The first part of this blog focussed almost entirely on the many insects I saw feeding on some ragwort plants (Senecio jacobaea) at Dundreggan in the middle of September. After lunch that day I continued walking in the birchwood near the lodge, and found a lot of other subjects of interest, so that I didn’t get very far at all during the course of the afternoon. I find this increasingly to the case – I cover less and less distance when I’m out in Nature, as I see more and more details in even small areas, often that I wasn’t aware of at all in times gone by.
In amongst the birches (Betula pubescens and Betula pendula) there are a few old Scots pines (Pinus sylvestris) and several stands of aspen trees (Populus tremula), and this is one my favourite areas of Dundreggan. As summer gives way to autumn, the different broadleaved trees and the bracken (Pteridium aquilinum) change colour in sequence, bringing vibrant hues of yellow, orange and gold to the forest. That process was just beginning this day, with the bracken being in the vanguard of the autumn colour display.
I headed towards the first of the aspen stands, as I wanted to see how much insect life there was on them, before they began their colour change – aspen is the first of the tree species in the forest to lose its leaves in the autumn.
Reaching the trees, I began looking closely at some of the leaves to see what was on them. Unlike many aspen stands, this one has a couple of trees with low branches that are easy to reach, and I’ve often found various galls on them before, as well as various caterpillars. Almost immediately I saw a familiar and distinctive caterpillar, which has a multi-coloured body and lots of hairs on it. I recognised it as the larva of the poplar grey moth (Subacronicta megacephala), which is a common species on aspens in our area.
I watched it for a little while, as it was quite active, moving around on the leaf. At one point it came close to a gall induced by a mite (Phyllocoptes populi) on the leaf – this is also a common species on aspen.
As I continued looking at some of the leaves on the aspen, I found more of these galls, usually just one or two on each leaf. However, I then discovered one leaf which had eight on it! Galls are abnormal growth on a plant that is induced by a different organism – in this case the mite – and it is in effect a parasitic relationship. This is because the gall causer benefits at the expense of the plant (i.e. the aspen here), which is induced to produce unusual growth that is of no benefit to itself.
The relationship appears to be relatively benign, though, as the tree doesn’t seem to experience any noticeably negative effects from its various gall causers. A number different organisms induce galls on aspen, and on this particular tree I’ve often seen the petiole galls induced by a midge (Contarinia petioli) in previous years. There were none of them visible now though, and it makes me wonder what factors cause one type of gall to proliferate in a given year, and a different type to be abundant the next year?
On one of the leaves where the blister galls induced by the mite (Phyllocoptes populi) were quite numerous, I spotted a different gall, also induced by a mite (Aceria dispar). This takes the form of a tightly rolled section of the edge of a leaf on the aspen tree, and once I’d seen one of these I began to notice them on other leaves well. The larva of the mite develops inside the rolled section of the leaf, out of sight from predators, and somehow induces the tree to produce this different growth form for its benefit. This is in contrast to another type of leaf rolls on aspen, in which the whole width of the leaf is rolled up on itself, several times over. That is the work of the aspen leaf roller moth (Anacampsis populella), and is not a gall, because the larva rolls the leaf itself, whereas the mite (Aceria dispar) makes the plant do the rolling, by means which are still not fully understood or satisfactorily explained.
I continued looking at some of the leaves on the aspen tree, particularly the undersides, as that is where most herbivorous invertebrates live, to reduce their visibility to their main predators – insectivorous birds. On one leaf it was very easy to see that an insect was at work, as almost half of the leaf had been eaten away. Turning it over, the culprit was immediately obvious – a green caterpillar with a distinctive horn or spike on its posterior. I didn’t immediately recognise the species (although I have seen it before), but Roy Leverton, who helps me with caterpillar queries, identified it as being the larva of the swallow prominent moth (Pheosia tremula).
As the second part of its scientific name indicates, this moth is a species whose caterpillars feed on aspen trees (Populus tremula), although it also occurs on various willows (Salix spp.) and some other poplars (Populus spp.).
Some other leaves on the aspen had orange-yellow spots on them, and these are signs of a rust fungus (Melampsora populnea) that fruits as small bright orange blobs on the underside of the leaves.
While I was moving around the aspen tree to look at its leaves, I noticed a bright orange shape on the forest floor nearby that I recognised as a waxcap fungus. Liz Holden later indicated that it was most likely the goblet waxcap fungus (Hygrocybe cantharellus).
Having spent quite a while with the aspen tree, I walked on, but I hadn’t gone more than a dozen paces when I saw another waxcap fungus. This one looked different, and when I sent the specimen to Liz she identified it as being the vermilion waxcap (Hygrocybe miniata).
Moving on again, my attention was drawn to the bracken that was scattered in patches amongst the trees, particularly where the fronds were displaying bright shades of yellow, before dying back for the winter.
There are lots of juniper bushes (Juniperus communis) interspersed amongst the birches in this part of the woodland, and some of them usually have juniper aphids (Cinara juniperi) feeding on them. I had a close look at one bush where I’d seen the aphids before, and sure enough they were there again this day. It can be difficult to spot the tiny aphids amongst the dense foliage of the junipers, but they are usually tended by wood ants (Formica lugubris), which are much more conspicuous.
Thus the best way to find the aphids is by looking for a group of wood ants on the foliage, which will be clustered around them. The aphids suck the sap of the juniper and secrete a clear liquid waste product, called honeydew.
The honeydew is a primary food source for wood ants, and the ants tend the aphids assiduously, stroking them with their antennae to stimulate them to excrete it. In return, the ants provide protection to the aphids from predators and parasitoid wasps, making this a mutualistic, symbiotic relationship. Wood ants actually ‘farm’ aphids, sometimes moving them physically from one place to another on their host plant, to aid their feeding and the consequent production of honeydew.
Not all the honeydew excreted by aphids is harvested by wood ants, and drops of it will spatter down on to the leaves or needles. There, it attracts the attention of other insects, including common wasps, various hoverflies and bumblebees, which land on the leaves and consume it. The aphids therefore provide the means whereby many different organisms are able to benefit from the photosynthetic ability of the plant (juniper in this case) to harness the sun’s energy and convert it into organic compounds.
These common juniper aphids (Cinara juniperi) feed on the new growth of the juniper, presumably because it is easier to penetrate the softer plant tissue and access the sap underneath. We have another, much rarer aphid that occurs on the junipers at Dundreggan – the giant juniper aphid (Cinara smolandiae). Dundreggan is the only place in the UK where this species has been found so far, and it feeds lower down on the juniper stems, where the wood is much tougher. It too is tended by the wood ants.
After watching the ants and aphids for a while, I continued on a little further to a place where one of the focalisers of this week’s Conservation Week at Dundreggan had told me he’d seen a slime mould. I’ve had a long-standing interest in slime moulds, and have written several blogs about them in the past – some examples can be seen here and here. As it turned out, when I reached the spot, there were several different colonies of the same slime mould within a few feet of each other.
I didn’t immediately recognise this slime mould, but it had flowed over some decaying plant material on the ground, and had reached its plasmodium phase, in which it was ascending some stems of a blaeberry plant (Vaccinium myrtillus).
For part of their life cycle, slime moulds exist as single-celled organisms, usually in decaying vegetation or dead wood, Then, something – most likely environmental factors such as temperature changes or rainfall patterns – causes them to converge and form a multi-cellular plasmodium. This can move or flow across the landscape, and will often rise up plants. The sporocarps that release the spores, by which the slime mould reproduces, emerge out of the plasmodium, and this can be seen in the photograph here.
It had taken me most of the day to reach the site where these slime moulds were, but in fact it was only about a 10 minute walk from the buildings at Dundreggan. This was testament to how much of interest I’d found along the way – from the insects feeding on the ragwort, to the waxcap fungi, the wood ants and the aphids, and finally the slime mould, it truly had been a day of wonders.
In between taking the photographs I’d also shot some video footage, so here’s a brief compilation of that to finish with. It includes some footage of the wood ants tending the aphids on the juniper, and also has a sequence of other wood ants – the same species (Formica lugubris) – tending different aphids (Pterocomma tremula) that were feeding on a young aspen tree (I didn’t take any still photos of them though, which is why I haven’t mentioned them before this).