An inordinate fondness for aphids

In late August and early September I was at Dundreggan for meetings on several different days, and had some extra time on each of them before I had to return home. I used those occasions to visit a patch of devilsbit scabious (Succisa pratensis) just outside the lodge, where a rare mining bee (Andrena marginata) had been observed harvesting pollen from the flowers in 2007 and the following two years. However, after an incursion by sheep had resulted in the scabious being eaten, the mining bee had not been seen again, so we’ve taken steps to encourage the healthy growth of the scabious, hoping that this revitalisation of its habitat will promote a return of the bee. Thus, I was visiting the scabious regularly on sunny days when it was in flower in August and early September, to see if the bee was there.

Devilsbit scabious flowers later in the summer than most other plants, and it therefore plays an important role in providing a source of nectar and pollen for insects, at a time when there are not so many other sources for them to draw upon. The mining bee times its emergence as an adult for when the scabious is in flower, but during my visits this year, there was no sign of it at Dundreggan, although it has been seen at a couple of other sites in Glenmoriston.

 

 

 

 

 

 

 

While I was searching for the bee, my attention was drawn by some small green shapes on one of the scabious flower stems, and as I looked closer I realised with some excitement that they were aphids.

I’ve had a growing fascination with aphids during the last 3 years, and although I knew that there are aphids that feed on devilsbit scabious plants, I’d not seen them before. These ones here looked like particularly attractive aphids, as they were bright green in colour, and were clustered densely together on the stem of the scabious, thereby providing a good opportunity for photography. Getting my high magnification macro lens out, I prepared to spend quite a while with them …

 

 

 

 

 

 

 

I emailed some of the photos and a few specimens to Ed Baker, an expert on aphids who helps me with their identification, and he replied that they are a common species called the rose aphid (Macrosiphum rosae).

The species derives its name from the fact that it feeds primarily on roses. However, in some situations it will also move to a secondary host (which can be teasel, valerian, or in this case, devilsbit scabious) partway through the summer season. This phenomenon is called host alternation, and is utilised by a considerable number of aphid species. Often the primary host is a tree or woody plant species, and the secondary host will be a completely unrelated herbaceous plant.

This illustrates some of the complexity of the ecological life cycle of aphids. In addition to host alternation, there is considerable variation within the morphology of individual species (for example both winged and wingless forms of an aphid species occur at different stages of the life cycle of a single species), while many species have sophisticated reproductive cycles, whereby sexual generations alternate with asexual, all-female generations, which are able to reproduce without mating.

In the case of the rose aphid, further complexity is added by the fact that the species occurs in two different colour forms. While almost all of the aphids I saw on the scabious plants were green in colour, a small group of plants were covered in reddish-brown aphids. I thought that these must be a different species, but when I sent some specimens to Ed, he confirmed that they were in fact the same species (Macrosiphum rosae), just the red colour variation of them.

As to why this species has two different colour forms, I don’t think anyone has come up with a satisfactory explanation to date. Perhaps it’s just a similar phenomenon to the variety of hair colour – from blonde and brunette to red and black – that people have?

 

 

 

 

 

 

 

While I was searching for more of the aphids on the scabious plants, I came across several caterpillars that were feeding on the flowers. A pale green one with whitish stripes along the length of its body turned out to be a young caterpillar of the broom moth (Ceramica pisi). I was familiar with the fully grown version of this caterpillar, but this was the first time I’d seen one of its earlier instars, as the stages of development of caterpillars are known as, between the times when they moult, or shed their skin, as they grow in size.

A few days later, when I was back looking for the aphids on the scabious plants again, I spotted a couple of mature broom moth caterpillars on some of the scabious.

 

 

 

 

 

 

 

Their coloration was now quite different, with very distinctive alternating bands of black and yellow running the length of their bodies, making them one of the easiest moth caterpillars to identify in our area. A different caterpillar on another scabious plant was harder to identify, and Roy Leverton, who helps me with caterpillar identifications, said it was one of the pug moths, probably the grey pug (Eupithecia subfuscata), but that it could also be of the closely-related common pug (Eupithecia vulgata).

As I continued to look at the numerous scabious plants flowering in this area near the lodge, I also came across a number of plant bugs on some of them. A bright green one was identified for me by Joe Botting, who co-authors the excellent British Bugs website, as being a common mirid bug (Plagiognathus chrysanthemi), while a larger brown bug (Lygus wagneri), which I’ve seen at Dundreggan in late summer in previous years, was also a member of the Miridae family.

The brown bugs were particularly common on the scabious flowers, and on one of them I saw an adult and a green nymph together. The nymph is an earlier stage of development in the bug’s life cycle, and in the photograph here the wing buds can be clearly seen. By the time the bug reaches its adult phase of development these will be fully formed wings.

 

 

 

 

 

 

 

 

 

Another bug on a different scabious plant was the common froghopper (Philaenus spumarius), which is best known for the white froth, or ‘cuckoo spit’, that its larvae produce on various plants in early summer.

Although I was pleased to find the bugs, my main interest lay with the aphids, so I continued to search for them in this  abundant patch of scabious plants. On one of the scabious stems I spotted a brown aphid that was a different shape to the green ones. I recognised this as an aphid ‘mummy’ – an aphid that has been attacked by a parasitoid wasp. What happens is that a tiny female wasp inserts its ovipositor into an aphid, laying its egg there. When the wasp larva hatches out, it eats the aphid from the inside, creating this distorted shape, and killing its host in the process.

When the wasp larva is fully grown, it pupates and the adult wasp emerges from the aphid mummy. I sent this aphid mummy off to Ed Baker, as he has a keen interest in aphid parasitoids, and he told me that a female wasp (Aphidius rosae) emerged from the corpse a few days after he received it. Apparently there are no prior records for that parasitoid wasp in Scotland – it’s not rare but there are very few people searching for aphid parasitoids so he thinks it has just been overlooked.

Because of their sheer abundance, aphids are attacked by a wide range of parasitoid wasps, with some wasps specialising on individual aphid species, while others will parasitise a number of different types of aphids. These wasps are known as primary parasitoids, as they attack the aphids, which are herbivorous insects. However, they are themselves subject to attack by secondary parasitoids – other wasp species that specialise in laying their eggs in the developing larvae of the primary parasitoid wasps.

In addition to suffering the attentions of these parasitoids, aphids are also preyed upon by a range of other insects, including the larvae of both midges and hoverflies. As I continued to spot aphids on the stems of the scabious plants, I noticed a number of orange blobs in amongst some of them. These are the larvae of one of the midge species that prey upon aphids. I collected some samples of these larvae, which I’ve sent to Ed, but he’s not yet been able to produce a positive identification for them.

I also emailed these photos I took of the larvae to Bob Dransfield, who did a survey of aphids for us at Dundreggan in 2013. After checking the scientific literature, he informed me that there are a number of midge species whose larvae feed primarily on aphids, and that these ones I’d found were likely to be larvae of the most common species (Aphidoletes aphidimyza). I’m hoping that Ed will be able to confirm (or correct) this provisional identification after closer study of the specimens I sent him.

How is it I wondered, though, that these midge larvae can succeed in preying on the aphids? After all the midge larvae have no legs, whereas the aphids do, and I’ve seen them move around readily enough on the stems of their host plants. How is it then that they fall victim to the midge larvae? The answer appears to be that the midge larvae are actually able to wriggle around quite quickly themselves, while an individual aphid, with its rostrum (the drill-like feeding appendage, for sucking plant sap) fully inserted into the stem of the scabious plant, is relatively immobile. The aphids cannot extract their rostrums in time when a midge larva approaches, and they are literally like sitting ducks, there for the taking.

Once a midge larva has reached an aphid, it injects a toxin into the aphid’s leg joints,  paralysing them, so that the victim is completely helpless. The larva then proceeds to suck the juice out of the aphid, leaving behind a desiccated and shrivelled corpse. An individual midge larva will feed on many aphids during a day, even killing some that it does not consume the juices of, and for this reason larvae of the species Aphidoletes aphidimyza are now used commercially to control aphid infestations on greenhouse crops.

 

 

 

 

 

 

 

In a natural situation like this, predation by the midge larvae takes its toll on some of the aphids, but their overall population on these scabious plants was so large that it didn’t seem like it would make any significant reduction in the aphid numbers.

On one of the scabious stems I noticed something different amongst the aphids. This was pale yellow in colour, and was clearly another larva, although it wasn’t as smooth in its external shape as the orange midge larvae. It too appeared to be feeding on the aphids, and when I sent a specimen and the photos to Ed, he responded that it looked like the larva of a hoverfly. I’m hoping he’ll be able to identify this for me in due course, along with the midge larvae.

On another scabious plant, a male garden spider (Araneus diadematus) was sitting patiently, waiting for a flying insect to get caught in its web, ignoring the aphids completely.

 

 

 

 

 

 

 

I never did see any of the mining bees (Andrena marginata) on the scabious flowers during those several days when I was at Dundreggan. However, I spent a lot of time observing and photographing these aphids and their predators, the midge and hoverfly larvae. It reinforced and strengthened my interest in this remarkable group of insects, which most people probably only consider to be pests. However, the more I find out about them, and their complex life cycles and  relationships, the more intrigued and enthusiastic about them I become.

When the 20th century biologist and life-long atheist, J. B. S. Haldane, was asked by some theologians about what could be inferred about the mind of the Creator from the works of His Creation, he famously replied that he must have had “An inordinate fondness for beetles.” This was a reference to the fact that over 400,000 species of beetles are known to science, and they comprise 40% of the total number of insects that have been described to date. By contrast there are only about 4,000 species of aphids currently known in the world (although there are undoubtedly additional species that have yet to be identified and classified). However, despite their comparative paucity, and my status as a relative newcomer to the study of them, I think it’s accurate to paraphrase Haldane’s quote and describe my interest in them as an inordinate fondness for aphids!