Although I’ve made hundreds of trips to Glen Affric over the past 40 years, every time I go there, I have a different experience and see some new things. When I went out for a day in the middle of July, I stopped as I often do along the public road between Badger Falls and Dog Falls, as that is the richest and most biologically diverse area in the glen. As I walked along, my eye was drawn to the leaves of a young alder tree (Alnus glutinosa) that was growing right beside the road, as they were covered in brightly-coloured galls. This casual observation then led me to spending a couple of hours with the alder, as I discovered more and more of interest on this one tree – enough to fill this blog!
The galls were quite familiar to me, as they are common on alder trees, and are induced by tiny mites (Eriophyes laevis inangulis), the larvae of which develop inside the colourful bulbous swellings that protrude upwards from the affected leaves. The galls vary in colour from a pale yellowish-green to bright pinkish-red, and some of the leaves had dozens of them, covering much of the surface area. Interestingly enough, it is only some alders that harbour these galls, and it’s also only some leaves on each tree that host them. I’m not sure if there is a satisfactory explanation for this irregular distribution, although it must have something to do with where the adult mites deposit their eggs.
With regard to the colour of the galls my observation is that they often seem to be more yellow when they are younger, and that as the galls develop further they tend to become bright red, almost like a fruit that is ripening. I’m not sure if that is always the case, and if so, I don’t know what the evolutionary benefit of the colour change would be.
There is still much that is not known about the development of plant galls, but these ones had drawn my attention to this young alder tree, and I then began to look more closely at its leaves.
I noticed one leaf that had a number of irregularly-shaped holes in it – a clear sign of feeding by an invertebrate of some sort. Looking more closely, there was a beautiful metallic green beetle on the leaf, right in between some of the most damaged parts. It’s quite a distinctive beetle that I’ve seen on alder trees before and is known scientifically as Chrysomela aenea. It doesn’t appear to have a common name, although I’d suggest alder leaf beetle as being a good candidate for that.
Looking in more detail, I noticed that a number of the leaves on the alder had similar feeding damage on them, and I spotted several of the leaf beetles, both on the leaves and on the stems of the tree.
Some of the beetles were situated right next to the holes in the alder’s leaves, and there was an abundance of frass (as the waste product of larval invertebrates is called) also visible. My intuition was that it was the larvae of these beetles that had been feeding on the leaves, and the adults had likely only recently appeared after the metamorphosis of their pupation stage.
I then began searching the leaves to see if I could find any larvae. Although I didn’t see any, I came across what I recognised as a pupal case – the structure within which an invertebrate undergoes the transformation from its final larval stage, or instar, to its adult form. This looked to me to be about he right size for one of these leaf beetles to pupate in, and after photographing it, I collected it in a tube, hoping to get confirmation of its identity in due course.
I was planning to send this specimen, plus any others I collected, to Sholto Holdsworth, a specialist in Coleoptera (ie beetles) who works at the Natural History Museum in London and who kindly helps me with beetle identifications. However, even before I could get the package in the post to him, I noticed at home the next day that the pupal case had cracked open, and an adult of the beetle (Chrysomela aenea) had emerged from it, thereby confirming my intuition about its identity.
As I continued looking closely at various leaves on the young alder, I found what I thought were larval forms of the same beetle. One looked it might be going into its pupal phase, while another looking like it was shedding its skin ( which is called an exuvium in scientific language). Because insects, including beetles have an exoskeleton (unlike mammals and birds, where the skeleton is internal), they have to break out of their old skin when they have outgrown it. This usually occurs only in the larval stage of insect development, and not with the adults, as they do not normally grow at all. Instead, their main function is solely to mate and reproduce.
Eventually, after searching for quite a few minutes I found what I recognised immediately as a beetle larva on one of the alder’s leaves. It was a yellow colour with several rows of black spiky protrusions running along the length of its body, making it look like a miniature insect version of one of the armoured dinosaurs such as Ankylosaurus. That ancient extinct reptile was a herbivore, and its spiky carapace was used for defence against predation. In a similar fashion, the herbivorous beetle larva must have evolved its spiky form to discourage predators such as insectivorous birds and wood ants (Formica spp.) from eating it.
Finding this larva gave me a good sample of what I thought were the different life stages of this leaf beetle. However, I wanted to get the identification of the larvae confirmed, so I sent the different ones shown in the photos here to Sholto in London. I was delighted when he replied to say that all the larvae had pupated during their journey down to him, and that all were indeed of the beetle (Chrysomela aenea).
By this time I’d spent quite a while with the alder tree, and although my focus had been on the leaf beetles, I’d come across some other invertebrates as well. On one leaf with a lot of feeding damage from one of the beetle larvae, I saw a bright green bug. Its colour made it relatively inconspicuous on the leaf, and it was identified for me by Joe Botting, an expert on bugs, as the black-kneed capsid bug (Blepharidopterus angulatus). He said it’s a common species found on a range of trees but especially on alder and sallows (Salix spp.).
On another leaf I noticed a ball of yellowish fuzz with a bright green spider next to it. This was a female cucumber spider (Araniella cucurbitina) and she was guarding her egg mass. I’d already seen a few other cucumber spiders in the glen in the past couple of weeks, including some with egg masses like this, and it appears to be a particularly good year for them. I don’t know the reason for that, but there’s definitely significantly more of them this summer than I’ve seen for a number of years.
In July 2011 I spent a memorable couple of hours watching a cucumber spider with her egg mass near the River Moriston in Glenmoriston, while several wood ants (Formica lugubris) attempted to steal the eggs. I wrote a blog about that at the time, and fortunately for this spider there didn’t seem to be any wood ants in the vicinity of this alder tree. The cucumber spider is one of the most distinctive and colourful spiders in the Caledonian Forest, with its bright green overall colour contrasting with the red tip to its abdomen. Its main arachnid rival in terms of brilliant colouration is another spider that’s also named after a food item – the strawberry spider (Araneus alsine). That species is quite common on Trees for Life’s Dundreggan Estate, but as far as I’m aware has not been seen in Glen Affric, although it has been found near the village of Cannich.
By this time I’d looked at quite a number of leaves on the young alder, and I was continuing to find more invertebrates on them. On one leaf I spotted a small black beetle that I didn’t recognise, but when I sent the specimen to Sholto he identified it as a widespread species called Luberus longicornis – like most beetles it has no common name. It is associated with birches (Betula spp.) and willows (Salix spp.). and there were some of both those trees next to this alder.
Beetles are relatively easy to photograph, because they spend most of their time on something – a leaf, flower or a stem, for example. They seldom take to flight, so if I see one there’s usually enough time to get my camera out, set up the macro lens and flash, and take a photograph of it. It’s a different matter entirely for flies. They spend most of their time on the wing, only alighting on leaves or flowers for brief periods, and taking off before I can get close enough to take a photograph. An exception to that was the fly shown here, which stayed in place long enough for a couple of images, but it flew off before I could catch it for identification purposes. Peter Chandler, an expert on Diptera (as two-winged flies are known in scientific parlance) identified it for me from the photos as being a species of Phaonia, but he was unable to determine which species without examining a specimen.
Having taken so many close-up photographs of various insects on the alder, I took a few minutes to step back and look at the tree as a whole.
It was probably only about 5 metres tall, and had several trunks growing up next to each other. It appeared to be very vigorous and full of life, both in itself and with all the invertebrates living on it, and that was very heartening for me to see.
Alders in the Highlands have been affected by a fungal disease (Phytophthora alni) that causes them to die back, often from the crown downwards. In Glen Affric old alders along the lower section of the Abhainn Gleann nam Fiadh watercourse have been killed by this disease – a photograph of that featured in one of my blogs from 2012. It’s therefore encouraging to see that young alders like this one are flourishing in other parts of the glen.
The longer I spent looking at the tree, the more interesting things I found on it. I began to notice some leaves that had been partially rolled up – clear evidence that an invertebrate had been at work. In one case a leaf had some of the galls induced by the mite (Eriophyes laevis inangulis) on it, and had also been rolled, so this single leaf was supporting two species. Thanks to the help of Rob Edmunds, an entomologist who specialises in leaf miners and leaf rollers etc, I was able to get the identity of the roller confirmed as most likely being a a moth called the pale red slender micro-moth (Caloptilia elongella).
Some of the leaves were only partly rolled, but others were completely rolled into a tight cylindrical shape, so that the moth’s larva can feed inside, out of sight of potential predators. I opened one rolled leaf and it did indeed contain a small caterpillar that I was able to photograph.
One particular leaf that I saw had been partially rolled, and had some of the galls induced by the mite (Eriophyes laevis inangulis) on it. It also appeared to have had a significant section of its leaf blade eaten. This was a different pattern of feeding damage to what I’d seen on the tree’s leaves so far, making this leaf rather conspicuous. Turning it over, I found signs of what looked like some dramatic action from the point of view of invertebrates.
Near the open end of the rolled section of the leaf there was an exuvium, or shed skin, of an insect of some sort. Then, inside the leaf roll was what seemed like an invertebrate crime scene! It looked to me as though there were the remains of the body of a caterpillar, presumably of the pale red slender micro-moth (Caloptilia elongella), which had been taken over by a parasitoid that had grown inside the caterpillar and then pupated, with the adult bursting out of the carcass, in a similar fashion to the xenomorph in the original ‘Alien’ film. That scenario is of course just speculation on my part, and there’s not really any way of knowing exactly what happened within the rolled leaf.
Moving around the tree, I continued to turn over leaves, looking for anything of interest on their undersides. Many invertebrates spend their time there, instead of on the upper surfaces, as they are out of sight of predators such as insectivorous birds. Under one leaf I found a small spider, which was named for me as being a female Neriene peltata by Edward Milner, who helps me with spider identifications.
On several leaves I noticed small red invertebrates racing around almost frantically over the surface area. As I watched I could see that they were tiny red mites, but it was very hard to take any photographs of them as they moved so quickly and never seemed to stay stationary at all. Eventually I managed to get a few reasonable images of one of them, and I have subsequently discovered that they may be a species called the whirligig mite (Anystis baccarum). This common name is derived from the species’ constant movement in roughly circular paths. I collected one of the mites and am currently waiting to hear from a specialist whether they can confirm this provisional identification.
On another leaf I spotted a bug, or actually the nymph of a bug, as the pre-adult stages are known for this group of invertebrates. I could tell it was a nymph rather than an adult because it didn’t have any wings, but it had prominent wing buds, visible in the photograph here as the two lighter green bulges part way along the bug’s body. When I sent the photo to Joe Botting, he identified as a nymph of the black-kneed capsid bug (Blepharidopterus angulatus) – the same species I’d seen an adult of on the alder, and which featured earlier in this blog.
Having spent over two hours studying this one alder tree, I moved on further west into the glen for the rest of the day, and although I stopped to look at one or two other alders, none of them seemed to have the same abundance of life on them, and I saw no further sign of the leaf beetles (Chrysomela aenea) either. Was there something special about this one tree that made it different to the others I looked at? Or was it just another manifestation of my deepening connection and affinity with Nature in this glen that led me intuitively to look at this particular tree, and find all the invertebrates it was hosting? I don’t know the definitive answer to these questions, but I do find myself getting drawn to interesting things every time I go to Affric. By opening myself deeply to Nature, it seems that life, in all its various diverse forms, presents itself to me, almost as if to say ‘Look at me, I’m part of this special place.’
On my next visit to the glen, 9 days later, I returned to the same alder tree to have another look at it. I wanted to see if it still had an abundance of the leaf beetles, and also whether there might be any other species of note to be found there. It didn’t take long for me to find evidence of the leaf beetles, as on one of the first leaves I turned over there was a relatively young beetle larva (in the photo above), right next to the central vein of the leaf.
On another leaf with very conspicuous feeding damage I discovered the exuvium, or cast skin, of one of the beetle larvae –no doubt the individual that had eaten the various holes on the leaf. I wasn’t sure if this exuvium was from when the larva had outgrown its old exoskeleton and moulted to become a larger size, or if it was what was left after the larva had pupated and emerged in its adult form. There were also a number of beetles visible on the leaves, so there was definitely something about this tree that suited them.
I didn’t spend as long with the alder tree this time around, but I still noticed a number of different invertebrates. On one leaf I spotted a familiar looking spider – one of the orb weaver arachnids. When I sent it to Edward Milner he responded that it was a spider in the genus Metellina, but that it was still a juvenile and hadn’t yet developed the characteristics that are used to differentiate between the species of Metellina that we have in Scotland.
On the underside of another leaf I found something quite unusual – a white fuzzy mass that appeared to be enveloping a small green caterpillar. I look at it for a little while, puzzling over what it could be. It reminded me of something I’d come across in Glen Affric last year – the larva of a moth that had been parasitised by a wasp, which had laid its eggs in the caterpillar and the wasp larvae had subsequently erupted out of their victim, forming white cocoons sticking out from the caterpillar’s body.
In that case I’d sent the photograph of the caterpillar to Roy Leverton, an expert on moth caterpillars, and he’d identified it as being of the December moth (Poecilocampa populi). Subsequently, Mark Shaw, a specialist in parasitoid wasps in the Braconidae family, identified the erupting cocoons as being those of a wasp called Cotesia spuria. The cocoons are the pupal cases within which the wasp larvae pupate, having fed already on the body of the caterpillar, and adult wasps will emerge from them in due course.
My first thought therefore was that what I’d found on the alder leaf was another example of such parasitism by a wasp on a caterpillar. However, when I sent the photograph to Roy, he replied that it showed the larva of a sawfly, rather than the caterpillar of a moth. Sawfly larvae can be differentiated from the caterpillars of Lepidoptera (as moths and butterflies are collectively known in scientific terms) by the fact that they have six or more pairs of prolegs, whereas caterpillars have five or fewer pairs.
I’d also noticed that the larva was moving around, so it didn’t seem like it was suffering from parasitism. On a hunch therefore I did a search on the Internet for sawfly larvae that occur on alders and almost immediately discovered a species called the woolly alder sawfly (Eriocampa ovata). the larvae of which look exactly like the one I’d found. A subsequent email exchange with Andrew Halstead, an expert on sawflies, confirmed that identification. This species is known in the UK mostly from England, although there are apparently a few records in Scotland from near Inverness. Roy, who lives in northeast Scotland, said he had never come across this larva himself in all his years of studying Lepidopteran caterpillars, so that tends to confirm the supposition that it has a very limited distribution in Scotland.
With regards to the larva’s bizarre bodily decoration, I wonder if it has evolved this way to mimic the cocoons of parasitic wasps, like I’d seen last year, in order to appear unpalatable to potential predators? Another suggestion I’ve come across is that the larva resembles a bird’s dropping, especially when it is curled up, again making it unattractive as a food item. Whatever the reason, it was quite a remarkable find to make, and it summed up for me the feeling that this particular alder tree was somehow special. It was definitely a highlight of my day (strange though that may seem to some people!) and is also, for me at least, a good subject to finish this blog with, illustrating another of the small but fascinating wonders of biological diversity in the Caledonian Forest.