This month Dr Ben Aldiss explains all about the gall
We’re on the letter G for galls with Dr Ben Aldiss
Galls are a reaction by a plant to a parasite and can be seen as conspicuous growths on the stem, leaves or flowers. Everyone is familiar with the various galls on oak trees and the witches’ brooms of silver birch, but there are many others, all with fascinating stories behind them.
If you saw my introductory piece last June, you will know that my interest in natural history really took off in Great Snoring when I was about eight years old. During my frequent rambles to Walsingham and Wells in my hunts for moth caterpillars, galls inevitably drew my attention.
It would be a further two decades before this fascination was rekindled. I’d left Norfolk for Worcestershire and was teaching biology at Malvern College.
It was here that I was lucky to meet the legendary Arnold Darlington – one of those impressive naturalists with an encyclopaedic knowledge of wildlife along the lines of Norfolk’s much-missed Dick Bagnall-Oakeley or Ted Ellis.
He’d long since retired from teaching when I knew him, but what he didn’t know about galls wasn’t worth knowing. In fact, his book on the subject, first published in 1968, is still widely recognised as one of the best.
So what makes a gall? Arnold explained to me that the parasites
responsible should be called gall causers – not gall makers. His reasoning was that it is the plant itself that forms the gall – the parasite is simply the stimulus.
Gall causers produce chemicals that alter the growth of the plant in their favour, providing shelter and food. As we’ll see later though, this seemingly fool-proof protection is far from infallible.
Most galls cause relatively minor damage to their host plant, but one, the familiar marble gall of oak trees, grows in place of acorns and in some years occur in such numbers that the yield of acorns is significantly reduced.
The gall was deliberately introduced from the Middle East into Devon around 1830 as a source of tannic acid for the dyeing and ink-making industries. The insects causing the gall were tiny black wasps called Andricus kollari, some of which inevitably escaped into the wild and spread rapidly through Great Britain.
By the mid 19th century marble galls were the subject of huge press controversy, as it was feared they could ruin the acorn crop, depriving pigs of a valuable source of fodder.
Another well-known gall of oak is the oak apple. Unlike the hard, smooth, round marble gall, this one forms in place of a leaf bud and rapidly grows from May until it is fully formed in June or July. At this stage it is spongy, often a rosy pink colour, and up to 4cm in diameter. If you open such a gall, you’ll find around thirty small, whitish grubs inside, which if left unmolested, would emerge as tiny adult wasps later in the summer. The gall then turns black, shrivels and remains on the tree throughout the winter.
This may seem to be a relatively straightforward life-cycle, but no – in truth it is far more complex than this. The adults that emerge from the oak apple are of a single sex.
All from one gall will be male, whilst from another they’ll all be female. How this happens is far from certain.
Mating takes place in July, after which the female wasps dig into the soil at the base of the tree and lay eggs in tiny rootlets of the oak. Each egg hatches into a grub which causes a spherical brown root gall to form.
In this protected environment the grub grows slowly throughout the autumn and winter and throughout the following year, finally emerging as tiny wasps at the end of the second winter. Unlike the male and female wasps that hatched from the oak apples, these are wingless and all female.
They climb up the tree trunk in spring and lay unfertilised eggs at the bases of leaf buds to create the next generation of oak apples.
The galls on wild roses are perhaps the most beautiful of all those found in Norfolk, being a mass of fine hairs that turn a rosy red by September. They are commonly known as Bedeguar Galls, Moss Galls or Robin’s Pincushions – after the mythical woodland sprite Robin Goodfellow.
Each pincushion is the temporary home of around 60 gall-wasp larvae, which hibernate in the gall once it has shrivelled and turned black. They emerge as adult wasps the following May.
Gall-wasps and other gallcausing insects would seem to have the perfect lifestyle, but, as I mentioned earlier, it’s not always a rosy existence. Nearly all gallcausing creatures have their own predators and even parasites.
The wasp grubs causing the Robin’s Pincushion are somehow located and attacked by at least three other, even smaller wasps, whose grubs live in their bodies and eventually kill them. Amazingly, another truly tiny wasp is a parasite on these parasites.
Known as a hyperparasite, it waits until the Pincushion grubs have been attacked, then lays its own eggs. The resulting larvae then eat both the others!
From the fascinating galls in our local hedgerows, next month we turn our attention to the hedges themselves.
Nearly all gall-causing creatures have their own predators and even parasites
ABOVE: Oak galls ABOVE RIGHT: Bedeguar gall on a wild rose