A previous year’s gall on a Thimbleberry cane. The holes are where the occupants left when the gall matured. (Image: Brendan McGarry)
Do you ever go outside to get something from your vegetable garden and stand up a half an hour later in a haze of naturalistic wonder? My partner calls it distracto-boy, and suggests I have ADD — which may be a good moniker and a not impossible diagnosis. Mostly I just think I have (a largely) undivided attention for nature.
My most recent spiral was initiated by several large bumps on the stems of the Thimbleberries, Rubus parviflorus, I planted in our yard a few years ago.
Despite trying to train myself to not lose my mind whenever I see a blemish on anything I’m growing (because mostly this is just a good sign that a plant is being used by other species around it), I couldn’t help but feel an initial bit of horror. I knew these bumps were galls, but I didn’t know if this was a death knell for the Thimbleberries I’d been lovingly watching grow over the past three years.
Galls are abnormal growths found on plants, typically on twigs, leaves, or roots. They are not dissimilar to a tumor or a wart, except they are always caused by an external organism or a virus. Call them gallformers, (yes, there is a website by that name dedicated to galls). These growths can be created by a host of different gallformers — insects, fungi, bacteria, mites, nematodes, other plants, and even viruses. In many cases there are specific species that only target a single plant and have their life histories entwined with that host.
Typically galls house and feed the gallformer or its offspring. The thimbleberry gall was most likely created by a small wasp called the Thimbleberry Gall Wasp, Diastrophus kincaidii. This small, solitary wasp inserted its ovipositor, (what we’d generally think of as a “stinger”) into the stalk of the thimbleberry and induced a reaction through a cocktail of chemicals and a bundle of eggs. This initial contact is followed by the abnormal differentiation of the plant’s cells, a response that starts to form the gall structure. Eventually as the larvae inside grow, the gall reaches maturation, and they emerge as the gall begins to dry out, or dehisce. When a gall is formed on a deciduous leaf, these structures eventually decay along with the leaf, but in the case of my thimbleberries, when I looked a bit closer I saw old galls now dried along the older stems now covered in the bark of two year-old stems.
The key to gall formation is to harness a plant’s natural growth patterns and guide them in a different direction. Inducing this morphogenesis usually involves an arthropod’s saliva, such as in midges or aphids, or an injection from an ovipositor, as wasps and sawflies are known to do. Tricking a plant into a rapid response is best achieved in a period of fast growth like we see in late spring and early summer on the Hill. Plant hormones like auxins that regulate leaf out and growth are often roped into rapid production of galls by their parasites. Amazingly, in many cases, the gall not only provides a safe shelter, but the plant is also induced to generate a flow of nutrients for the creature it is hosting. By late summer many of these galls are just maturing or may have already nursed a mature insect into being. (But of course, with diversity there is always an if, or a but, so like many a Pikes/Pines, these descriptions are the most shallow of dives.)
A gall on a willow I have tentatively identified as being from the sawfly genus Euura. Sawflies are hymenopterans, related to ants, wasps, and bees. They look like wasps but they aren’t. Anyway – look at this fussy ball, covered in “hair” to make it less palatable, it truly looks nothing like the Scouler’s willow, Salix scouleriana, it’s on. But it’s also entirely willow plant cells transformed by the sawfly. (Image: Brendan McGarry)
Several common groups of plants on the Hill host obvious galls, like the rose family (of which thimbleberries are a part of), willows, poplars, and oaks. These can span from bizarre, multi-colored puffballs to small bumps on a leaf. In the week following my initial encounter I found several galls on roses and willows. Because different species of gall inducers have specific methods and associations with certain plants, the diversity of shapes, sizes, and host plants are entirely relevant to understanding who is living inside these abnormal organs. Oaks are particularly famous for their galls and a single oak individual can host multiple species just in its leaves and stems. If a gall is on the upper or underside of a leaf, a different shape or size, it’s entirely likely that different species are bedded down in each example. (Check out the sweet video below for a view into how varied galls can be!)
For the gardener who has specimen plants, a bunch of galls can be rather alarming. Thimbleberries are cane forming, multi stemmed members of the rose family, so I had a feeling it was no big deal for them and that control, if needed, would be as simple as removing this year’s galls and disrupting the lifecycle. But the real answer is that galls are hardly a problem in the vast majority of circumstances. After reading about them, it didn’t take much convincing for my ecologically bent heart and mind to see galls as symbols of the biodiversity I am trying to lift up with my gardening activities.
Personally, I see these strange growths as weirdly beautiful, benign houses for the creatures I want to share this planet with.
The horticultural and arboricultural world talks about so-called problems with plants as much as celebrates the beautiful connections between them and the communities of organisms they support. But don’t be confused by discussions of “control,” there are very few situations where a healthy plant is debilitated by gallformers even if they might seem unsightly at first. Situations where galls become problematic for a plant are often when they meet an organism they have no natural defenses against – a good example is of jumping gall wasps, Neuroterus saltatorius and Garry Oaks, Quercus garryana, growing in the northern parts of their range.
The real answer is that galls are just part of the whole deal with plants living in a community. What’s more, galls have a whole host of organisms that center in on them. These glaring structures are easy pickings for parasitic wasps, who inject their eggs into the larvae growing inside. Many birds feed on galls, gobbling down whoever is living inside whole with a side of plant cells, or by picking them apart to reveal a tasty treat. The drama! The biological controls!
I collected leaves from the same willow as the Euura gall above with these different gall on them. Because the leaf has been drying up, I am fairly certain the larvae inside are starting to consider emerging. Either way, you can peer in and see the curled up larvae inside this gall! (Image: Brendan McGarry)
By total happenstance, as I researched and wrote this, I found out it was Gall Week on iNaturalist.
While it’s pretty self explanatory, this week-long event signals the fact that it’s a good time to go out and look for galls and support community science simultaneously (here’s part one of a great multi-part youtube series on galls and a free pdf on them too).
In doing so, you’ll start to notice not just galls but all sorts of things on plants – from the strange u-shaped cutouts on leaves from leafcutter bees, to deciduous foliage that has started to succumb to fungal colonies as they reach the end of their tenure. All of these species have interesting stories that are constantly evolving, many of which are yet fully understood, even just off our back patios and in our parks.
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