Pemphigus obesinymphae

Identification, Life cycle & Distribution

The Pemphigus obesinymphae life cycle is rather different from other Pemphigus species. Instead of overwintering as eggs on the primary host (poplar), they overwinter as viviparae on the secondary host (roots of Brassicaceae). In spring these populations produce sexuparae, which fly to Populus after the initial leaf flush. The sexuparae on poplar produce the sexuales, that mate in the usual manner on the bark, and then lay eggs on the primary host, albeit rather later in the year than the closely related Pemphigus populitransversus. The eggs hatch rapidly, with no period of diapause, to give the fundatrices. These feed on the petioles of late-flushing leaves of Populus fremontii (Aoki & Moran, 1994), and Populus deltoides (Abbot & Withgott, 2004), inducing gall formation.

Pemphigus obesinymphae galls (see three pictures below) are usually more-or-less spherical, and are close to the leaf blade, though do not incorporate the leaf midrib (cf. Pemphigus populitranversus, which usually has rather more elongate galls, positioned about mid-way along the petiole). When mature the galls are 8-16 mm in diameter, with the lips of the transverse slit sometimes bulging outwards, and a circular ostiole (=small opening) about 1.2 mm in diameter somewhere along the slit. The ostiole allows defenders to enter and exit the gall.

Note: The aphid species living in spherical galls of this species on Populus deltoides in eastern North America, now known to be Pemphigus obesinymphae, were formerly considered to be a morph of Pemphigus populitranversus.

First & third images above copyright Lorri-Gong; second image copyright James Bailey;
all under a creative common licence.

Only two generations occur in the galls: the fundatrix, which is apterous, and her offspring, the fundatrigeniae, all of which are winged. The adult fundatrix of Pemphigus obesinymphae (see large brown, white-tufted, aphid in first picture below) has 4-segmented antennae. The apical rostral segment is without secondary hairs. The shape and distribution of wax plates is as usual in Pemphigus species. The eighth tergite has 10-19 hairs. The first-instar progeny of the fundatrix in the gall are heavily sclerotized with well-developed forelegs. These adaptations enable them to attack predators. The 2nd and 3rd-instar progeny are usually much-swollen (hence the names 'obesinymphae' and 'cherubic'), and resemble wingless adults. Why they are so swollen is unknown. Alatae do not leave the galls till August-October, migrating like Pemphigus populitransversus to cruciferous hosts on which overwintering occurs.

The alate Pemphigus obesinymphae fundatrigenia (see 2 alatae in second picture below) has 6-segmented antennae. Antennal segment III is shorter than segments IV + V together and the rhinarium of segment V is rather large and approximately square. Antennal segment III has 3-6 secondary rhinaria, with 0-1 rhinaria on IV and V. The secondary rhinaria are normal in type, extending at most round a fraction more than half the circumference of segment III. The apical rostral segment is about as long as the second hind tarsal segment (HTII). Separation from Pemphigus populitranversus requires examination of the embryos within the emigrant alatae. Embryos of Pemphigus obesinymphae in the emigrant alatae have the tip of their rostrum extending beyond the first segment of the fore-tarsi (cf. embryos of Pemphigus populitranversus in the emigrant alatae, in which the tip of the rostrum does not extend beyond the first segment of the fore-tarsi). There are small oval to transversely elongate spinal wax plates on abdominal tergites I-VI, and a large elongate one on VIII, but plates on most posterior tergites are reduced in size. Siphunculi are ringlike, with only half of the "ring" sclerotized.

First images above copyright James Bailey; second image copyright Millie Basden;
all under a creative common licence.

Pemphigus obesinymphae host alternates from cottonwood (Populus), its primary host, to the roots of Brassicaceae, its secondary host. Abbot and Withgott (2004) describe how it is distributed across the United States on two hosts: the eastern cottonwood (Populus deltoides), on which it is sympatric with Pemphigus populitransversus, and the Fremont cottonwood, (Populus fremontii) on which that species is absent. The two aphid species were confused for many years. Pemphigus obesinymphae is widely distributed in the USA, and probably in northern Mexico, whilst Pemphigus populitransversus is only found in eastern USA.

Other aphids on the same host

Primary host

Pemphigus obesinymphae has been recorded from 3 poplar species (Populus deltoides ssp deltoides, Populus deltoides ssp occidentalis, Populus fremontii).

• Blackman & Eastop list 25 species of aphid as feeding on Eastern cottonwood (Populus deltoides ssp. deltoides) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 3 as occurring in Britain (Show British list).

• Blackman & Eastop list 11 species of aphid as feeding on Fremont cottonwood (Populus fremontii) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists none of these as occurring in Britain (Show British list).

Secondary hosts

Pemphigus obesinymphae has so far only been recorded from 1 species of the Brassica genus (Brassica oleracea) - but doubtless occurs on many others in the Brassicaceae (=Cruciferae) family.

• Blackman & Eastop only list 18 species of aphid as feeding on cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, savoy, kohlrabi (Brassica oleracea) worldwide, to which we have added Pemphigus obesinymphae from the work of Chen et al. (2009) (Show World list). Of those aphid species, Baker (2015) lists 11 as occurring in Britain (Show British list).

Acknowledgements We are grateful to James Bailey, Lorri-Gong & Millie Basden for making their images of Pemphigus obesinymphae available for use under a Creative Commons License. We have used the species account given by Aoki & Moran (1994) together with information from Abbot & Withgott (2004), Chen et al. (2009) & Roger Blackman & Victor Eastop in Aphids on Worlds Plants. We fully acknowledge these authors, and those listed in the reference sections, as the source for the (summarized) taxonomic information we have presented. Any errors in 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 Abbot, P. & Withgott, J.H. (2004). Phylogenetic and molecular evidence for allochronic speciation in gall-forming aphids (Pemphigus). Evolution 58(3), 539-553. Full text Aoki, S. & Moran, N.A. (1994). Pemphigus obesinymphae, a new American aphid species with defenders and swollen nymphs (Homoptera: Aphidoidea: Pemphigidae). Journal of the New York Entomological Society 102(2), 251-260 (p. 252) Full text Chen, N. et al. (2009). Molecular identification and population dynamics of two species of Pemphigus (Homoptera: Pemphigidae) on cabbage. Insect Science 16, 115-124. Full text