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Aspen bark aphidIdentification & Distribution
Identification & Distribution:Apterae of Pterocomma tremulae are dark warm brown or olive brown and are dusted with powdered wax. The abdomen has large marginal and pleurospinal sclerites and marginal tubercles on the abdominal tergites. The pale yellowish siphunculi are slightly swollen. The body length of Pterocomma tremulae is 2.5-4.3 mm.
The terminal process is 1.5-2.0 times as long as the base of the last antennal segment. The second antennal segment has 8-12 hairs (this distinguishes it from Pterocomma populeum which has 4-6 hairs on that segment.) The third antennal segment has 4-79 secondary rhinaria and the fourth antennal segment has 0-9 secondary rhinaria (cf. Pterocomma populeum which has none on the third antennal segment).
Pterocomma tremula is usually found on suckers and two-year-old twigs of aspen (Populus tremula). No sexual morphs have been recorded, which is surprising given its northerly distribution. It is nearly always attended by ants. Pterocomma tremulae is found in northern, central and eastern Europe and west Siberia.
Biology & Ecology:
The main sign of a Pterocomma tremulae colony is usually the presence of large numbers of ants in a cluster on the branch.
Pterocomma tremulae is always attended by ants, whether by a Formica species or by Lasius fuliginosus. At Dundreggan in Scotland, tending was done by hairy wood ants (Formica lugubris), as can be seen in the pictures below.
Large numbers of ants could be found tending relatively small colonies of aphids on the twigs and branches of aspen (Populus tremula).
Although Formica lugubris were very attentive towards the aphids, it was noticeable that they were not very persistent in defending aphid colonies. The ants tended to scatter at even a relatively minor disturbance.
Nevertheless they seemed to be effective at keeping away most aphid predators. The only exception to this were cecidomyiid flies which unusually were laying their eggs directly on the aphids, rather than on adjoining vegetation. The picture below shows an adult Pterocomma tremulae with at least six eggs laid on its back, as well as one or two eggs laid on young nymphs.
Everything in a quite extensive literature indicates that Aphidoletes lay their eggs on the plant near aphids, not directly on the aphids. For example, Lucas & Brodeur (1999) working on potato aphids (Macrosiphum euphorbiae) found that Aphidoletes aphidimyza laid its eggs on the host plant in aphid infested sites with high plant trichome densities (in other words pubescent parts). We have instead found that Aphidoletes species often lay eggs directly on aphids - not just with Pterocomma, but also with Macrosiphum rosae, Cavariella theobaldi, Chaitophorus populialbae and Cinara laricis (see are these mites or eggs? ).