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This month Dr Ben Ald­iss ex­plains all about the gall

EDP Norfolk - - INSIDE -

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

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