Pemphigus betae

The aphid forms pouch-shaped galls on the leaf blades of its host; among its primary hosts are the balsam poplars, Populus section Tacamahaca (e.g., P. angustifolia James, P. balsamifera L.), and hybrids involving these species, and cottonwoods in Populus section Aigeiros (e.g., P. deltoides Marsh, P. fremontii S. Watson) (Fig. 1) (Palmer 1952; Harper 1959; Floate et al. 1997).

The taxonomic status of P. betae remains unclear. Pemphigus betae was originally described from specimens on sugarbeets, the gall form being unknown. Pemphigus balsamiferae Williams, described from the gall form, is currently considered a synonym of P. betae (Maxson 1916; Remaudiere and Remaudiere 1997). Grigarick and Lange (1962) considered P. betae to be synonymous with Pemphigus populivenae Fitch, which also forms pouch-shaped galls on the leaf blades of species in Populus section Tacamahaca. Summers and Newton (1989) used the name P. populivenae for aphids on sugarbeets in California. Harper (1959) designated aphids that formed galls projecting below the plane of the leaf blade on Populus hosts as P. betae (Fig. 1) and used the name P. populivenae for aphids forming galls that projected above the plane of the leaf blade. However, according to Maxson and Knowlton (1929), in heavy infestations the latter can occur in the position typical of P. betae. In addition, Pemphigus knowltoni Stroyan has been reported from Utah, where it forms galls similar to those of P. betae on P. angustifolia (Stroyan 1970). In Europe the species on beets and other Chenopodiaceae is named Pemphigus fuscicornis (Koch) and has no known associated primary host form (Blackman and Eastop 2000).

Collectively, these results identify at least three co-occurring populations forming similar galls on the same host trees.The concordance of the mitochondrial and nuclear data suggests that the three groups are reproductively isolated. Specimens from group A (Fig. 2) key to P. betae. Specimens from group C key to P. populivenae. Specimens from group B are intermediate. No specimens matched the state reported for P. knowltoni. The available evidence suggests that group B is an undescribed species. Other than P. betae, P. populivenae, and P. knowltoni, no North American Pemphigus species are known to regularly form elongate galls along the leaf midrib of species in Populus section Tacamahaca. Although group C was identified as P. populivenae, there are several inconsistencies. Galls of this species are reported to project above the plane of the leaf blade (Harper 1959), but galls sampled in the present study resembled those of P. betae, i.e., they project below the leaf blade. Galls of P. populivenae can apparently resemble those of P. betae when populations of the former are high (Maxson and Knowlton 1929). However, galls in the position normally ascribed to P. populivenae are uncommon in southern Alberta (Harper 1959; K.D.F., personal observations), whereas group C samples were common in our collections. These discrepancies are resolved if P. populivenae forms galls that can project either above or below the leaf blade regardless of population density. Alternatively, group C may represent an undescribed species morphologically similar to P. populivenae. Groups B and C co-occur in morphologically pure zones of P. balsamifera (samples from Cypress Hills and Lundbreck Falls, Alberta). Group B was not detected at sites where P. deltoides (section Aigeiros) or hybrids involving it occur.

It is apparent that several taxa form similar galls on the leaf blades of species and hybrids of Populus section Tacamahaca. The form of a gall in a given position (petiole, petiole2blade junction, along midvein), may depend as much on host physiology or morphology as on the inducing aphid species. Anecdotal observations that we made during recent collections indicate that midvein galls containing morphologically similar aphids tend to be more globular when close the leaf base than those located more distally, especially on thicker leaves, sometimes approaching the state characteristic of Pemphigus populiglobuli Fitch. Maxson and Kowlton (1929) noted galls similar to those of P. populivenae produced by aphids they identified as Pemphigus populicaulis Fitch and P. populiglobuli. Reliance on gall position and form as indicators of species identity is clearly not a viable approach for use with this group of Pemphigus species, given the current state of knowledge.

- RG Foottit, Kevin Floate, Eric Maw: (2010) Molecular evidence for sympatric taxa within Pemphigus betae (Hemiptera: Aphididae: Eriosomatinae)^©