Casuarina Glauca Galls Project
The other day i started my very first Inat project and gave it the really catchy title 'Casuarina Glauca Galls' https://www.inaturalist.org/projects/casuarina-glauca-galls
As an enthusiastic explorer of Casuarina glauca/ Swamp Oak Forests I'm intrigued by these rather mysterious tiny structures.
Mysterious because I've hit a brick wall with identifying the inducers of many of the galls I've observed beyond Pterygota or Hymenoptera. It may be that I don't have access to the information (I'm not a scientist) or it may be that it doesn't exist. The latter seems unlikely given the prevalence of the galls around here. However I've been assured that when it comes to galls there may be little or no information to be found.
Nevertheless I find it hard to believe that this aspect of the ecology of a tree so common in Australia's biggest population centre could be so overlooked and under-observed. Perhaps it's just that I'm used to identifying plants not galls.
Though Swamp Oaks may be common the Swamp Oak Floodplain Forests of N.S.W are predicted to become extinct at some point in the future, so it seems like a good idea to document as much of the biodiversity surrounding C. glauca as possible.
While I expect this might be a mostly solitary pursuit, please join and keep an eye out for glauca galls. If you happen to see any do contribute to the project. It would be wonderful to have more observations from other people and places. I hope one day we'll be able to identify more of them, but for now I'm just going to enjoy the mystery.
Here's a bit of information about a gall you won't find on Casuarina glauca, although it is known as 'Casuarina gall':
https://www.inaturalist.org/projects/casuarina-glauca-galls/journal/73965-the-deceptively-named-casuarina-gall-cylindrococcus-spiniferus
Observations
Photos / Sounds
What
Ants, Bees, Wasps, and Sawflies (Order Hymenoptera)Observer
vavilovian_mimicDescription
Casuarina glauca gall
Photos / Sounds
What
Ants, Bees, Wasps, and Sawflies (Order Hymenoptera)Observer
vavilovian_mimicDescription
Casuarina glauca galls
Comments
Hi there
I'm no expert on galls by any means, but I might be able to provide some insights on what to expect from gall formation in a more general sense
From a phylogenetic standpoint, the phenomenon you refer to as fluffy galls could likely be an inheritable disease or susceptibility to fungal attack in the host plant. I say this because this formation looks quite similar to the Witches Broom disease which attacks the secondary growth clusters (leaf bases) of certain Protea species here in South Africa, which one plant has a probability of passing on to its progeny
If I had to take a wild guess, I'd say the disease or fungus in the host plant proliferates in the subdermal tissues and effects a gradual channeling of the inner pith layers through the gaps in the vascular bundles and ever closer towards the epidermis, stopping just somewhere short. The result of this ongoing process would likely start to manifest as what you refer to as the club galls, wherein the usually fleshy leaves become evermore brittle as the vascular bundles are throttled and the pith swells. At this point, its possible that a tiny insect such as a gall wasp might have the capacity to select for plant parts in just the right stage of this process, perhaps based on surface texture or maybe olfactory cues. Once the wasp has selected the ideal 'swelling', it should be able to mine through the now drier, chalky interior of the gall where it lays eggs. The larvae then hatch, feed on whats left inside the gall, and then dig their way out to pupate and fly off from the surface (i.e the dried up gall with holes in your last photo)
Like I say, this is all just hypothetical and I could well be way off the mark as Australian gall ecology might be startlingly different from what you find in South African vlei's and grasslands for all I know. Nevertheless, this should hopefully provide a useful template to compare against other gall observations you will make in future, I certainly hope it does help :)
I'd certainly not be able to try ID the species of gall wasp responsible for this, you'd need a proper entomological boffin born and bred in your country for that. Your gall observations are really cool, so please do share more with us! You're welcome to follow me if you'd like to see all the plants and critters we have here in SA
All the best with your swamp adventures! :D
Regards
Ant
@anthonywalton
These are really great ideas, Ant, you've opened my mind to some really interesting possibilities. You might be on to something.
Casuarina glauca does get witches brooms and there is actually a wasp that seems to have a preference for them, Selitrichodes utilis! It is not an obligate relationship, but whenever i've seen Selitrichodes galls densely it's on witches brooms. See https://inaturalist.ala.org.au/observations/145570832. It has been suggested by a Casuarina expert that perhaps the witches broom branchlets are softer and easier for adult wasps to penetrate. These particular witches brooms are probably formed by phytoplasma, but I don't think anyone knows that for sure.
It seems very plausible to me that the fluffy ball galls could be fungal or something like that. It does seem to make more sense than wasps making galls on top of other wasp galls. On the other had I did finally dissect some fluffy ball galls and they do seem to have distinct chambers like (i think) an insect gall would have.and one had a larvae.This doesn't really prove anything but it does make me lean toward thinking the fluffy ball galls are insect galls. I will upload observations soon
Yeah i definitely need to make friends with a local entomologist : )
Thank you so much for your generous thoughts, and encouragement, you've given me a lot to think about. I will certainly be following your South African adventures!
Jade
Glad I could lend a small hand :)
And as you say, yes - it could very well be that the witches brooms on your side have been visited by a disease/fungus bearing midge, more than likely the same kind that will later revisit that plant once its 'tool' has taken effect to make a nest out of it. Outside of progenic dispersal, this could plausibly be the cause of additional spread of WB here in South Africa too, though I haven't studied nearly enough of the topic to say for sure
If you'd like to know more about the insects that most often cause galls on plants and the relationships between plant and insect, this should prove some very insightful reading for you: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1570-7458.2008.00726.x
I know there's a site where you can get pretty much any scientific articles for free and completely circumvent this publisher patronage network, I'm sure you will find this article there too - please tell me the name of the site if you find it :)
Enjoy
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