This blog entry dates back to a memorable day in early July, when I spent several hours in a beautiful but tiny native woodland remnant downstream from the Dundreggan dam in Glen Moriston. The area is about a mile before the entrance to our Dundreggan Estate itself, and I sometimes stop there on my way to the estate, as it’s such a special area – I’ve written about it in a previous blog entry.
It contains a good mixture of tree species, including Scots pine, oak, aspen, hazel and juniper, and also has a high concentration of wood ant nests. Because of the narrowness of the river there, the gorge-like conditions combine with the spray from the cascading waters to create a temperate rainforest micro-climate, which is rich in mosses, ferns and arboreal lichens.
On this damp and overcast day in early July, the woodland was lush, abundant and vibrant green, with the fullness of summer growth. It was also wind-still, which made it possible to photograph the aspen trees – on windy days their leaves become a blur in photographs, as they move in the slightest breeze. Aspen is one of the last trees to get its new leaves in the spring, with the buds sometimes not bursting until late May. On this day, the leaves were still bright green in the first flush of new growth.
I had only moved about 2 metres from my car when I made the first interesting discovery of the day – on the underside of a hazel leaf there was a female wolf spider (Pardosa sp.) guarding her eggs. The leaf was partially curled up, due to the silk she’d used to provide protection for the egg mass, and it was this unusual shape of the leaf that had drawn my attention. Little did I know then that this discovery was a harbinger for the main treat of my day, later on!
As I moved into the woodland, I continued to look closely at the leaves on the trees, hoping to spot more invertebrates on them. On a nearby aspen tree I saw a stonefly on one of its leaves, and I subsequently had it identified as the small yellow sally (Chloroperla torrentium), a species well-known to anglers whose nymphs are fed upon by fish such as Atlantic salmon (Salmo salar).
Just beyond the aspen was the first of the oak trees, and my eye was drawn by a mis-shapen leaf on a low branch at head level. Closer inspection revealed that the leaf had a couple of galls in the middle of it, induced by a wasp (Andricus curvator). A number of wasps in the genus Andricus lay their eggs on oak leaves and the larvae somehow stimulate the abnormal growth of plant tissue around them, providing protection from predators. The exact mechanism by which galls are caused remains one of the mysteries of botany.
Nearby, on another young oak tree another flash of brighter yellow-green on the underside of some leaves drew my attention, and my first thought was that it was more galls. However, as I looked closer I realised it was the egg mass of a spider, with a couple of wood ants (Formica lugubris) trying to dislodge them from the leaf.
The eggs belonged to a cucumber spider (Arienella cucurbitina), that was hanging about 10 cm. below the eggs themselves. This is one of our most distinctive spiders, being bright green in colour and with a red tip to its abdomen – its common name is wonderfully evocative of its main colour. Having seen wood ants dragging the egg sacs from other spiders back to their nests before, I was intrigued to see the actual act of egg theft under way on this occasion, and I settled down to watch the action.
I was surprised to see that the spider didn’t seem unduly concerned by this attempt to steal her eggs, as, knowing how strong wood ants are for their size, I thought they would easily be able to pry the egg mass loose from the leaf. The spider did move a bit closer to the ants, but she made no attempt to confront them, perhaps knowing that two ants were more than she could handle.
After a while, it looked like the ants were succeeding, as they had managed to pull the egg mass away from the leaf by over a centimetre. However, they (and I) didn’t reckon on the strength of the spider’s silk, as the strands were so strong that they were unable to dislodge the eggs any further. After a further while, a third ant joined the fray, but still they were unable to pry the eggs loose.
By this time I had been watching the action for well over hour, so I wandered around in the woodland nearby to see if I could find anything else of interest. On an aspen a few metres away, a bright red colour on one of the leaves caught my eye, and it turned out to be another gall. Although I’ve been photographing galls on aspen for some years, I hadn’t seen this particular gall before.
It was quite spectacular-looking, being situated at the point where the leaf stem or petiole meets the blade of the leaf itself, and I was later able to identify it as being caused by a mite (Eriophyes diversipunctatus). Looking around on the same aspen tree, I noticed more of these galls on other leaves, as well as a different type of gall, on the face of the leaves themselves. These are induced by midges in the genus Harmandiola – there are several different species that cause galls in aspen leaves.
On another aspen tree nearby I noticed an unusual leaf formation on one of the branches. The leaves were all drooping together in a tent-like formation, providing a sheltered and inconspicuous feeding location for aphids that were sucking the sap of the aspen, and which were in turn being tended by more wood ants. I subsequently had the aphids identified as being a species (Pterocomma tremulae) that we’ve also recorded on aspens on Dundreggan.
The aphids suck the sap of the aspen and secrete a honeydew liquid that is an important food source for the wood ants (Formica lugubris). The ants ‘farm’ the aphids, protecting them from predators, and even moving them around the tree from place to place. The tent-like formation has proved to be something of a puzzle though. Although some sap-sucking aphids that feed on aspen do cause leaves to take on this shape by feeding at the base of the leaf stems or petioles, there are apparently no records of this species of aphid doing that. Can this be a previously unrecorded behaviour, or did another aphid species that I didn’t notice cause the drooping leaves? The species that do cause this type of drooping leaves formation are not tended by ants, so that adds to the mystery.
I’ve now got a specialist in aphids lined up to do a survey at Dundreggan next summer, so hopefully he’ll be able to have a look at these trees as well. If the same tent-like formations occur next year, he may be able to solve the mystery and find the explanation for what is causing the unusual leaf growth. I’ll also be checking the aspens at Dundreggan where these aphids occur, to see if there’s any evidence of these tent-like shapes there.
With all these wood ants around, there were several nests nearby, so I spent a while watching the ants on one of them. By this time the sun had come out, so there was frenetic activity of ants rushing back and forth across the nest. An aspen leaf that had fallen on to the nest, with some pine needles on top of it, provided a good opportunity to see the ants clearly without the visual distraction of the nest itself, with all its bits of needles and plant debris.
By this time over an hour had gone by since I’d left the cucumber spider and the wood ants that were attempting to steal her eggs, so I went to have a look at what had happened. My expectation was that the ants would have succeeded in their act of theft, but when I got back to the oak, the cucumber spider was beside her eggs, and there was no sign of the ants, so it looked like they had given up on their raid.
The spider appeared to have attached the eggs more tightly to the leaf again, so I thought that all was well for her. However, as I watched, she moved away a little, and one of the ants returned to have another go at stealing. I spent another 15 minutes or so observing the scene, but then it was time to leave. I never did find out the outcome of all the ants’ efforts that day – the attempted egg heist was still underway when I left!