The wonder tree

In late July, we had two specialists, Bob Dransfield and Bob Brightwell, come to Dundreggan for a week to do a follow-up survey for aphids to that which Ed Baker had done there in 2012. As usual when we have biodiversity surveyors at Dundreggan, I spent a couple of days there with them, and in this case that turned out to be the first and last days that the Bobs, as they call themselves, were there. In comparison to the phenomenal abundance of aphids we experienced in 2012, there have been comparatively few apparent this year, but the Bobs did find a number of species that hadn’t been recorded last year, particularly on herbaceous plants.

In between my visits they found a solitary specimen of one aphid species (Dysaphis sorbi) on a rowan tree (Sorbus aucuparia) that hadn’t been found in 2012. That species induces galls on rowan trees, whereby the leaves curl in on themselves, with the aphids feeding inside, out of sight. The single aphid they found was tiny, and when they showed it to me, it was extremely hard to photograph. However, during the last day of their survey, we found a rowan tree beside the Allt Ruadh, or Red Burn, that was host to a large number of these aphids, and which were being attended by wood ants (Formica lugubris).

As we looked at the tree, we saw more and more of the galls induced by the aphids, and in addition, lots of other life, attracted by the aphids and the honeydew (a waste product for the aphids) that is a resource for many species of insects. It turned out to be the highlight of the whole day, and I probably spent about 2 hours at the tree altogether, observing the activities of the various species attracted by the aphids, and taking photographs of them.

I started with the aphids themselves, which weren’t that easy to see or photograph, as they were feeding inside the curled leaves, and are also, like all aphids, very small in size. However, with a little patience, and the use of the macro flash unit for my camera, I was able to get some reasonable images. The locations of the aphids were very obvious, both because of the curled leaves, and also by the presence of the wood ants, which rely on the honeydew from aphids such as these as one of their primary food sources.

The aphids were feeding on the midrib of one of the rowan’s leaves, and it appeared to me that their feeding was causing the leaf to curl in on itself. A similar phenomenon occurs with another aphid species (Pachypappa tremulae) that feeds on aspen trees (Populus tremula), which had been recorded on Dundreggan in 2012. This may be an adaptation to protect the aphids from predators and parasites, as there are many species that prey on, or parasitise, aphids.

The Bobs were very impressed by the colony of aphids, having never seen such a concentration of this species before.  The aphids were also feeding not just on the leaves but also on the stems of the ripening berries on the tree, and they wondered if this behaviour had been recorded before, as they weren’t aware of it themselves. There were large numbers of wood ants tending the aphids, and others going up and down the trunk of the tree itself, to and fro from the aphid feeding sites.

 

 

 

 

 

 

 

There was a steady stream of wood ants going up and down the trunk of the tree, no doubt seeking to take their turn at harvesting the honeydew from the aphids. While some of the ants just passed each other on the trunk, others paused briefly to touch antennae with ones going the other way, providing chemical cues to each other which form a major communication method amongst the members of a single ant nest.

The aphids suck the sap of the tree using their rostrum, which is a fused combination of mouthparts that can penetrate through the tree bark and reach the phloem underneath, where the sap flows. The ants stimulate the excretion of honeydew from the aphids by stroking them with their antennae, and will move around a colony, touching different aphids, to get a steady supply of the sweet liquid waste product from them.

Because of the numbers of aphids and ants I wasn’t able to keep track of the individual ants, but I assumed that they were each consuming their fill of the honeydew, and then descending the tree, to be replaced by others that were continuously passing them, on the way up the tree’s trunk. In addition to consuming honeydew directly from the aphids themselves, the ants were also feeding on it from where it had dripped on the rowan’s leaves, and they were joined in this by a range of other insects.

 

 

 

 

 

 

 

 

 

 

 

 

 

As I watched, I saw more and more insects were attracted by the bounty of the honeydew, flying in to feed from the liquid that had dripped on to the rowan’s leaves. Unlike the wood ants, none of these other insects attempted to obtain honeydew from the aphids directly, but there was enough on the leaves to provide for them. Hoverflies, flies from various different groups and a common wasp all arrived to feed on the bounty of the aphids’ waste product.

The more time I spent observing the activity on the tree, due to the presence of the aphids, the more life I saw there. One of the wood ants going up or down the tree had become caught in the web of an orb web spider (Araneus diadematus) that was spun from the trunk of the rowan tree.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I watched this scene for a while, but the spider studiously ignored the ant – perhaps it knew from past experience that the ant was either unpalatable, or possibly dangerous – the spider was missing a couple of its legs, so must have been involved in a previous altercation of some sort. The ant would be unable to free itself from the web, so would suffer a lingering death, eventually dying of hunger or starvation, if the spider continued ignoring it.

While I continued to watch the ants going up and down the tree, my eye was caught by something translucent on the other side of the trunk. When I took a closer I saw that it was a slug, and from this particular angle, its body was backlit in such a way that it appeared to be opaque, with the light shining partially through it. I’d never seen a slug in such a light before, and this only served to increase my growing sense of wonder at the life that was on the rowan tree.

In order to get a better view of the slug, I photographed it from the other side as well, where it could be seen more clearly. I think this may be the tree slug (Limax marginatus), but I’ll need to get that identification confirmed or corrected by an expert. Interestingly enough, there was another spider, much smaller in this case, sitting on the tree trunk, immediately below the slug, and the two invertebrates seemed quite content to be in close proximity, with neither bothering the other.

When I looked again at the branches of the rowan where the aphids were feeding, I noticed some galls on the leaves there. These were induced by a mite (Eriophyes sorbi).

 

 

 

 

 

 

 

These galls take the form of an erineum, or a felt-like patch, on the underside of the leaf. The galls are also visible from the upper side of the leaves, where they appear as pale, discoloured blotches. The galls are not connected in anyway with the presence of the aphids, but they added to the diversity of life that was living on the tree.

 

 

 

 

Nearby, there was a case on the underside of another leaf of the rowan. This would have been produced by the larva of a micro-moth, probably one in the genus Coleophora, but I’ll have to seek expert opinion to get it identified correctly.

While I was taking photographs, the Bobs were busy photographing the aphids as well. Bob Dransfield has a camera with a home-made adapter to direct the built-in flash forwards for macro photography. He gets excellent results with it, and they have produced an excellent online visual resource about aphids which is accessible here. This was their last day of surveying on Dundreggan, so we reluctantly left the rowan tree in the late afternoon and headed back to the lodge.

However, I was back at Dundreggan again the next day, to spend time with the next biodiversity surveyors, Joe Botting and his partner Lucy Muir, who had just arrived to do a survey for Hemiptera (true bugs). I took them up to see what I was calling by this time the ‘wonder tree’, and some of the photographs on this page were taken on that second day, when I spent another couple of hours with this rowan and all the life it was supporting.

 

On the second day, I spent quite a while watching the wood ants, and taking photographs and some video footage of them, going up and down the trunk of the rowan tree. They seemed tireless in their activity, and full of energy – the aphid honeydew was no doubt the source of that. I’ll finish this blog off therefore with some of the video footage, and I hope that it, and the still photos here, will convey something of the sense of wonder I experienced from my time with this rowan, and the varied life it was supporting.

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See also the follow up to this blog – Return to the Wonder Tree.