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Identification & Distribution:The apterae of Aphis jacobaeae (see first picture below) are dark green and are not wax powdered. The thorax has rather extensive dark lateral sclerites, whilst the abdomen has small marginal sclerites on tergites 2-4, small sclerites just behind the siphunculi, dark bands across tergites 7-8, sometimes a small median sclerite on 6, and dark intersegmental muscle sclerites. The marginal tubercles are prominent. The siphunculi have a conspicuous apical flange. The legs are dark except for the extreme bases of their femora. This feature will usually distinguish Aphis jacobaeae from Aphis fabae which also occurs on ragwort, but has pale tibiae. The body length of Aphis jacobaeae apterae is 1.8-2.2 mm.
The alate Aphis jacobaeae has a similar pattern of sclerites on the abdomen, but the marginal and post-siphuncular sclerites are larger. The micrographs below are dorsal and ventral views of an adult aptera of Aphis jacobaeae. Note that the ventral view (second picture below) provides a good view of the prominent marginal tubercles along the edges of the abdomen.
The ragwort aphid does not host alternate. Sexual forms occur in autumn. It feeds on ragwort (Senecio jacobaeae) living basally or higher on the stem or on the flowers. They are usually attended by ants which may build earth tents over the aphids. Aphis jacobaeae occurs in western and central Europe and into Russia.
Biology & Ecology:
We have found Aphis jacobaeae in a range of habitats including chalk downland (especially common), old pasture, shingle ridge grassland and mixed woodland. The picture below shows a dense colony of Aphis jacobaeae on ragwort growing on old shingle ridge grassland. Most of the aphids in this colony appear to be alatiform nymphs, destined to migrate to hosts anew.
The dynamics of aphid populations are influenced both by plant quality and the actions of natural enemies. Müller et al. (2005) sought to assess the relative importance of fertiliser addition to plants and exclusion of predators for growth of Aphis jacobaeae populations in the field. NPK fertilizer was applied weekly to ragwort plants at two sites. There was no effect of fertilization on aphid populations at either site. Exclusion experiments demonstrated that predators were the main determinant of aphid colony growth. Ants were left unmanipulated in both sites and their performance on the aphid colonies did not significantly differ between sites or between treatments.
Vrieling et al. (1991) looked at the interactions between ragwort, the aphid Aphis jacobaeae, the ant Lasius niger and the cinnabar moth (Tyria jacobaeae) and ragwort. Ragwort is protected from herbivores by high levels of toxic alkaloids. The larva of the cinnabar moth is able to sequester the alkaloids, rendering it harmless to the larva. But the larva is then toxic to vertebrate predators - hence the aposematic colouration (see picture below). Large numbers of larvae may completely defoliate ragwort plants. Ants defend ragwort plants infested with aphids against larvae of the cinnabar moth, so such plants escape regular defoliation by cinnabar moth larvae. However, only plants with a lower pyrrolizidine alkaloid get colonized by aphids and ants. Natural variation in cinnabar population levels and hence herbivore pressure leads to the maintenance of genetic variation in pyrrolizidine alkaloid concentration observed in natural populations of ragwort.
The aphid also sequesters pyrrolizidine alkaloids, but it does not invest in aposematic colouration. Aphis jacobaeae has dark green cryptic colouration and is also defended by attendant ants. The ants may also build an earthen shelter over the aphid colony as shown below. This may be partly for protection, but also probably serves to maintain high humidity levels.
Farmers and horse owners are antagonistic towards ragwort, because the toxic alkaloids can result in poisoning of livestock (albeit cases of poisoning are rather rare). Ants (attending aphids) on the ragwort are probably highly beneficial for livestock owners because they make it even more unlikely that cattle and horses will eat ragwort (try eating a mouthful of angry ants).
The absence of cinnabar larvae, due to aphid-tending ants, will also favour plants with rather lower alkaloid levels. Moreover ragwort control, by cutting and/or pulling up ragwort, exacerbates the problems as the broken stems produce new plants. We have encountered one appalling approach to ragwort control on nature reserves, namely the use of herbicides and burning. This was in the fragile, but heavily-used, habitat of Barrow dunes in Somerset (see picture below).
We fully endorse the views of Neil Jones (Common myths about ragwort) who comments on the abundance of misleading and erroneous information about ragwort: