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A shieldbug extravaganza, part 1

Findhorn Hinterland: 28 August 2020 10 Comments

A shieldbug extravaganza, part 1
Final instar nymphs of parent bugs (Elasmucha grisea) on the leaf of a silver birch (Betula pendula) on the Findhorn Hinterland.

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.

The cluster of three silver birch trees (Betula pendula) on the Findhorn Hinterland.

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.

Red-legged shieldbug or forest bug (Pentatoma rufipes) on a leaf of one of the silver birches on 2nd July 2020.

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 …

Caterpillar of the nut-tree tussock moth (Colocasia coryli) on the underside of a leaf of one of the birches.

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.

Final instar nymphs of parent bugs (Elasmucha grisea) on the underside of a birch leaf.

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.

Closer, frame-filling view of the final instar nymphs of parent bugs (Elasmucha grisea) on the birch leaf.

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.

Here, the smaller more green-coloured nymph is a 4th instar, in amongst the other 5th or final instar nymphs of parent bugs (Elasmucha grisea).

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.

Cluster of final instar nymphs of adult parent bugs (Elasmucha grisea) covering a catkin of the birch tree.

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.

Here, the final instar nymphs have been joined by an adult parent bug (Elasmucha grisea) on the birch catkin.

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.

Here, the final instar nymphs of the parent bugs have begun to disperse from the birch catkin.
Closer view of the adult parent bug (Elasmucha grisea), showing its characteristic shield-like shape.

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.

In this photograph, the nymphs are moving up off the catkin to disperse on to other nearby parts of the birch tree.
Final instar nymphs of parent bugs (Elasmucha grisea) with the exuvia, or cast skin of one, on a birch leaf.

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.

The smaller nymph to the right of centre in this photograph is a 4th instar nymph, in amongst the larger 5th or final instar nymphs.

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.

This is a 3rd instar nymph of a different species of shieldbug – the birch shieldbug (Elasmostethus interstinctus).

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.

Adult birch shieldbug (Elasmostethus interstinctus) on a catkin of the silver birch tree.

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).

High magnification close up of the adult birch shieldbug (Elasmostethus interstinctus), showing its eyes and antennae, and the colour and the stippled texture of its body.

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.

Newly-hatched nymphs of birch shieldbugs (Elasmostethus interstinctus) and their empty egg cases on a leaf of the silver birch.

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.

Closer view of the newly-hatched nymphs of birch shieldbugs (Elasmostethus interstinctus) and their empty egg cases. The pale orange shape at the bottom is an egg with an embryo still inside it.

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.

Final instar nymph of a parent bug (Elasmucha grisea) beside newly-hatched nymphs of birch shieldbugs (Elasmostethus interstinctus) and their empty egg cases on the leaf of the silver birch.

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!

Adult parent bug (Elasmucha grisea) & 5th or final instar nymphs on another leaf of the silver birch.

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.

Final instar nymphs of parent bugs (Elasmucha grisea) on a leaf of the silver birch tree.

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!

First instar nymphs of birch shieldbugs (Elasmostethus interstinctus) on the stem of a catkin on the silver birch.

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.

Closer view of the 1st instar nymphs of birch shieldbugs (Elasmostethus interstinctus) climbing up the stem of a birch catkin.

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 …

1st instar nymph of a birch shieldbug (Elasmostethus interstinctus) on a catkin of the birch tree.

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.

1st and 3rd instar nymphs of birch shieldbugs (Elasmostethus interstinctus) on a catkin of the silver birch.

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.

This silver birch aphid (Euceraphis betulae) was well-camouflaged on the underside of a leaf of the silver birch.

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.

Eyed ladybird (Anatis ocellata) on a leaf of the silver birch, with a catkin visible behind.

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.

Golden argent micro-moth (Argyresthia goedartella) on a leaf of the silver birch.

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.

2nd instar nymphs of birch shieldbugs (Elasmostethus interstinctus) on a catkin of the silver birch.

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.

1st, 2nd and 3rd instar nymphs of birch shieldbugs (Elasmostethus interstinctus) beside a bud on a twig of the silver birch.

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?

Fourth instar nymphs of parent bugs (Elasmucha grisea) on a leaf of the silver birch.

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.

Red-legged shieldbugs or forest bugs (Pentatoma rufipes) mating on a catkin on the silver birch.

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:

The red-legged shieldbugs (Pentatoma rufipes) were moving around on the birch tree, whilst still joined together as a mating pair.

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.

Final instar nymphs of parent bugs (Elasmucha grisea) and the mating red-legged shieldbugs (Pentatoma rufipes) on a leaf of the silver birch.

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.

Final instar nymphs of parent bugs (Elasmucha grisea) clustered together on a leaf of the silver birch.
Life on a spear thistle
A shieldbug extravaganza, part 2

Tagged With: biodiversity, birch, Findhorn, insects, shieldbug

Reader Interactions

Comments

  1. Arabelle Hurlstone says

    2 September 2020 at 10:44 am

    I had been told a birch tree supposedly hosts the most insects of all tree species so your blog now proves this. I have 2 in my garden in Newport, Essex and the birds love them. The neighbours not so much especially at this time of year when the tree’s tiny seeds seems to have a knack for finding their way into corners of our homes. There is no way I would get rid of these trees but unfortunately that might be their fate when we eventually move out (feet under or in front!)…If it wasn’t for people like you taking the time to observe these things, we just wouldn’t know the richness hiding under tree leaves. Thank you.

    Reply
    • Alan Watson Featherstone says

      2 September 2020 at 10:57 am

      Thanks for your comment Arabelle. Although birches are good trees for supporting insects, both oaks and willows host a wider range of insects than birches do. Birch seeds have evolved to be easily and widely dispersed by the wind, so they can get into all sorts of places.

      Reply
  2. Alan Watson Featherstone says

    30 August 2020 at 4:53 pm

    Many thanks for your comment and feedback Cornelia. Maybe I can start a new genre of writing themes – thrillers of biodiversity discoveries!

    Reply
  3. Cornelia Featherstone says

    30 August 2020 at 1:20 pm

    Wow, Alan what a gripping account of the intricacies of bug life. Reads like a thriller 🌟.

    Reply
  4. Dorothy says

    29 August 2020 at 8:13 am

    Absolutely fascinating stuff with cracking photos. Thanks for this.

    Reply
    • Alan Watson Featherstone says

      30 August 2020 at 4:51 pm

      Many thanks Dorothy – I’m glad you’ve found the blog fascinating. There’s more to come in Part 2!

      Reply
  5. John Lowry says

    29 August 2020 at 3:20 am

    Alan — thank you for another intriguing blog — will be out this Sunday and am going to follow your example and closely examine the underside of the leaves of some local trees. I am endlessly amazed at the variety of life forms that surround us, and find it sad that so many of us (myself included) never take the time to seek them out and observe them. We are surrounded by life and just need to pay more attention, which I hope to do more of as my retirement date approaches.
    thanks again, John Lowry

    Reply
    • Alan Watson Featherstone says

      30 August 2020 at 4:50 pm

      Thanks for your comment John, and I hope that your leaf-turning proves to be as fruitful as it does for me. It’s been one of the gifts of the COVID-19 lockdown for me to spend more time in my immediate local area and find more of the remarkable biodiversity that is all around us. I have no doubt it will be the same in Ontario, and that there’s a lot waiting for you to discover there.

      Reply
  6. Brendan says

    28 August 2020 at 8:54 pm

    Great photographs!

    Reply
    • Alan Watson Featherstone says

      30 August 2020 at 4:47 pm

      Many thanks Brendan!

      Reply

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    alanwatsonfeat1 Alan Watson Featherstone @alanwatsonfeat1 ·
    16 May

    This is a red birch midget micro-moth (Phyllonorycter ulmifoliella) on a leaf of a silver birch (Betula pendula) on the Findhorn Hinterland today. The second photo includes part of my thumbnail & provides a sense of scale, showing how tiny this micro-moth is!

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    alanwatsonfeat1 Alan Watson Featherstone @alanwatsonfeat1 ·
    16 May

    Here's a mid-instar nymph of a red-legged shieldbug (Pentatoma rufipes) with an alate or winged silver birch aphid (Euceraphis betulae) riding shotgun on it, on a twig of a silver birch (Betula pendula) on the Findhorn Hinterland today!

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    alanwatsonfeat1 Alan Watson Featherstone @alanwatsonfeat1 ·
    16 May

    Mid-instar nymph of a red-legged shieldbug (Pentatoma rufipes) on a silver birch (Betula pendula) on the Findhorn Hinterland today.

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