and damages the parchment on which it is written, sometimes turning the page to lacework where the letters dissolve completely. Despite this it remained popular up until the 20th century and can still be bought today.

Shick-shack day…

…otherwise known as Oak Apple Day is named after a gall of the same name that resembles a spongy apple, replete with a rosy glow. The day is observed on 29th May to celebrate the restoration of the monarchy in 1660. Traditional celebrations include wearing an oak apple or sprig of oak leaves in reference to the occasion when Charles II hid in a hollow oak tree to evade the Roundhead army in 1651. Those found without the correct attire on oak apple day incurred the penalty of being whipped with nettles, or pinched, which gives the day its alternative name of pinch-bum day.

Whatever your opinion of the monarchy or its restoration, next time you’re in one of the remaining fragments of the Great North Wood, take a moment to examine an oak branch and see if you can spot any of the myriad shapes of the galls I’ve mentioned or others like the spangle gall, artichoke gall or ram’s-horn gall.

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