Oak trees and Knopper Galls
By Charlotte Owen, WildCall Officer
Next time you walk past an Oak tree, take a closer look and you might notice something a bit unusual. Perhaps some of the acorns are ripening nicely but others seem to have mutated into an abnormal, knobbly mess. What’s going on?
It’s all down to the larvae of a miniscule wasp. There are more than 30 different species of gall wasp that lay their eggs on oak trees, and each species produces a different type of gall - essentially a protective casing that shelters the wasp larvae as they feed and grow inside. The wrinkly mutant acorns are known as Knopper Galls and are made by the Gall Wasp Andricus quercuscalicis, which looks like a tiny black fly just a few millimetres long. Earlier in the summer, the females of this species laid eggs on the buds of developing acorns and a fascinating transformation was triggered as soon as they hatched: responding to a chemical signal released by the larvae, the oak tree was persuaded to sculpt its tissue into knobbly Knopper Galls in place of normal acorns. When there are just one or two larvae inside, the result is a fairly simple, folded structure but when several inhabitants are competing for space the galls look more contorted, like mini explosions frozen in time.
Knopper Galls are greenish-yellow and sticky at first but ‘ripen’ to a woody brown in early autumn, at which point they drop to the ground. The larvae pupate inside and spend the winter tucked up in their galls before emerging as adults the following spring. Interestingly, every single one of these emerging adults will be female. They form the first phase of the Gall Wasp’s two-generation life cycle, and the freshly-emerged spring females need to seek out a Turkey Oak to initiate phase two, by laying their eggs on its catkins. These develop inside a second type of gall, from which both male and female wasps will emerge. They form the second, summer generation and will go on to lay eggs on native Oak trees, creating more weird and wonderful Knopper Gall sculptures.
Comments
Nature is fascinating. I’ve seen these gall knoblers and wondered what they were so thanks for info.
20 Aug 2020 12:28:00
Agreed. The variety of life is just fascinating. And thanks for the amazing pictures
20 Aug 2020 13:32:00
How amazing! I have often wondered what the gall knoblers were, thinking they must be some kind of disease. I am astonished at how incredible and complex nature is. We know so little! Thank you.
20 Aug 2020 15:39:00
We have several oak trees in our street . The oak tree outside our house always has a heavy infestation of the gall wasp and therefore the Knoppler gall acorns . The other Oaks in our street do not seem to have these infestations . Do these infestations harm the Oak tree ? Very interesting article , Thanks
20 Aug 2020 18:34:00
@Byran Kiely: Although the galls have a negative effect on the reproduction of the tree, they don’t harm the overall health of the Oak.
20 Aug 2020 20:36:45
Fascinating, as everyone has remarked. Never heard of a Turkey Oak before though. Are they radically different from ‘ordinary’ oaks and is their distribution ratio similar?
20 Aug 2020 21:03:00
@Heather Hedges: The Turkey Oak was introduced as an ornamental tree in the 1700s, it is faster growing than native Oaks but of less value to wildlife. The Turkey Oak has an elongated leaf and ‘hairy’ acorn cups compared to native oaks. More info here
21 Aug 2020 08:10:00
Someone told me Oak Galls were used to make ink- is this correct?
21 Aug 2020 10:47:00
@ Jennie Lamb: I haven’t heard about using Knopper Galls for ink, but you can use Oak Apple Gall and Oak Marble Gall as they have a high tannin content
21 Aug 2020 15:03:17
I believe oak galls were used to make ink eg the Magna Carta
I recently found the following website very useful:
https://www.woodland-ways.co.uk/blog/primitive-crafts/oak-gall-ink/
A possible winter project!
22 Aug 2020 09:47:00
Their life cycle is fascinating but I don’t understand it. The phase two wasps must surely be from unfertilised eggs, so do both male and female wasps in phase 2 have only half the usual number of chromosomes? If so, what does this mean for the females emerging in phase one?
22 Aug 2020 11:29:00
Hiya there, can you please tell me if the resulting Gall that is left is beneficial nutritionally to the Britain’s Wildlife that usually feed on the acorns or do they not touch the Gall at all.
The abnormal growth of the galls create more nutrient rich material inside than in the surrounding plant tissue. This inevitably has benefits for other tiny insects who essentially have free, safe accomodation and easy food. In addition, gall insects may stimulate the production of nectar within the gall which lures nectar-feeding insects who may provide security from other predators....in much the same way as ants farming aphids and protecting them from predation. Some larger parasitic wasps certainly benefit from using the galls as living larders for their own young. Some of our mammal species may also use certain galls as a supplement to their diet during harder times, for example, rodents such as Wood Mice.25 Aug 2020 14:11:00
What do the galls on the Turkey Oak catkins look like?
09 Oct 2022 14:28:00
Sussex Wildlife Trust:
We don't have any images of these to show you, so you'll have to google I'm afraid!
Found one of these with grandchildren at local park, know about oak apples but had never seen one of these, thank you for the information.
15 Aug 2023 07:59:00
Old small oaks in Suffolk covered in knobblers. A lot single occupancy. Great to see so many.
30 Aug 2023 10:36:00
Just looked for the knopper galls and soon found two on oak in a local park in Falkirk. I knew they were galls but have now learnt about the life cycle of the wasps – thank you!
01 Sep 2023 13:34:00