For just over two years now I’ve been a trustee of the Findhorn Hinterland Trust, a local charity that manages about 35 hectares of land surrounding the Findhorn Community, where I live. The site includes sand dunes and dune heath rich in lichens, dune scrub consisting mostly of gorse, species-rich grassland and an old pine plantation that is gradually being restored to native woodland.
I was invited to join the board of trustees because of my 30 years of experience with Trees for Life (the charity I founded), and also for my knowledge of ecology and the restoration of natural ecosystems. At the time of joining I said I had a particular interest in identifying and documenting the biological diversity of the area, and that has subsequently become one of my main contributions to the Trust.
I’ve organised a number of specialist surveys of the area, including ones for Diptera (two-winged flies) and spiders, and have also been building up records myself of various groups of organisms, such as fungi, aphids and beetles. The closer I look, the more I’ve been finding of interest on the site, and this was epitomised for me recently when I spent some time with a spear thistle (Cirsium vulgare) in the middle of July.
Spear thistle is a plant that I haven’t paid as much attention to in the past as I have to its close relative the creeping thistle (Cirsium arvense), which is more common and abundant here. Another reason for this is that creeping thistle commonly hosts two different species of aphids – a group of organisms that I have a particular interest in – whereas it’s rare to find any of them on a spear thistle.
So it was that when I came across this spear thistle (which was in fact actually a cluster of three different thistles growing closely together) on the Hinterland area in July I felt drawn to have a closer look. One of the most obvious features of this species are its highly tapered leaves, which end in sharply pointed tips. In this they are said to look like the tips of spears and it is this resemblance which gives the thistle its common name.
The thistle is well known for being the national flower of Scotland. However, it’s never actually been specified which of the various thistle species growing in the country is the subject of this accolade. The images that are used to represent the national flower though are most commonly those of the spear thistle. This is probably because of the size of its flowers, which are larger than those of other thistles, such as the creeping thistle, and the dramatic visual contrast their richly coloured but delicate blossoms make with the spiky balls from which they emerge. It was the presence of the flowers that attracted me to look more closely at this group of thistles, as I was hoping there might be some insects visiting the blossoms. As it turned out, I found lots of interest there and ended up spending a couple of hours with the plants altogether!
Spear thistle has been shown to produce large quantities of nectar for insects, to entice them into pollinating its flowers, and it is one of the top plants that have been studied in the UK in terms of its nectar production. I was hoping therefore that there would be quite a lot of insect activity associated with it, and I wasn’t disappointed. Almost immediately I noticed some small shapes amongst the spines on one of the unopened flower buds.
I recognised these as being the nymphs of a bug, and I recollected having seen something like them on spear thistles before. Looking more closely I noticed that they were on most of the flower heads, both those that were already in flower and the others that hadn’t opened yet. I also spotted what were clearly some adult bugs in amongst the nymphs, and when I was back at home later I was able to identify them quickly as being examples of the spear thistle lacebug (Tingis cardui).
As its common name suggests, this species is specific to spear thistles, and only occurs on them, although a closely-related species is found on creeping thistles. The adults have an intricate lace-like pattern on their wings, and it’s this that gives rise to their English name. The species occurs throughout most of Europe and also in North Africa, and they were certainly abundant on this cluster of thistles.
As I watched them on the thistle flower heads, another insect caught my eye as well. This was no bug however, but a fly, and it had beautiful, seemingly rainbow-coloured eyes, which were what drew my attention to it. As I watched it, the fly seemed to be taking its time to explore the flower bud it was on, so I suspected that it had a special interest in, and relationship to, the thistle as well.
I also noticed that it had a distinctive tube-like structure at the rear of its abdomen, so I suspected that it must be a female, and that was its ovipositor, or egg-laying organ. After doing some research on the Internet when I got back home, I provisionally identified it as the thistle gall fly (Terellia serratulae), and this was subsequently confirmed by Peter Chandler, an expert in Diptera, or two-winged flies, who helps me with their identification.
Peter said that there are not many records in Scotland for this species, although there are at least a couple from this part of the Moray coast, which I confirmed by checking its distribution on the NBN Atlas. The larvae of this fly induce galls to form on the thistles, and this female was presumably searching for good sites to lay her eggs. I watched the fly for several minutes, taking various photographs without seeming to disturb its searching behaviour at all.
After a few minutes the thistle gall fly left, but almost immediately another fly landed on one of the thistle flower buds. This one didn’t look as distinctive, but it still seemed to take a particular interest in the thistle. When I sent photos of it to Peter he identified it as a more widespread species, the dumbell-spotted flutter fly (Palloptera modesta), and said that its larvae also develop in the flower heads of thistles (and knapweeds). I suspect it was laying eggs when I spotted it.
Three days later I made a return visit to the same thistles in order to get some more photos for the blog I was planning to write about the life I’d seen there. I was also interested to see what, if any, changes had taken place in the intervening period, such as some more of the flowers having opened. In fact, the thistles looked very similar to how they’d been on my earlier visit, but my experience there this second time was completely different.
Almost immediately I spotted a spider sitting motionless on one of the lower leaves, and it looked to me like the nursery web spider (Pisaura mirabilis). This is one of the most distinctive and easily recognisable spiders in Scotland, and its identity was confirmed for me later by Edward Milner, a friend who is a spider expert and helps me with identification queries.
While I was looking at the spider, I noticed something red moving around out of the corner of my eye. Shifting my attention to it, I saw that it was a harvestman, Mitopus morio, the large harvestman that is common throughout Scotland, and that it was covered in the larvae of parasitic mites that are bright red in colour. I’ve often seen one or two of these mites attached to this species of harvestman, but this one, remarkably, had nine on it!
In the past I’ve sent specimens of these mites to Joanna Makol, a specialist in parasitic mites who works at a university in Poland, and she’s identified them as being in the genus Leptus, and usually a species called Leptus phalangii. I suspected that was what these ones would turn out to be as well.
The mite larvae attach themselves to the leg joints of the harvestman, feeding on its haemolymph (the invertebrate equivalent of blood). As parasites, they do not kill their host, and eventually drop off to pupate into adults that live as predators of other small invertebrates. Because of the way in which the harvestman’s body is structured, and in particular the articulation of its legs, it’s impossible for it to remove the mites.
Harvestmen like this appear to survive the effects of the mite larvae all right, but they must be weakened, to some extent at least, by the loss of haemolymph. After I’d photographed it, I collected the harvestman in a specimen tube and later, when three of the mites had dropped off it, I released it unharmed (and lightened of its load of parasites!) and sent the mites to Joanna in Poland for identification.
It seemed as though this experience had got my eye in for mites, as the next thing I spotted on the thistles was an adult mite, moving very quickly across the underside of one of the leaves. This was also reddish in colour and I recognised it as being one of the so-called whirligig mites (Anystis spp.), which gain their common name from their rapid, almost circular-like pattern of running. I suspected this was the most typical and abundant species (Anystis baccarum), but unfortunately I’ve not yet found an expert who can identify this type of mite for me.
I was rather surprised not to have seen any flies or other pollinating insects such as bees yet on the flowers, but eventually I did notice one fly that had landed on a leaf of the plant. It looked quite like the thistle gall flies I’d seen earlier, with the same rainbow-coloured eyes, but it didn’t have the brownish tube at the rear of its abdomen, so I wondered if it might be a male of that species (Terellia serratulae). That identification was confirmed for me by Peter Chandler when I emailed him a photograph of it later the same day.
Because of my interest in aphids I’ve got into the habit of checking the underside of the leaves of plants, as that is where many aphids feed, to reduce their visibility to the birds and other creatures that prey on them. There were no aphids on these thistles, but I’d already found the whirligig mite on the underside of one leaf, and underneath another I discovered a nicely-coloured spider. This was identified by Edward Milner as the common candy-striped spider (Enoplognatha ovata).
There were still just as many of the spear thistle lacebug nymphs on the flower buds of the thistle as there had been three days earlier, and while I was looking at some of those I noticed another spider, clambering around amongst the spines on one of the buds. This looked different to the two spiders I’d already seen, and was subsequently identified by Edward as being a juvenile female crab spider in the genus Philodromus. He said it was probably a common species, Philodromus aureolus, but it’s impossible to determine the species in the juvenile stage, as the identifying characteristics haven’t developed yet.
I’d been surprised not to have seen any pollinating insects on the thistles, so I made another visit to the same patch a week later, at the beginning of August. I had to pass that way anyway then, in order to carry out a monthly servicing of some spider pitfall traps that we’ve installed on the land, as part of our ongoing survey of spiders there. When I got there, the first thing I noticed was that some of the flowers were now wide open, and I realised that they had only been partially open before, with the stamens all bunched together tightly.
Now, by contrast, the stamens were all spread out, and the pollen grains attached to them were clearly visible, and of course much more accessible to visiting insects. It seemed clear to me that this was why I hadn’t seen any pollinators before – the flowers hadn’t opened up fully enough to enable them to access the nectar that they come to feed on.
Almost as if to prove the point, and exactly on cue as I watched, a buff-tailed bumblebee (Bombus terrestris) landed on one of the flowers and buried its head in amongst the stamens. It was searching for the nectar that the plant provides as the lure to attract pollinators like this, and as it moved around on the flower its body quickly became coated with pollen grains.
The structure of the thistle’s flowers and the hairiness of the bee’s body are a classic example of co-evolution, in which these two different organisms have evolved in such a way that their interactions provide benefit to both of them.
Visiting bees have to reach down past the pollen grains on the stamens to access the nectar, resulting in the grains getting stuck to the hairs on the bees’ bodies, and especially their legs.
When the bumblebee flies off to another thistle flower, some of the pollen attached to its legs and body will brush off as it feeds there, thus achieving the thistles’ goal of cross-pollination between different plants. The bumblebee in turn receives the energy it needs for flight and reproduction etc from the nutrient-rich nectar provided by the thistle.
As I watched, several other buff-tailed bumblebees visited the various flowers on the thistle. They weren’t the only visitors though, as I spotted a tiny beetle crawling amongst the stamens on one of them. I’ve yet to get the beetle identified, but I’m including the photograph here anyway, as it shows the challenging topographical terrain that the beetle has to negotiate, because of its very small size, relative to the flower. I don’t know whether this tiny beetle will also provide the service of pollination for the thistle, but hopefully that will become clear once I’ve got a positive identification for its species.
Elsewhere on the thistle I came across another, larger beetle, or at least the corpse of one. It was covered in silk strands so it must have got caught in a web from one of the spiders living on the thistle and been unable to escape. Although I couldn’t see enough to be certain, I assume the spider would have eaten the soft inner parts of the beetle, just leaving its indigestible exoskeleton behind.
The thistle certainly seemed to be a good hunting ground for arachnid predators, as I came across another specimen of the same species of harvestman (Mitopus morio) as before. This one also had some red mite larvae attached to it, but only two instead of the nine I’d seen previously. Harvestmen and spiders are both Orders in the same Class, Arachnida, in the biological system of classification of living organisms. This also includes mites, and all three of these Orders are characterised by having eight legs (whereas insects have six legs) and a predatory lifestyle.
When I first stopped to look at this group of thistles, it was the nymphs of the spear thistle lacebug that initially drew my attention. They were still there almost two weeks later, although there did appear to be proportionally more adults than I’d seen the first time around. It was good to reconnect with them again, and to appreciate the journey of discovery they had led me on, as I spent more and more time with this cluster of three spear thistle plants. Somehow, my experience seemed deeply symbolic of what has happened for me (and I know for many others as well) during this time of the COVID-19 lockdown in 2020.
Unable to make the journey to my usual sites such as Glen Affric because of the travel restrictions that have been imposed as part of the lockdown, I’ve spent more time instead getting to know Nature in the immediate surroundings of where I live. I’ve explored the Findhorn Hinterland area in much greater detail than before, and with this group of thistles in particular I’ve come to a deeper appreciation of just how much life, and interesting ecological relationships, there can be on what many people might consider to be a rather unassuming and lowly plant.
In writing this blog I’ve gained a greater understanding of some of the diverse and varied life the spear thistle supports, and I hope that it may have a similar effect for others as well. I’ll finish now with a photograph that for me sums up and symbolises the richness of the experience I’ve had – the buff-tailed bumblebee covered in pollen on one of the thistle flowers.