Pemphigus monophagus Maxson

Gall. Pale greenish yellow. Composed mostly of swelling on upperside of leaf base with petiole broadened and somewhat twisted; orifice consisting of a slit on underside between petiole and leaf. Gall may be either globular or with more or less transverse indentation as shown in figure.

Collections. In twisted petiole and leaf galls on Populus angustifolia and P. tacamahaca (There appears to be no alternate host since the alate viviparae leaving the gall as soon as it opens are already sexuparae and must deposit the sexuales on the winter host.) Colorado: Buckhorn Creek (near Masonville), at Redfeather, Horseshoe Park (in Rocky Mountain National Park), Colorado Springs and Walden. Fundatrices July 4 to Sept. 23, apterous summer viviparae July 12 to Sept. 23, alate viviparae (sexuparae) Aug. 18 to Sept. 23, sexuales Aug. 10 (born from alatae which were imprisoned); rather rare. The fundatrix and apterous viviparae (very few) are found in the galls with alate sexuparae.

Pemphigus monophagus Maxson

The gall is globular, pale greenish yellow, consisting of a swelling of upper side of base of leaf lamina with petiole broadened and somewhat twisted, and a slit-shaped opening underneath between petiole and leaf (fig. 131N, and Maxson 1934). Fundatrix honey-yellow to pale olive-green, producing a generation of creamy white apterae in the gall. Monoecious, alatae (BL 1.4-1.9 mm) emerging from galls in August-September to deposit sexuales on the bark of the trunk. On Populus angustifolia, P. balsamifera and P.trichocarpa in western North America. Aoki & Kurosu (1988) discussed the evolution of monoecy in this species.

Pemphigus monophagus, n. sp.

Described from holotype and 16 paratypes from gall on Populus balsamiferae at 8,200 feet elevation, August 17, 1924, in Rocky Mountain National Park, Colorado.

The Gall. A more or less globular gall composed of leaf blade located at the base of the leaf with nearly circular opening on the under side closed by a thickened portion of the petiole that lays in a plane almost parallel to the blade of the leaf. The opening may be at the right or left of midrib. In a gall open on the right side of the midrib the thickened part of the petiole bends sharply to the left; then from a point near where the petiole passes into the midrib there is an abrupt bend to the right. Thus the thickened parts form a letter S. When the gall opens on the left of the midrib the S is backwards. There is usually a shallow furrow passing over the gall near its middle suggesting a double gall.

Taxonomy.-This species is placed in the genus Pemphigus because of the great structural similarity. In the number of generations and the sequence of forms it differs from the typical Pemphigus. There are but four generations, three of which, the fundatrix, fundatrigenia and sexupara, all develop in the same gall. Observations lead to the conclusion that all of the young of the fundatrix develop into apterous viviparae, these in turn producing the sexuparae. Further study may place this species in a new genus since it resembles Pemphigus much as Cornaphis does Asiphum. What appears to be this species was taken by J. J. Davis on August 5, 1913. near Flat Head Lake in Montana. Material received from R. Glendenning collected at Lillooet and Merrit, B. C., on Populus trichocarpa also appears to be this species.

Pemphigus monophagus Maxson, 1934.

Stem mothers feed near the base of the leaf blade to form a roughly globular gall mainly comprising the swelling of the upper surface of the leaf blade and partially comprising the swelling and twisting of the leaf petiole (Palmer 1952). The gall has a circular opening on the underside of the leaf and is frequently bisected by a shallow groove, suggesting a double gall (Maxson 1934). This species does not have a secondary host, completing its entire life cycle on the Populus host. The only apparent generation of alatae are the sexuparae, which exit the gall in late August and early September. First instars of this species include a morphologically distinct subset of instars that are probably members of a soldier caste, although this has not been confirmed either experimentally or by direct observation (Aoki and Kurosu 1988).

Primary hosts include P angustifolia and P balsamifera (Palmer 1952; Alleyne and Morrison 1977) (P balsamifera = P tacamahaca in Palmer 1952). Palmer (1952) reports P monophagus to be "rather rare," although high densities of galls were reported on leaves of P. balsamifera at Pigeon Lake, Alberta (Alleyne and Morrison 1977).

Pemphigus monophagus Maxson

Tough, leathery galls formed on or adjacent to the mid-rib on leaf blade of Populus balsamifera

This was not found in the areas surveyed in Quebec. It was first reported from Populus angustifolia and P. tacamabaca in Colorado by Maxson in 1934. Galls of this aphid were extremely numerous on leaves of balsam poplar, P. balsamifera L., at Crystal Springs on the west side of Pigeon Lake in Alberta during the summers of 1972 and 1973. The galls are rather tough and leathery and are produced as projections on the upper leaf surface on both sides of the mid-rib (Fig. 14) as a result of the feeding activity of the stem mother on the lower mid-rib region. This aphid is monoecious, the alates emerging in late August and early September.