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Pterocomma pilosum

Hairy willow bark aphid

Identification & Distribution  Biology & Ecology 

Identification & Distribution:

The wingless viviparae of Pterocomma pilosum are greenish, greyish or brownish. The terminal process of the antenna is 1.0-1.9 times as long as the base of the last antennal segment. The base of antennal segment six has 5-10 long hairs (cf. Pterocomma rufipes  which has only 1-3 long hairs on the base of antennal segment six). All abdominal segments have marginal plates but the pleurospinal plates are discontinuous or absent. Pterocomma pilosum siphunculi are yellowish and cylindrical (cf. Pterocomma rufipes which has slightly or markedly swollen siphunculi). The body length of the viviparous aptera is 2.3-4.0 mm.

 

Winged viviparae have more or less fused abdominal cross-bands on abdominal segments 2-8 and marginal sclerites. The pictures below are micrographs of an apterous Pterocomma pilosum in alcohol (dorsal and ventral views).

 

Two subspecies of Pterocomma pilosum are recognised:

  • Pterocomma pilosum pilosum: has no or very small marginal tubercles on the prothorax and abdominal tergites 1-7.
  • Pterocomma pilosum konoi has well-developed marginal tubercles on the prothorax and abdominal tergites 1-7.

They feed on both broad leaved willows (Salix cinerea and Salix caprea) and on narrow leaved willows (Salix alba), where they live in large colonies along the twigs and branches or partly concealed in crevices in the bark. The hairy willow bark aphid is nearly always attended by ants. Sexual forms occur in October. Pterocomma pilosum is found in Europe eastward to Pakistan. It has also been found in North Africa, and has been introduced to North America.

 

Biology & Ecology:

Colour variation

The apterae and especially the nymphs of Pterocomma pilosum are cryptically coloured to match the background colour of the bark on which they are resting. The picture below shows a dark blackish green adult aptera and a light green adult aptera. The dark nymphs with orange siphunculi belong to another species (see below in associations).

Other apterae of Pterocomma pilosum are yellow-brown as in the picture below:

The alatae, on the other hand, have bold black cross-bands on most abdominal segments which is classical aposematic (warning) coloration.

This could be Mullerian mimicry, with distasteful alatae (from accumulating a chemical such as salicylic acid, the active ingredient of apirin, which is present in the bark of Salix trees) mimicking a widely used warning pattern. Alternatively it could be Batesian mimicry, where the alate is not distasteful but is copying insects which are. Whichever it is, the adult alatae are sending out a warning signal, whilst the nymphs are trying not to be noticed.

 

Associations

The mixing together of cryptic and aposematic forms is taken a step further when bark aphids form multispecies groups. Both the nymphs and the wingless adults of Pterocomma pilosum are cryptic, but in southern Britain they regularly form mixed colonies with the aposematic black, white and orange Pterocomma salicis. 

The consequences of mixed-species colonies for the visibility of the aphids to predators is intriguing. We discuss this further in our aphid colour page.  Mixed species groups occur with other bark feeding aphids, for example Cinara cuneomaculata  occurs in mixed species colonies with Cinara laricis.  For leaf-feeding aphids, Shearer (1976)  has demonstrated that aphids reared in aggregations are heavier than ones reared on other parts of leaf, indicating that such aggregations act as "sinks" diverting nutrients towards aphid colonies.

Molnar et al. (2003)  looked at the niche separation between Pterocomma pilosum konoi and Pterocomma rufipes  on willow. In spring, the population of Pterocomma pilosum konoi increased earlier. In autumn both species reached their peaks simultaneously, but there were fewer individuals of Pterocomma pilosum konoi for a longer time. The latter species was more vulnerable to aphid parasitoids in each season. Molnar does mention that small numbers of Pterocomma salicis  were also present, but he does not report mixed colonies of any of the three species.

 

Ant attendance

Molnar et al. (2000)  studied attendance of willow aphids (Chaitophorus and Pterocomma species) by ants. They found the mutualistic interactions between ants and aphids were stronger for Chaitophorus vitellinae than for Pterocomma species. Mostly jet black ants (Lasius fuliginosus) were responsible for these interactions, but Lasius niger and Lasius brunneus also played a role in attending aphids.

We have found jet black ants (see pictures above and below) attending Pterocomma pilosum on a number of occasions. In each case attendance was close and sustained, and these ants actively defended the colony.

 

We have also found southern wood ants (Formica rufa) attending Pterocomma pilosum (see pictures below). Attendance was similarly 'energetic', with the ants attacking the (human) intruders.

The ant often cover the aphid's body with its own whilst antennating the aphid.

 

Whether they always attack potential natural enemies, such as adult syrphids ovipositing, is another matter. It is not unusual to find ant-attended colonies surrounded by syrphid eggs, which suggests that adult syrphids may be able to evade the attending ants - whether the emergent syrphid larvae are similarly capable is not known.

 

Natural enemies

There are rather few records of parasitism of Pterocomma pilosum. Rakhshani et al. (2007)  gives Euaphidius cingulatus as a specific parasitoid of Pterocomma species, and this appears to be the parasitoid that we found (see pictures below of the mummies). They noted that hyperparasitoids were abundant and may well have a critical impact on the efficacy of the primary parasitoid.

 

The pictures below are female and male specimens of the parasitoids that we reared from the Pterocomma mummies. They agree in all respects with the key characteristic of Euaphidius cingulatus (No. antennal segments: female=19, male=21; face is densely pubescent; the petiole is long, with dorsal central longitudinal carinae).

 

The large larva in the picture below, which is feeding in a mixed species colony of Pterocomma pilosum and Pterocomma salicis, is probably the syrphid Didea fasciata. The adults of that species are found in woodland, where males aggressively defend sunlit patches. Their larvae are found among aphid colonies on both coniferous & deciduous trees.

Bagachanova (1990)  recorded larvae of a number of species of Syrphidae feeding on Pterocomma species including Scaeva pyrastri, Didea alnetim, Metasyrphus nitens, Syrphus ribesii and Syrphus vitripennis.

The image below shows (dead?) nymphs of Pterocomma pilosum hanging from a branch by their rostrums. This is the only time we have seen anything like this - presumably they have died from a disease or possibly from a toxin produced by the host plant (so don't try this at home).

Acknowledgements

We are extremely grateful to Jon Martin of the Natural History Museum, London for identifying the Pterocomma pilosum we found living under loose bark on Salix cinerea. We have ourselves made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, we have then confirmed identifications by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Blackman (1974) , Stroyan (1977) , Stroyan (1984) , Blackman & Eastop (1984) , Heie (1980-1995) , Dixon & Thieme (2007)  and Blackman (2010) . We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure  provided by Blackman & Eastop (2006).

Useful weblinks 

References

  •  Bagachanova (1990). The fauna and ecology of the syphids (Diptera, Syrphidae) of Yakutia (in Russian). Yakutsk Nauchnye Tsentr SO AN SSR. 164pp. Full text 

  •  Molnár, N. (2003). Population dynamics features of willow-feeding aphids. Acta Phytopathologica et Entomologica Hungarica 38 (1-2), 125-135. Full text 

  •  Molnár, N., Kovács, É. and Gallé, L. (2000). Habitat selection of ant-tended aphids on willow trees. Tiscia 32, 31-34. Full text 

  •  Rakhshani, E. et al. (2007). Parasitoid (Hymenoptera,Braconidae, Aphidiinae) associations on willows and poplars in Iran. Acta Zoologica Academiae Scientiarum Hungaricae 53(3), 281-292. Full text 

  •  Shearer, J.W. (1976). Effect of aggregations of aphids (Periphyllus spp.) on their size. Entomologia Experimentalis et Applicata 20(2), 179-182. Abstract