Gall wasps (Cynipidae), I have to confess I love gall wasps, there are loads of them in the UK, and each one creates a different gall. Telling the gall waps apart requires quite alot of expertise, the wasps are no bigger than fruit flies and look very similar, luckily the galls that form on the trees are very different. So if you know the Gall, you know the gall wasp that created it.
Marble gall Oak apple gall
So here's two I found on the same Oak tree, left is the classic marble gall, on the right is the oak apple gall, Both of these are last years galls that have clung on over winter with the occupants having left last summer of 2019.
So who was inside them, the left was a wasp called Andricus kollari, and on the right was a wasp called Biorhiza pallida, The marble galls contain 1 - 4 grubs, the apple galls contain 6 - 12 grubs.
Try not to think about Gall wasps as you might the classic vespa wasp, most gall wasps cannot sting and are tiny, 1.5 mm - 8 mm max, often a dull brown or black and pretty dull looking.
The wasps in both cases lays there eggs in the bud of the tree and this irritates the tree which in response grow the gall, similar to a grain of sand making an oyster grow a pearl. the grubs then hatch and chew their way around the gall until ready to pupate and hatch out.
However this is trickier than it sounds, it's all about perfect timing, The grub has strong mandibles, but the adult fly does not. The grub must anticipate when is the right time to chew a small exit hole, then pupate and crawl out through the hole as a fully developed fly.
If it doesn't chew a hole big enough, the fly is trapped inside the gall, if it doesn't have time to chew an exit hole at all, it will pupate into a fly and is sealed inside without the mouth parts to create a hole, sad. If it chews the hole too early, this gives time for other parasitic wasps to shove an ovipositor down the hole and bury a parasitic wasp grub in the poor gall wasp's grubs skull...nature, red tooth and claw.
Experiment time
I wanted to prove what I just said was true, so I went out and came back with two marble galls, one with no exit hole at all, and the other (experience tells me) had a exit hole that was too small.
Scientific high tech tool at the ready (the Xmas nut cracker) I broke them open. results below...
The galls of Andricus kollari the marble gall wasp, which requires two different oak trees to complete its life cycle, English Oak Quercus robur or Sessile oak Quercus patraea and Quercus cerris Turkey Oak.
Oh dear, as I suspected this grub was stuck inside the gall and it was dead, this is the Gall with no exit hole.
So here's two I found on the same Oak tree, left is the classic marble gall, on the right is the oak apple gall, Both of these are last years galls that have clung on over winter with the occupants having left last summer of 2019.
So who was inside them, the left was a wasp called Andricus kollari, and on the right was a wasp called Biorhiza pallida, The marble galls contain 1 - 4 grubs, the apple galls contain 6 - 12 grubs.
Try not to think about Gall wasps as you might the classic vespa wasp, most gall wasps cannot sting and are tiny, 1.5 mm - 8 mm max, often a dull brown or black and pretty dull looking.
The wasps in both cases lays there eggs in the bud of the tree and this irritates the tree which in response grow the gall, similar to a grain of sand making an oyster grow a pearl. the grubs then hatch and chew their way around the gall until ready to pupate and hatch out.
However this is trickier than it sounds, it's all about perfect timing, The grub has strong mandibles, but the adult fly does not. The grub must anticipate when is the right time to chew a small exit hole, then pupate and crawl out through the hole as a fully developed fly.
If it doesn't chew a hole big enough, the fly is trapped inside the gall, if it doesn't have time to chew an exit hole at all, it will pupate into a fly and is sealed inside without the mouth parts to create a hole, sad. If it chews the hole too early, this gives time for other parasitic wasps to shove an ovipositor down the hole and bury a parasitic wasp grub in the poor gall wasp's grubs skull...nature, red tooth and claw.
Experiment time
I wanted to prove what I just said was true, so I went out and came back with two marble galls, one with no exit hole at all, and the other (experience tells me) had a exit hole that was too small.
Scientific high tech tool at the ready (the Xmas nut cracker) I broke them open. results below...
On the left, is a gall with an exit hole I thought was suspiciously small.
On the right, the gall had no exit hole what so ever.
The larvae died before it chewed its way out to the edge and cut its exit hole. remember these are last years galls, not this years, so this little larvae should have exited last September, 2019.
This was not a surprise to me, i've been opening galls since i was a kid and you nearly always fine a dead grub inside a gall with no exite hole,
Below is something much rarer...
This is the gall with an exit hole which appeared too small, once I cracked it open I expected to see either a dead larvae, a dead pupae or a dead gall wasp.
Remember the larvae must chew an exit hole big enough to get out of once it has metamorphosed into and adult wasp, I.e. while it is still a larvae, because the adult wasp has no chewing mouthparts.
Experience told me this hole was too small, and sure enough, inside was a dead adult. meaning it create an exit hole, pupated into an adult but couldn't squeeze out of the hole. Here is the dead wasp, still inside the gall.
This what really surprised me....it wasn't dead.
It crawled out and sat on my finger tip. It was really skinny and would probably not survive, the abdomen, the last part behind the head and thorax should be round and fat, it should be in peak condition, most of the fat grub's (two pics above) energy should have been transfered to this wasp during pupation. But this one has been stuck inside this gall since September (8 months) the abdomen was thin and glassy transparence, poor thing.
This is how is should have looked, stock photo.
I could go on here, with Andricus quercuscalisis the Knopper gall wasp, Diploepis rosae the robin's pin cushin gall wasp, the spangle gall wasp Neuropterus quercusbaccarum, the red currant wasp Cynips divisa, there are 360 different species of gall wasp in Europe, 4 different spangle gall wasps in the UK alone (Chinery 2016).
On top of this there are then Sawfly which also make galls, one of which is very commonly seen on willow, plus gall making moths, aphids and leaf miners, et al.
In fact so many galls in the UK I could have done 24 blogs on just galls. Dull but true.
This is the gall with an exit hole which appeared too small, once I cracked it open I expected to see either a dead larvae, a dead pupae or a dead gall wasp.
Remember the larvae must chew an exit hole big enough to get out of once it has metamorphosed into and adult wasp, I.e. while it is still a larvae, because the adult wasp has no chewing mouthparts.
Experience told me this hole was too small, and sure enough, inside was a dead adult. meaning it create an exit hole, pupated into an adult but couldn't squeeze out of the hole. Here is the dead wasp, still inside the gall.
It crawled out and sat on my finger tip. It was really skinny and would probably not survive, the abdomen, the last part behind the head and thorax should be round and fat, it should be in peak condition, most of the fat grub's (two pics above) energy should have been transfered to this wasp during pupation. But this one has been stuck inside this gall since September (8 months) the abdomen was thin and glassy transparence, poor thing.
This is how is should have looked, stock photo.
I could go on here, with Andricus quercuscalisis the Knopper gall wasp, Diploepis rosae the robin's pin cushin gall wasp, the spangle gall wasp Neuropterus quercusbaccarum, the red currant wasp Cynips divisa, there are 360 different species of gall wasp in Europe, 4 different spangle gall wasps in the UK alone (Chinery 2016).
On top of this there are then Sawfly which also make galls, one of which is very commonly seen on willow, plus gall making moths, aphids and leaf miners, et al.
In fact so many galls in the UK I could have done 24 blogs on just galls. Dull but true.
Chinery, C. (2016) Britain's Plant Galls. Hampshire. Wild Guides ltd.
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