Dear Readers, this talk exemplified why I am loving this series so much. James Heal is such an enthusiast that although I knew next to nothing about plant galls at the beginning, by the end I was desperate for spring to come so that I could go out gall-hunting. Heal endeared himself to me greatly by saying that he had a dream of giving up his job in finance so that he could become a gall-mite specialist. Who wouldn’t rather be a gall-mite specialist is my question, and I’m an accountant too. But let’s see first of all what a gall is.
We’ve all probably seen plant galls, even if we haven’t been aware of it.
A gall is abnormal growth on a plant under the influence of another organism. So the ‘silk buttons’ above are actually created by the plant itself, due to chemicals produced by the invading organism. The gall involves the enlargement or proliferation (or both) of the cells or vascular tissue of the plant. This is produced for the nutrition and protection of the gall-inducing organism.
The great thing about galls (or one of the great things) is that they can be used to identify what caused them. Gall midges, for example (of which more later) are extremely difficult to identify to a species level from the insect, but the galls can be diagnostic.
So, how do you know what gall you’re looking at? Heal suggested a three-part approach:
Firstly, identify your plant. This might seem easy (‘It’s an oak’) but what kind of oak is it? There are some galls that we’ve all seen on lime leaves, even if we didn’t know what they were, but to identify the insect that caused them we need to know if we’re looking at a small-leaved lime, a broad-leaved lime or the very common hybrid between the two. Why is life never simple, I ask myself. Probably because it would be boring.
Having identified your species, it makes sense to take a good detailed note of where exactly on the plant you found the gall – leaf rib, body of the leaf, stem, bud?
Secondly, take a number of photos (if it’s a leaf, take both sides). If you can zoom in or magnify, that’s a good idea too. Many galls can be identified right down the species just through a good photo. If you want to become a serious galler (or Cecidologist) you might consider taking specimens and even rearing the inhabitants of the galls until they emerge, blinking, into the big wide world, but it’s not necessary for most people.
Thirdly, get yourself a good guide. If you are just beginning, Heal recommends this one:
But if you’re more serious, this is the one to go for, and apparently a new edition is due out soon.
And if you are really, really serious, the New Naturalist on plant galls has everything you ever wanted to know. This one is, I think, out of print, but you can get one-off reprints of New Naturalists if you go to their website, or second-hand bookshops will often have them. After all, plant galls are not the most apparently interesting of subjects, though after Heal’s talk I imagine there might be a run on the title.
Then, Heal moved on to talk about the different organisms that cause galls, and there are a fair few of them. After all, if you are a delicate little larva, how nice it must be to be surrounded by a robust protective covering while you munch away to your heart’s content, and many unrelated insect groups have taken this route.
First, we have the gall midges (Cedidomyiidae), members of the fly family (Diptera). These are tiny creatures, many of whom are less than a millimetre long. They have these remarkable antennae that look like strings of beads. Heal showed a number of photos of the midges causing the leaf to curl around: while we might see the caterpillars of species like the peacock butterfly making themselves a shelter by stitching the leaves of nettles together, with these midges the plant itself is persuaded to grow in an unnatural way. You can see a picture of ash mid-rib gall, caused by a gall midge, below.
Then there are the gall wasps (Cynipidae), who produce some of the most well-known and spectacular galls, such as the oak knopper gall in Photo Two, and the Robin’s pincushion on roses. Within that multicoloured mass of ‘hairs’ is a many-chambered gall, each containing a tiny wasp larvae. Heal points out that if you open up a gall caused by an insect, what pops out might not be the creature that made the gall but another species entirely that is either a parasite, a predator or some other kind of free-loader.
Then there are the gall mites (Eriophyidae), which are Heal’s favourite group, partly because they are so understudied and so it’s likely that there are some undiscovered species lurking in our back yards. They are not typically ‘mite-y’ looking, but, as Heal put it, they look more like carrots, long and slim. The gall that you might have seen most often is the Nail gall on lime leaves, which is rather beautiful in my opinion. Some of the galls contain microscopic ‘hairs’ which can be diagnostic for species. Some gall mites are pests of food crops, but others, such as the bindweed gall mite, are used for biological control of noxious weeds.
So, these are the three main groups of insects who can create galls, but there are many others; sawflies on willow, aphids on elm and some species of moth. Then there are the fungi that can induce galls. Mistletoe technically is a gall-causer – the tree produces distortion and swelling at the point of infection, induced by the parasite. And then there are endless bacteria and viruses that can cause galls.
So this was a most interesting talk by James Heal – he pitched it perfectly for beginners, I wouldn’t be the least bit surprised if a whole new bunch of gallers has been inspired by his presentation. I certainly learned a lot, and it’s left me with a whole lot of things to ponder. If you’d like to listen to the whole thing (which I would recommend) you can find the link here.
Photo One by Lairich Rig / Silk button spangle galls on oak
Photo Two by M J Richardson / Knopper gall on oak
Photo Three by By Alvesgaspar – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3187062
Photo Five by Trish Steel, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Photo Six By Rosser1954 Roger Griffith – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6902951
Photo Seven By Unknown author – Australian Insect Common Names: http://www.ento.csiro.au/aicn/system/c_2775.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5810228
Photo Nine by Lairich Rig / Leaf galls on crack willow