A shieldbug extravaganza, part 2

After my two visits to the birch trees with an abundance of shieldbugs on them that I wrote about in Part 1 of this blog I went across to the west coast of Scotland for a few days, so it was over a week later before I returned to have another look for them. I didn’t know whether they would still be there, or if they would have all metamorphosed into adults and dispersed already, so I approached the trees without any great expectations of what I would find.

Any concerns that I might have had about having missed out on the development of the shieldbugs were immediately dispelled when I reach the trees on 11th August. Almost immediately I found a cluster of the parent bugs (Elasmucha grisea) in the same place as I’d seen them before. Now, however, they were no longer nymphs, but had become adults, although they were still clustered together as tightly as I’d seen them previously.

They must have newly emerged as adults, as they were still pale in colour. After the adult shieldbugs break free from the exuviae (or cast skins) of their final instars as nymphs it takes some time for their adult coloration to fully develop and for their bodies and wings to harden. In this brief period they are known as teneral adults, and the difference between one in this phase and a fully-developed adult can be seen in this photograph of the two next to each other.

Here’s another couple of photographs that show the teneral phase and a fully-developed adult parent bug, on different leaves of the same tree:

As I looked around at some of the leaves on the birch tree I spotted more and more of these teneral adult parent bugs. I noticed one that was on a leaf beside the exuvia it had moulted from – I guessed it must have emerged from its old skin not long before I found it. Seeing them side by side like this, the size difference between the final instar nymph and the adult parent bug was quite obvious.

Then, remarkably, I came across one of the teneral adults that was still in the process of emerging from the skin of its final instar form.

The exuvia was still covering the rear of the bug’s abdomen, whilst the emerging adult itself was a brighter and paler green colour than the other tenerals I’d seen. It seemed to be motionless, and I wondered if it was resting from the effort involved in breaking out of its old skin. I realised that I was watching a major event in the life of this insect, the closest human equivalent of which may be being born from our mother’s womb. Somehow it felt like a great privilege to witness this profound and almost miraculous process of transformation …

On a different leaf nearby I came across the exuvia of another parent bug nymph, this time with no teneral adult in sight. This one must have moulted earlier, and looking at the perfectly-preserved outline of the former nymph’s legs I wondered again at the effort that must have been expended by the adult as it extracted its legs from the tight casing of its old exoskeleton.

There’s no way of course that I as a human can really understand or know what the experience of a bug like this is as it goes through its moulting process. However, it’s a useful exercise for me in developing a more biocentric perspective to imagine what it must feel like to break out of what has become a restrictive old skin into the greater freedom of a new bodily form with the ability to fly (which of course the nymphs do not have).

As I continued to look around the birch tree, I discovered that the emergence process of the adult parent bugs was staggered, as there were still clusters of final instar nymphs on some of the leaves.

Seeing a large group of the nymphs (or in some cases a mixture of nymphs and teneral adults) together like this made me wonder if this behaviour had evolved to provide an experience of ‘safety in numbers’ to reduce the risk of predation. However, I also came across several instances of a solitary nymph on a leaf by itself – were these just adventurous individuals, or ones that had been disturbed somehow and wandered away from their fellows, and therefore more likely to be picked off by a bird?

There’s no way of really knowing the answer to these questions without spending a lot of time studying the behaviour of bugs like these, and so far I haven’t come across references to any research that has been done on this. For the time being at least, it will remain a mystery to me.

As I continued to look at various leaves on the birch tree, I came across several examples of something I hadn’t seen before – swollen and blackened sections of the midribs on the undersides on some of the leaves. I suspected these were galls, but a type that I hadn’t previously seen. Searching on the Internet later I discovered that they are indeed galls, and are induced by the larvae of a midge (Massalongia rubra). How insects such as this midge actually stimulate their host plant (ie the birch tree in this case) to produce this anomalous growth (which is of no benefit to the plant itself) is another mystery that biologists have not yet been able to provide a satisfactory explanation for.

While my main focus on this birch tree was the shieldbugs, I found other things of interest as well. My attention was drawn to a leaf that was already mostly yellow, in a premature example of the autumn colour change that would follow in a few weeks. On the underside I found a winged silver birch aphid (Euceraphis betulae) with its partially green body and semi-transparent legs providing near perfect camouflage against the green and yellow colour of the leaf.

Like shieldbugs, aphids are sap-sucking insects, and the autumn colour change is a prime time to spot many of them. This is because the tree withdraws the chlorophyll from its leaves before shedding them, and as it does so the flow of sap provides a good feeding opportunity for aphids. As a result it can be easy to find them on the yellowing leaves of a birch like this.

However, it was the shieldbugs that continued to draw my attention, not least because they were so abundant on the leaves of the birch. In addition to the parent bugs, there were also plenty of birch shieldbugs (Elasmostethus interstinctus) on the tree, although it was mainly nymphs that I saw. One 2nd instar nymph looked like it had moulted very recently from its first instar stage, as its colouring wasn’t fully developed yet.

I found a group of 2nd instar nymphs clustered around a bud for next year’s leaves, whilst elsewhere I came across a final instar nymph, which was readily distinguished by its prominent wing buds. These different life stages of the birch shieldbug, and all the other shieldbugs that occur in Britain, are expertly illustrated on the Shieldbug Illustrated Life Stages chart that is available on the British Bugs website.

By this time it was quite late in the afternoon, but I felt that there was still more to discover about the shieldbugs, so I returned to the birch trees again the next day, the 12th of August.

The first thing I found then was not a shieldbug, but a ladybird, and a rather unexpected one at that. It was the larch ladybird (Aphidecta obliterata), which is known as a specialist predator of aphids on conifers particularly larches (Larix app.), but also pines (Pinus spp.). The nearest conifers to these birches are some Scots pines (Pinus sylvestnis) about 100 metres to the west, so it seems reasonable to presume that this individual had been blown from there by the prevailing southwesterly winds that we have at Findhorn.

Soon afterwards I found another ladybird on the tree – the ten-spot ladybird (Adalia decempunctata). Like the larch ladybird (and indeed many ladybird species) it is a predator of aphids, and occurs on a variety of broadleaved trees, so its presence on the birch was not so surprising. As I’d already noticed, there were lots of silver birch aphids (Euceraphis betulae) on the tree, so there was plenty for the ladybirds to eat.

It wasn’t long though before I found some shieldbugs, and in this case it was a group of newly-hatched nymphs of the birch shieldbug (Elasmostethus interstinctus), clustered together beside the empty egg cases they must have just emerged from. There seems to be considerable colour variation amongst the early instar nymphs of the birch shieldbugs, as other first instar nymphs that I saw on the tree were much redder than these ones.

I was struck by the fact that almost all the life stages of the birch shieldbug seemed to be present simultaneously on the birch tree, from the newly-emerged nymphs I’d seen beside their egg cases, to adults. On one leaf I found an adult beside a 2nd instar nymph and both were right next to a cluster of small downy birch aphids (Betulaphis quadrituberculata), which were dwarfed in size by the shieldbugs.

An adult birch shieldbug can be up to 11.5 mm in length, but the small downy birch aphid really does live up to (or rather, down to!) its name as an adult reaches a maximum size of just 2mm. As such it’s much smaller than the silver birch aphids I’d seen earlier, which have a body length of up to 4.2 mm. Despite the size difference between these two aphid species, both will be prey for the ladybirds I’d seen on the tree.

As I’d discovered during my visits to these birch trees at the beginning of the month, the adult birch shieldbugs didn’t appear to be afraid of my presence, and this meant I could focus in on them very closely with my high magnification macro lens. The photographs I was able to take with it reveal some of their physical characteristics, such as the stippled texture of their bodies and the compound eyes on their heads.

Shortly afterwards I was able to get just as close to one of the teneral adult parent bugs on another part of the birch tree. There are obviously a lot of similarities between the two shieldbugs, but there are also subtle differences when they are viewed in close up detail like this. Neither of them would seem out of place if they featured as alien creatures in a science fiction film!

As I spent more time with the cluster of three birch trees, it became really obvious that the shieldbugs were concentrated on one of the trees – the one on the south-facing side of the group, which is also where the vast majority of the catkins were located. I’ve not been able to find any references or research papers that might explain this, but my theory would be that the birch tree has more sap flowing to areas where the catkins are growing, as that is where the tree’s investment in reproduction is concentrated. As sapsuckers, the shieldbugs would appear to be congregating there to take advantage of the greater quantity of sap that is available, in comparison to other parts of the tree.

This side of the tree with all the catkins was also notable for having the buds of next year’s catkins visible in abundance on it. I noticed a small spider crouched on one of those buds, and I thought that it looked like a crab spider. When I sent it to the spider expert Edward Milner for identification he replied that it was a juvenile spider in the crab spider genus Philodromus, and was most likely Philodromus aureolus. However, he said that the features necessary to distinguish it from other species in the genus hadn’t developed yet.

It was the shieldbugs though that drew most of my attention, and I was astonished at just how many of them there were on the tree. Unlike my previous visits at the beginning of August there seemed to be more birch shieldbugs now than parent bugs, and on several occasions as I was photographing them, some would get brushed off the tree’s leaves on to my clothes. In those cases I carefully placed the bugs back on the birch’s leaves, but with one particularly enterprising birch shieldbug nymph that crawled all over my hand I photographed it first before replacing it.

It had been another couple of very interesting and educational days for me watching these shieldbugs, often at such close distances, and I left for home at the end feeling inspired and touched by all that I’d seen. I’ll finish this blog with another photograph of the teneral adult parent bugs – a head-on view of a pair of them looking perhaps just as curiously at me as I was at them!