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Pemphigus populi

Club-shaped poplar-gall aphid

Identification & Distribution  Biology & Ecology  Damage & Control 

Identification & Distribution:

In spring Pemphigus populi form green or greenish-yellow globular outgrowths (see first picture below) of the mid-rib of the leaves of poplar (Populus) species, especially black poplar (Populus nigra). They are situated near the base on the upper side of the leaf. The outgrowths become club-shaped with the basal part narrower than the apex when mature.

 

Images copyright Dr László Érsek, all rights reserved.

Developing Pemphigus populi immatures are pale greenish yellow, whilst the mature alatae are greyish green with abundant wax (see second picture above). They have small but distinct marginal wax gland plates. The terminal process of the antenna is 0.25-0.4 times as long as the basal part of the sixth antennal segment. They have 3-9 secondary rhinaria on the third antennal segment, 1-3 rhinaria on the fourth segment and 0-2 on the fifth segment. The first tarsal segment almost always has 2 hairs (rarely 3). The apical segment of the rostrum is about 0.5-0.7 times as long as the second segment of the hind tarsus. The body is 1.5-2.4 mm long.

Pemphigus populi host alternates from various poplar species, mainly black poplar (Populus nigra) and lombardy poplar (Populus nigra 'italica'), to the roots of various species of Fabaceae, especially meadow vetchling (Lathyrus pratensis), black medic (Medicago lupulina) and tall melilot (Melilotus altissimus).

 

Biology & Ecology:

Pemphigus populi fundatrices hatch from fertilised overwintering eggs in late April, and induce development of the characteristic club-shaped gall (see picture below).

Image copyright Dr László Érsek, all rights reserved.

The adult fundatrix (see picture below) has abdominal wax gland plates, consisting of a few rather large facets, which secrete the grey wax which covers the fundatrix.

Image copyright Dr László Érsek, all rights reserved.

The fundatrix produces pale greenish yellow nymphs (see picture below) which develop within the gall.

Image copyright Dr László Érsek, all rights reserved.

These nymphs mature to wax-covered greyish green alatae (see image below). Pemphigus aphids have developed an efficient way of handling their excreted honeydew. They use a very hydrophobic (=water-repellant) wax that coats every element inside the gall: the gall itself, the aphids and the liquid honeydew. The secreted powdery wax has three distinct roles: (i) it is hydrophobic; (ii) it creates a microscopically rough inner gall surface made of weakly compacted wax needles, making the gall ultra-hydrophobic; and (iii) it coats the honeydew droplets, converting them into liquid marbles (see picture below of two such 'marbles'), that can be rapidly and efficiently moved. This produces a virtually non-stick environment, which is of vital importance in this enclosed space (Pike, 2002 ).

Image copyright Dr László Érsek, all rights reserved.

Pike (2007)  made a fascinating comparative study of the ecology of three gall-dwelling species: Pemphigus populi, Pemphigus spyrothecae, and Pemphigus busarius.  The three species have clear differences in their levels of social behaviour: Pemphigus populi is highly asocial. It has no soldiers, has the shortest galling phase, and exhibits synchronised development of a small number of individuals in a sparsely populated gall. Pemphigus spyrothecae on the other hand is a highly social species with aggressive morphologically-specialised soldiers, the longest galling phase, and unsynchronised development of a large number of individuals in a densely-populated gall. Pemphigus busarius is intermediate in these various characteristics and behaviours.

Pike noted that the opening of the galls of Pemphigus populi is highly unusual. Wide fissures open in the gall starting at its apex and often extend such that the gall's inner surface is turned entirely outward. The offspring of the fundatrix are almost entirely alate, and these all emerge immediately the galls open. Immediately following emigration from the gall of the alatae, Pike found 42.0% of galls contained predators, 47.3% contained inquiline aphids, and 22.7% contained aphidopathogenic fungus. Hence 87.3% of the 150 monitored galls were affected by competitors and natural enemies that would have been fatal to these aphids, had they continued to inhabit their galls.

El-Akkad & Zalat (2000)  described an infestation of Pemphigus populi on Lombardy poplar in Sinai, Egypt. Ecott (2003)  found galls of Pemphigus populi not uncommonly on black poplar at Hainault Forest, Essex. Some of the galls were broken open, apparently by predators. Osiadacz & Halaj (2014)  report Pemphigus populi for the first time from Poland. They concluded that this most likely reflects a rapid, almost invasive, increase in range of this species.

Acknowledgements

We especially thank Dr László Érsek for the images shown above.

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed 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

  •  Ecott, B.A. et al. (2003). Pemphigus populi (Hemiptera: Aphidiidae) in Hainault Forest, Essex. Cecidology 18(2): 56 Full text 

  •  El-Akkad, S. & Zalat, S. (2000). Populus galls induced by Pemphigus aphids in Sinai. Egyptian Journal of Biology 2, 15-19. Full text 

  •  Osiadacz, B. & Halaj, R. (2014). First records of gall-inducing aphid Pemphigus populi (Hemiptera: Aphidoidea, Eriosomatidae) in Poland with gall-based key to Central and North European species of the genus. Entomologica Fennica 25, 16-26. Full text 

  •  Pike, N. et al. (2002). How aphids loose their marbles. Proceedings of the Royal Society of London B 269(4), 1211-1215. Full text 

  •  Pike, N. et al. (2007). Ecological correlates of sociality in Pemphigus aphids, with a partial phylogeny of the genus. BMC Evolutionary Biology, 7: 185. Full text