Issues in Aphid Biology
|"It has long been an axiom of mine that the little things are|
infinitely the most important"
A first look at the extraordinary life of aphids underground
Dransfield, R.D. & Brightwell, R.On this page: Aphis taraxacicola Aphis crepidis Aphis hypochoeridis Dysaphis radicola
A few aphids, such as Cinara and Lachnus species, opportunistically feed on roots, but for some aphid species this is their normal habitat. Given their concealed way of life, relatively little is known about root aphid ecology. Many root aphid species are regarded as quite rare, but may simply be under-recorded. Root aphids, such as Pemphigus bursarius, (which host alternates between poplar leaves and lettuce roots) Eriosoma ulmi, (alternates between elm leaves and grass roots) and Anuraphis subterranea (which moves between pear leaves and umbellifer roots) are important agricultural pests. Most of those we mention below are not serious pests, but some are important components of our grassland biodiversity.
Dandelion root aphid (Aphis taraxacicola) - hidden communities on yellow composites
We first came across the dandelion root aphid when walking along the road past the local churchyard. A dandelion (Taraxacum officinale) was (just) managing to grow between the wall and the tarmac, and had a powdering of soil over its basal rosette of leaves. Careful brushing away of the soil revealed the colony of mottled dark green aphids shown below.
This is the dandelion root aphid, Aphis taraxacicola. Strictly speaking it does not live on the roots per se but on the root collar. The adults are mottled dark green, but the immatures are somewhat paler, especially the first instars (we'll come to the slightly larger straw-coloured aphid in the picture shortly). The aphid colony is tented over with soil particles by ants (see below left), often by the common black garden ant Lasius niger (see below right), rendering the aphids themselves invisible to the casual observer.
So do we find predators and parasitoids operating under the soil particle tent?? Well, certainly for the Aphis species (such as Aphis sambuci) that live at the base of plants, the answer seems to be yes. We have found parasitoids busy going round the colony parasitizing the aphids (below left), and we have found parasitized 'mummies' (below right). The straw-coloured aphid in the middle of the very first image above was simply an aphid in the process of mummification.
As for predators, attending ants will certainly reduce the number of predators, but we have still found syrphid larvae amongst the colonies (see below).
Coccinellid larvae of the genus Scymnus are reputedly important predators of root aphids, but we have so far only found them predating conifer aphids and the pellitory-of-the-wall aphid (Aphis parietariae).
There are other aphids that live on the roots of dandelion - Trama troglodytes and Trama rara. These live on the roots themselves rather than the root collar and show some extensive adaptations to life underground including greatly reduced eyes. The Natural History Museum website has some pictures and more information.
Hawk's-beard root aphid (Aphis crepidis) - a rarity on shingle vegetation
The hawk's-beard root aphid (Aphis crepidis) pictured below is very closely related to Aphis taraxacicola, but lives on a different host, namely Crepis biennis and Crepis capillaris. The aphid is dark bluish green to yellow-green, and morphologically indistinguishable from Aphis taraxacicola.
We have yet to confirm this identification microscopically, but no other root-feeding Aphis is known to occur on Crepis. Stroyan (1984), reports that Aphis crepidis is very little known in Britain (only Cambridge and Derby), but comments that the species is perhaps overlooked, a common problem for all the root feeders.
Cat's-ear root aphid (Aphis hypochoeridis) - not uncommon but seldom recorded
The cat's-ear root aphid, Aphis hypochoeridis, is easy to distinguish from the two species above (apart from living on a different host) because its colour in life is bright yellow to pale greenish yellow with dark siphunculi. It also differs from the other species in certain structural attributes.
This was one of our easiest finds of the root aphids - there were several colonies of the species on cat's ear (Hypochoeris radicata) growing on the grassy bank outside a neighbour's house. The aphid colonies were tented over with soil particles by ants (see below).
This may be done either by the common black garden ant Lasius niger or, as in this case, by a Myrmica species.
Apple-dock aphid (Dysaphis radicola) - but not host alternating to apple in Britain
We came across the apple-dock aphid when searching for interesting aphids on salt marsh and shingle vegetation at Rye Harbour in Sussex. The only indication of an aphid colony were a few grains of sandy soil around the base of the plant.
Parting those grains revealed a smallish colony of gray aphids (see below).
This turned out to be not an Aphis species (with a finger-shaped cauda) but a Dysaphis species (with a short rounded cauda). The wingless adults are greenish gray to leaden gray and wax powdered as can be seen in the picture above.
As its common name suggests, Dysaphis radicola usually host alternates from apple (Malus) to the roots of dock (Rumex). On apple the aphid rolls and reddens the margins of the leaves, making a longitudinal roll near the mid-rib. This host alternating aspect of its biology may be lost in some populations, and the aphids stay on dock all year. It has never been recorded on apple in Britain. The factors promoting loss of host alternation in some localities are thought to be lack of the original primary host (not the case here since apple is common in Britain), poor success on the primary host and excessive competition on the secondary host.