Pemphigus immunis

Identification & Distribution

The galls of Pemphigus immunis can be found on poplar trees. In shape and texture they are walnut-like, thick-walled, green-brown or red (see first two pictures below) and quite large, 2.5-4.0 cm in diameter. The gall is strongly curved so the broad apical opening lies at the base of the gall (see base of red gall in second picture below). The glaucous-green fundatrix produces large numbers of green waxy offspring which all develop to alatae (see third picture below). The alatae of Pemphigus immunis usually have no secondary rhinaria on the base of antennal segment VI (cf. Pemphigus borealis which usually has two or more secondary rhinaria on the base of antennal segment VI). The primary rhinarium on antennal segment V is very large (more than half encircling the segment) and often has small round islands. There are 7-9 secondary rhinaria on segment III, 2-4 on segment IV, 0-1 on segment V, and none on the base of VI.

Note: We think the galls below are Pemphigus immunis (which usually has a textured surface, and occurs singly), but they could instead be Pemphigus borealis (which usually has a smooth rather shiny gall and occurs in groups of 2-4). With these galls the only way to be certain about identification is to check the identity of the adult alatae leaving the gall using the characters specified above.

The alatae migrate to the secondary host (Euphorbia) and found colonies on the roots. The resultant apterae are coated with copious amounts of wax.

The primary host of Pemphigus immunis is poplar (Populus), especially black poplar (Populus nigra) and Euphrates poplar (Populus euphratica). Alatae emerge from the gall from April to August and host alternate to the roots of Euphorbia species such as madwoman's milk (Euphorbia helioscopia) and petty spurge (Euphorbia peplus). Pemphigus immunis is found in much of Europe (but not Britain, Denmark, Norway, Sweden, Finland, Estonia, Latvia or Lithuania), north Africa, south-west and central Asia, Pakistan, north-west India and China.

Biology & Ecology

Life cycle

The biology of Pemphigus immunis (=lichtensteini) in Europe was described by Roberti (1938), and in Pakistan by Habib and Ghani (1970). It was then reviewed by Ghosh (1984). The fundatrices hatch from the yellowish-white eggs in April and May, and start to induce the galls. Each fundatrix undergoes four moults, then upon reaching the adult stage they produce 300-500 larvae. These nymphs develop within the galls to winged fundatrigeniae (see picture below, along with some immatures) which leave the galls during June-August.

Image above copyright Marko Šćiban, all rights reserved.

In Europe sexuparae develop in September and lay 6-8 larvae of males and oviparae in a protective place such as dried galls, scars and cracks in the bark, or even among the dried leaves and grasses at the base. Male and female adults develop in 3-4 days, copulate and the female lays a single egg in a bark crack. In Pakistan sexuparae and sexuales were found in December under cracked bark and in old galls of Populus nigra. It was further noted that Pemphigus immunis will deposit eggs under the bark of an introduced hybrid Poplar, Populus euramericana, but never form galls on this plant - although Populus nigra, growing in the same locality, were heavily infested.

Other aphids on the same host

Primary hosts

Pemphigus immunis has been recorded from at least 9 poplar species.

• Blackman & Eastop list 41 species of aphid as feeding on black, or Lombardy poplar (Populus nigra) (Show World list). Of these Baker (2015) lists 17 as occurring in Britain (Show British list).

Secondary hosts

Pemphigus immunis has been recorded from 6 Euphorbia species (Euphorbia esula, Euphorbia falcata, Euphorbia helioscopia, Euphorbia lamprocarpa, Euphorbia peplus, Euphorbia stricta) - and possibly from Taraxacum montanum.

• Blackman & Eastop list 9 species of aphids as feeding on madwoman's milk (Euphorbia helioscopia) (Show World list). Of these Baker (2015) lists 4 as occurring in Britain (Show British list).

• Blackman & Eastop list 13 species of aphids as feeding on petty spurge (Euphorbia peplus) (Show World list). Of these Baker (2015) lists 9 as occurring in Britain (Show British list).

Acknowledgements We are extremely grateful to Marko Šćiban (HabitProt) for the images of Pemphigus cf. immunis in Serbia. 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 Ghosh, A.K. (1984). Homoptera: Aphidoidea. Part 3. Subfamily Pemphiginae. In: The Fauna of India and Adjacent Countries. Zool Survey of India, Calcutta. 429 pp. Full text Najmi, J. et al. (2018). Gall-inducing aphids of Populus spp. in Razavi Khorasan Province, with introducing two new species for Iranian fauna. Journal of Plant Protection 32 (2), 46. Full text Roberti, D. (1938). Contributi alla conoscenza degli afidi d'Italia. Part. I. I Pemphigini del Pioppo. Boll. lab. Zool. gen. agr. Portici 30, 169-239.