Updated on 20th July 2015 with the correct name of the aphids found on the juniper bushes.
Towards the end of the weeklong survey that Ed Baker carried out in June for aphids at Dundreggan I was able to join him again for another day. He’d found quite a number of other species of aphids since I’d been out with him earlier in the week, and he showed me some of the more significant of those, so that I could recognise them in future, and also take some photographs of them.
First of all we went to the Allt a Choire Bhuidhe burn, which is easily reached on one of the visitor footpaths we’ve established in the birch–juniper woodland. There, on an alder tree (Alnus glutinosa) overhanging the cascading waterfalls on the burn, was a distinctive species of aphid (Clethrobius comes) that I hadn’t seen before. This is called the hairy birch aphid, and it occurs on both birch and alder trees, making it relatively unusual amongst aphids, most of which are confined to one species of tree, or to closely-related species within the same genus.
The hairy birch aphid has a distinctive shape, and it also adopts a characteristic posture when feeding, with the rear of its abdomen pointing up in the air. Aphids that feed on trees generally fall into two groups – those that suck sap from the leaves, and those that that suck sap from stems and twigs. The hairy birch aphid is in the latter category, and its posture must help it to drill into the stem with its rostrum, the sheath that encloses its sucking mouthparts, which are called stylets.
Like all aphids, this species has two tubes protruding from near the end of its abdomen. These are called siphunculi, or cornicles, and are used for exuding defensive fluids, which in some species are quite waxy. One of the hairy birch aphids had a white globule of this wax-like substance on each of its siphunculi, and another blob on one of its antennae, giving it a very distinctive and unusual appearance.
Aphids secrete a clear liquid known as honeydew, which contains excess sugars from the sap that the aphids feed on. Ants, including wood ants (Formica spp.), tend certain species of aphid, harvesting the honeydew they produce. Other species, including the hairy birch aphid, are not tended by ants, and it is thought that this is due to their honeydew containing substances that are unpalatable to the ants. Aphids that are tended by ants gain some protection from parasites and predators, so the relationship is a mutualistic or symbiotic one.
While I was photographing the aphids on the alder tree, I noticed a beetle with bright red markings crawling on the rocks immediately underneath the tree. I hadn’t come across a beetle like this before, where the colouration of its elytra, or wing cases, was the reverse of that on ladybirds. I sent a photograph of it to a beetle expert and he named it as Scaphidium quadrimaculatum. This species hadn’t been recorded on Dundreggan before, so I was pleased to be able to add another name to the list of beetles known to occur on the estate.
Just beside the footpath that leads to the cascading waterfalls on the Allt a Choire Bhuidhe burn there are a couple of patches of nettles (Urtica dioica). Stopping to look at them on our way past, we noticed large numbers of green aphids on one or two of the plants. Ed immediately recognised these as a nettle-specific species, the dark green nettle aphid (Aphis urticata).
I took a series of photographs of these aphids, and it was only later on, when I was looking at them at the end of the day, that I discovered I had recorded one of the other features of the life cycle of most aphid species.
This is that the females can give birth to live young (the scientific name is vivipary), and that they do so without mating with males. This latter attribute, of the females alone contributing to the next generation of aphids, is called parthenogenesis, and it is this combination of reproduction without males and giving birth to live young that enables aphids to multiply in numbers so quickly. In many cases, the live-born young will already have an embryo growing within her for the next generation. This type of reproduction typically takes place from the spring onwards, and later in the year, usually in the autumn, males are also produced and then sexual reproduction occurs. The mated females produce eggs that overwinter and hatch out as females the following spring, when the asexual reproductive cycle begins again.
Moving on from the nettle patch, we looked at some more of the aphids (Cinara juniperi) that we’d seen on the juniper bushes previously. These were quite abundant, and in amongst the living aphids, we also found some aphid ‘mummies’. These are aphids that have been parasitised by very small wasps, which lay their eggs inside the aphids and the wasp larvae then develop, feeding on the aphids and killing them. The parasitised aphids’ bodies often become swollen and distorted as result, and we found one where the wasp that had grown inside it had cut a neat circular hole to escape when it had pupated.
We were on our way to look at another juniper bush where Ed reckoned he’d found a different species of aphid, and as we passed the large wych elm (Ulmus glabra) growing near the lodge at Dundreggan, we looked at some aphids that had curled one of the elm’s leaves. This is a species (Eriosoma ulmi) that we’d had a record of previously, as Graham Rotheray, who did a survey of Diptera, or two-winged flies, for us in 2010 had seen these aphids there, and the hoverfly (Pipiza luiteitarsis) that feeds on them. This is another aphid species that secretes a waxy substance from its siphunculi, for defensive purposes.
When we reached the juniper bush Ed was taking me to, he showed me the aphids feeding on it, which were different from the species (Cinara juniperi) we’d already seen. Initially, he thought these might be a species called Cinara mordvilkoi, which is known to feed on junipers elsewhere in Europe, but has never been recorded in the UK before. However, after consulting with some experts in other European countries and checking the specimens again himself, their identity was confirmed as being the giant juniper aphid (Cinara smolandiae).
This is also a species that hadn’t been recorded in the UK before (it has only been found in Sweden, Finland and north-west Russia up until now), so it is another important ‘first’ for Dundreggan, and reinforces the importance of the juniper population on the site. Ed also found some parasitised examples of this aphid and has subsequently been able to rear the parasitoids from them – they are a species of wasp called Pauesia laricis.
Having found some aphids (Aphis farinosa) on an eared willow (Salix aurita) on West Affric recently, I was keen to see if the same species was feeding on an eared willow I knew of beside the Red Burn on Dundreggan. We went to have a look, and although we didn’t find that species, we did see another one that Ed identified as being Cavariella theobaldi. He also found some mummies of this aphid species and reared the parasitoid, which turned out to be another species (Praon cavariellae) not previously recorded in the UK.
Ed also found some other mummies of this same species of aphid (Cavariella theobaldi) and was able to rear the parasitoids from them too. They were a different species (Ephedrus helleni) but again it’s one that there’s no records of in Britain up till now. These finds do not necessarily mean that Dundreggan is special in having those species – it’s more likely a reflection of the fact that very little systematic survey work has been done before on aphids and their parasitoids in Scotland.
By this time it was late in the day and I had to head for home, so I said goodbye to Ed, as he was leaving the next morning to return to Wales, where he lives. However, I was out again at Dundreggan a few days later, so I continued looking for aphids myself, and re-visited some of the places where we’d seen them when I was out with Ed. This included some aphids (Tuberculatus annulatus) on an oak (Quercus petraea) near Dundreggan Lodge, and there was a parasitised mummy there too.
Ed told me he’d reared a parasitoid (Trioxys pallidus) from this aphid at Dundreggan, and had also collected a specimen of a secondary or hyper-parasitoid wasp (Alloxysta citripes) that was parasitising the primary parasitoid. In the ecology of aphids, the relationships become more complex the more we find out about them. In this case the parasite was taking advantage of another parasite, whereby the secondary parasite lays its egg in the larva of the primary parasite, which is itself exploiting the aphid that is sucking the sap of the oak!
Nearby that oak is the foxglove plant that was host to large numbers of aphids (Aphis armata) that I wrote about in part 1 of this blog.
I spent a little while taking some more photographs of these aphids, and marvelling at the remarkable lifecycle of these tiny insects. I’d learned a lot from the time I’d spent with Ed, and it had deepened my interest in and enthusiasm for this intriguing but relatively little-known group of invertebrates. I’ll certainly be looking out for aphids whenever I’m in the forest in future!