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Identification & Distribution:In life Dysaphis sorbi apterae are reddish-brown to dark green. The pale yellowish siphunculi are cylindrical and slender. Winged females developing on the primary host have dark antennae and dark slender cylindrical siphunculi, plus an ochreous to reddish yellow abdomen with a dark trapeziform dorsal patch. The antennae are slightly longer than the body.
The clarified slide mounts below are of adult viviparous female Dysaphis sorbi (on secondary host) : wingless, and winged.
Dysaphis sorbi is facultatively host alternating, with rowan (Sorbus aucuparia) as its primary host and bellflowers (Campanula) as its secondary host. Winged females are not produced till June and colonies may be found on rowan all year. On the primary host aphids may be found either in leaf galls or on the stems bearing developing berries.
Biology & Ecology:
Early colonies (see picture below) of Dysaphis sorbi crumple the leaves.
The mature leaf nest of Dysaphis sorbi (see picture below) is characterised by the leaf lamina senescing and turning brown, but the mid rib remaining green.
Dysaphis sorbi has been the subject of a number of behavioural studies in relation to the coevolutionary hypothesis on warning signals in plants. This states that red and yellow leaf colours in autumn signal to herbivores like aphids that the trees are well defended, and therefore should attract fewer colonizing aphids. Schaefer & Rolshausen (2007) artificially coloured leaves of rowan (Sorbus aucuparia) and monitored numbers of aphids landing on the leaves. These aphids included both Dysaphis sorbi and Rhopalosiphum oxyacanthae, the species recorded at Dundreggan. They found that contrary to the hypothesis there was no difference in the number of aphids landing on the different coloured leaves.
They did, however, find that there was a strong positive correlation between aphid numbers settling and fruit production of rowan. This latter observation may relate to our observation that some of the aphids at Dundreggan in July had left the leaf galls and were feeding at the base of the developing berries (see pictures above). Rearing these aphids produced only alates which presumably would then have migrated to the secondary host (Campanula). Rearing the aphids from the leaf nests produced only apterae.
One surprising aspect about Dysaphis sorbi highlighted by Heie, 2009 is that unlike most host alternating aphids, the males are only produced on the secondary host. The alate females which bear the apterous oviparae with which the males mate develop on both primary and secondary hosts. This means that host alternation is necessary for fertilization of overwintering eggs.
Colonies produce abundant honeydew which often attract a feeding frenzy of insects coming to consume it. In Dundreggan, Scotland we found numerous ants (Formica lugubris), adult hover flies (including Melanostoma scalare) and wasps (Vespula spp.).
However, there were rather few predators around (a single syrphid larva) and no evidence of parasitoid activity.