A shieldbug extravaganza, part 1

On 1st August 2020 I was making my monthly round of the Findhorn Hinterland area to check the series of 6 pitfall traps we’ve installed for an ongoing survey of spiders there. To reach the first couple of trap sites I had to pass a prominent cluster of three large, multi-trunked silver birch trees (Betula pendula) that have grown closely together, and I often stop to have a look at them, to see if there is anything of interest on their leaves.

I do this due to the fact that these are some of the largest and oldest birch trees on the Findhorn Hinterland, and on several occasions in the past I’ve found aphids and a variety of other insects on them. When I was checking the spider traps a month previously, at the beginning of July 2020, I’d come across a red-legged shieldbug, or forest bug, (Pentatoma rufipes) on one of them, as well as a nymph of the same species.

Given the relatively warm and sunny summer we’ve had at Findhorn this year, I was hopeful that there might be more insects of interest this time as well, especially as it felt like we were reaching the peak of summer weather, with the temperature reaching a (hot for Findhorn!) 28 degrees C some days. Little did I suspect though that my sighting of the forest bug was a portent for what I would discover this time …

Reaching the trees, I began turning over some of the lower leaves, to see if there was anything on their undersides – that is where most insects are to be found, as it reduces their visibility to the insectivorous birds that prey on them. It wasn’t long until my search bore fruit, when I found a brightly-coloured caterpillar resting underneath one leaf. I didn’t recognise it, but when I sent the photograph to Roy Leverton, an expert lepidopterist, he identified it as the caterpillar of the nut-tree tussock moth (Colocasia coryli), which feeds on a range of deciduous trees, including birch.

As I examined a few more leaves, my eye was caught by a mass of brightly-coloured shapes on one, and when I looked a little closer I realised it was a large number of shieldbugs clustered together. I wasn’t sure initially what species they were but I recognised that they were nymphs, not adults. I suspected they were huddled together for reasons of ‘safety in numbers’ and perhaps to give the impression to potential predators that they were a single larger organism, and therefore risky to try and eat.

I’d read about, and seen photographs of, such aggregations of shieldbugs before, but this was the first time I had come across the phenomenon myself, so I was really delighted with this discovery. I spent some time looking at them through my hand lens, marvelling at the intricate patterns and colours of their bodies, and how they combined en masse like this to create a strong and visually dramatic impact.

I was also pleased to note that they didn’t seem perturbed by my presence, or by the flash of my camera. I was therefore able to take quite a number of photographs, approaching closer and closer to reveal more of the micro-topography and textures of their chitinous bodies with my high magnification macro lens. I also noticed that there were some younger instar nymphs in amongst the majority, which were in their 5th or final instar, or stage of development, before becoming adults.

Later that day, back at home I was able to quickly identify this species of shieldbug as being the parent bug (Elasmucha grisea) by looking at the excellent Shieldbug Illustrated Life Stages chart on the British Bugs website. At the birch trees themselves I spotted another cluster of the same nymphs, just a few feet from the first one, and this time completely covering a catkin on one of the trees.

As I watched them, an adult of the same species sidled into view around the catkin, and I wondered if it might be a parent of the nymphs. Later on, I checked with Joe Botting, who co-runs the British Bugs website, and he confirmed that adults of this species do indeed stay around to look after their offspring for a while. The other possibility was that it might have been one of the final instar nymphs that had freshly moulted into its adult form, but he said this could be discounted as the colours would not have been as pronounced as they were. The body shape of the adult is quite different to that of the nymphs, and tapers to a rounded point at the bottom. This gives it a resemblance to the shield of a medieval soldier, and is the source of the common name for this group of bugs.

I don’t know if the adult parent bug was warning the nymphs of potential danger or not, but as I took some photographs, they began to disperse, climbing up the catkin, and then on to the twig it was attached to. This seemed to be in marked contrast to the cluster of nymphs I’d seen previously on the leaf, which hadn’t reacted at all to me taking photographs of them.

I continued looking at some nearby leaves on the birch tree, and soon found a leaf with two more nymphs on it. The colours of these ones were not as dark, indicating that they had newly-moulted from their 4th instar forms, and this was confirmed by the presence of an exuvia, or cast skin, of one of them on the leaf.

Because they have hard exoskeletons, insects such as shieldbugs are unable to grow continuously in the way that mammals or birds do. Instead they have distinct stages of growth, called instars, and they moult from one instar to the next by bursting out of the exoskeleton they have outgrown, leaving it behind as a cast skin which is known as an exuvia. Shieldbugs like this go through 5 instars as nymphs, before becoming adults.

I began to feel like I was on a roll with my discoveries on this birch tree, as I spotted a smaller shieldbug on another nearby leaf. This was clearly a nymph as well and I suspected it was of the birch shieldbug (Elasmostethus interstinctus), which as its common name indicates, is particularly associated with birch trees. Back at home later on, a quick look at the Shieldbug Illustrated Life Stages chart confirmed that it was indeed a nymph of the birch shieldbug, and was a 3rd instar.

Then almost straightaway, I spotted an adult shieldbug on a catkin just a few inches from the leaf with the nymph on it. This was smaller than the adult parent bug I’d seen previously, and differently marked, and I recognised it as being an adult birch shieldbug (Elasmostethus interstinctus).

Like the parent bug nymphs that I first encountered on this tree, it seemed unconcerned by my presence. I was able to examine it in some detail, taking several high magnification photographs to illustrate the stippled black dots that cover its body. In close up, it looked almost like an alien creature, with its compound eyes protruding from either side of its head, which is small in comparison to its body.

By this time I was really into studying the leaves of this birch tree, to see what else I could find. Sure enough, turning over another leaf revealed a mass of bright red spherical-looking shapes on its underside. My immediate thought was that they must be eggs of some sort, but when I looked at them with my hand lens I saw they were actually tiny shieldbug nymphs that must have just hatched out from their eggs.

There were a number of clear, spherical empty egg shells amongst the nymphs, and there was even one late starter – an egg with the embryo of a nymph visible inside it, not yet ready to hatch out. My intuition told me that these were probably nymphs of the birch shieldbug (Elasmostethus interstinctus) so I checked about them on the Shieldbug Illustrated Life Stages chart later. The shape and pattern of markings seemed to match, but the first instar nymphs shown there were not as bright red as these ones I’d just found. When I mentioned this to Joe Botting in an email he confirmed that they were indeed 1st instar nymphs of the birch shieldbug, but he said that their colour can be quite variable.

The leaf with these bright red nymphs on it was quite close to where the nymphs of the parent bug had been dispersing from the birch catkin that I’d photographed them on earlier. As I watched the birch shieldbug nymphs, I was surprised to see one of the final instar parent bug nymphs crawl along the petiole or stem of the leaf and come to a rest right next to the red nymphs, almost as though it was posing for me there! The size difference between the nymphs of the two species was quite significant, although the comparison was not really fair, given that the parent bug was a 5th instar but the birch shieldbugs were just newly-hatched first instar nymphs!

By this time I’d spent quite a while looking at, and photographing, all these shield bugs, and I really needed to move on to complete the servicing of the other spider pitfall traps, which are located in different parts of the Findhorn Hinterland area. Before I left though, I spotted another group of parent bug nymphs on one of the birch’s leaves, and they were clustered beside and behind an adult, thereby confirming that it is common behaviour for the adult bugs to stay with their offspring, presumably to help protect them.

The next day, 2nd August, I decided to return to the birch trees as I felt there was a lot more to be seen and that I could learn from observing these shieldbugs. As soon as I reached the trees, I noticed a cluster of parent bug nymphs on a leaf that I hadn’t seen the previous day. Whereas the previous day had been quite blustery, this one was almost windstill, so I was able to get some video footage of the nymphs. The insect on the left in the first part of the video is a silver birch aphid (Euceraphis betulae) – a common species in Scotland.

Looking around I found more and more leaves with clusters of nymphs on them. I stopped counting after I’d seen 7 or 8 different clusters, and those were just on the low-hanging leaves that I could reach. They formed a tiny proportion of the trees’ overall foliage so it made me wonder just how many shieldbug nymphs there were altogether – if I could somehow examine all the leaves of the three birches to count them!

I searched for the leaf with the newly-hatched birch shieldbug nymphs that I’d seen the previous day, but I wasn’t able to locate it again. That may have been because the nymphs had all dispersed from the leaf overnight. I did come across a group of bright red 1st instar nymphs that could have been the same ones – they were climbing up the stem of a catkin on the birch.

I watched them for a little while as they moved up the stem together, almost like a group of red-uniformed soldiers marching in step on parade. I assume their bright red colour must be aposematic coloration, which is used by many insects as a warning to potential predators that they are dangerous to attack and eat. However, given that there is considerable variation in the colour of these first instar nymphs I’m not sure how valid that explanation is. So far, I haven’t been able to find any detailed information about the colour variation in these nymphs that addresses this …

Certainly, their red colour makes these nymphs stand out visually and I soon spotted a solitary one on another nearby catkin. It must have got separated from its other co-hatchlings and wandered off in a different direction. There was certainly nothing to be gained from it being on the catkin – like all true bugs, the birch shieldbug is a sap-sucking insect, and there’s no access to sap in a catkin for such a small nymph as this.

The birch tree was heavily-laden with catkins, especially on the south-facing side where I’d been finding all these shieldbugs, and on another one I noticed a couple of the first instar nymphs, together with a much larger 3rd instar nymph. This latter was more green and black in its coloration, with just small patches of red on it. If the red colour on the first instars is indeed aposematic, it would seem that by the time the nymphs reach their third instars they must be less vulnerable to predators and/or are better able to defend themselves, and therefore don’t need to advertise their unpalatability so vividly.

As I continued to look at the leaves and catkins of the birch I noticed other insects as well. There were plenty of silver birch aphids (Euceraphis betulae), which as their common name indicates are specific to silver birches (a closely-related species occurs on downy birches). These are large in size for aphids and the winged adults usually have a dusting of blue-ish wax on them, which is just visible on this one. Like the shieldbugs, aphids are sap-suckers, but in this case they utilise cryptic coloration, or camouflage against the background of the leaves where they feed, to reduce the risk of predation.

On another leaf, I came across an eyed ladybird (Anatis ocellata), which was probably searching for aphids like the one I’d just seen, as ladybirds are voracious predators of them. This species is different from most ladybirds, which are mainly bright red (another example of aposematic coloration), by being dull orange in colour with 15 or 18 black spots, each surrounded by a pale cream ring. Its common name is derived from the perceived similarity of these ringed spots to eyes.

There were also a few micro-moths visible on some of the leaves and I recognised them as being the golden argent (Argyresthia goedartella). This is one of the few micro-moths that I can identify easily, and it has beautiful markings of white and bronze-gold on its wings, with a distinctive inverse ‘Y’ shape in the middle. This is a common species, occurring throughout the UK, and is on the wing from June through August.

However, by far the most abundant insects I was seeing on the birch tree were the shieldbugs, particularly the birch shieldbugs. They appeared to have spread out and dispersed over many of the leaves and catkins that were within easy reach for me, and interestingly enough I came across nymphs at various stages of development. The 2nd instar nymphs on this catkin were slightly larger than the 1st instars I’d seen earlier, and their markings were a little different as well.

I noticed in a few instances that the nymphs seemed to be congregating on the tree’s twigs at the junctions where leaves or next year’s buds emerged from them. I wondered if these were choice feeding sites, where it was easier for the nymphs to access the tree’s sap, or if it was just the case that the proximity of twig, leaf or bud offered more cover and protection from predators?

There is apparently no parental care provided by the adult birch shieldbugs for their young, which are left to fend for themselves as soon as they hatch out from their eggs. This is in marked contrast to the parent bugs, where one of the adults will guard the brood of eggs before any hatching takes place, and, as I’d already observed, adults also stay with the nymphs during their development. For anyone who is interested, some more detailed scientific information about the parent bug’s parental care is available here. On this birch tree, the parent bugs must have started breeding earlier than the birch shieldbugs, as the youngest parent bug nymphs I came across were these 4th instars.

I’d got into a rhythm of turning over the leaves on the birch, and looking at the catkins, as there always seemed to be something new to discover. It wasn’t long before I found the next subject of interest, which turned out to be a pair of red-legged shieldbugs or forest bugs (Pentatoma rufipes) mating on a catkin. These are larger than both the parent bugs and birch shieldbugs, with a body length that reaches 14 mm, and, as their common name suggests, are distinguished by their bright red legs. Having begun this blog with the red-legged shieldbug I’d seen on this tree at the beginning of July, it seemed quite special to find it again now, as it confirmed that there were three different species of shieldbugs living on the tree at the same time.

I also managed to shoot some brief video footage of this pair on the catkin:

I continued to take some photographs of the mating pair, but they seemed slightly unwilling to have their amorous encounter recorded for public viewing as they began moving around on the tree, trying to find some cover whilst still joined together. I think this was just an automatic defensive behaviour, as they were obviously more vulnerable to predation in their conjoined condition than they would be as separate individuals.

As I watched, their meanderings took them to one of the leaves where a cluster of the final instar nymphs of the parent bugs were huddled together. I was very pleased to photograph this image here, showing the two species on the same leaf. Somehow it seemed to symbolise the abundance of life, and the diversity of shieldbugs, that this birch tree was supporting.

These two visits to the birches to document the shieldbugs were a memorable experience for me, and I planned to return again in the following days to see what else I could discover about them. I’ll write about that in Part 2 of this blog, and will finish here with a close up photograph of the parent bug nymphs clustered tightly together on one of the birch leaves.