Pemphigus populi-ramulorum Riley

Gall. Green. Normally globular with transverse mouth, about 25 mm in diameter. In cases of heavy infestation some galls are elongated and some occur on petioles where they differ from galls of P. populi-transversus by protruding upper lip. Normally located on stem of twig at base of petiole.

Collections. As winter host on Populus sargenti and P. “balsamifera” (prob ably P. candicans or deltoides), in twig and sometimes petiole galls; summer host unknown. Throughout region. Fundatrices June 8 toJuly 22, alate funda trigeniae June 8 to Aug. 14; rather common.

Pemphigus populiramulorum Riley

Galls are formed by swelling of new growth of twigs, or rarely at the bases of petioles; they are quite large (up to 2.5 cm diam.), rounded, green to brownish, with a transverse or vertical slit-shaped opening which often has protruding lips. The primary host is usually Populus deltoides ssp. occidentalis (= monilifera), but it is also recorded from P. balsamifera and P. fremontii. The fundatrix is pale creamy yellow (Palmer 1952). Alatae (BL 2.2-2.5 mm) leave galls in July-September (Harper 1959b), flying to an unknown secondary host. In western North America (Utah, Colorado, South Dakota).

Pemphigus populiramulorum Riley

Gall on Populus sargentii Dode on new growth of twig

Very few galls of P. populiramulorum were found in Alberta. This aphid was first reported in the area in 1954 and since then a few galls were found each year. The gall (Fig. 6) is formed not on the leaf but on the new growth of the twig; as the aphid feeds, the twig swells and encloses it. The gall is round and has a transverse or vertical slit similar to that in the gall of P. populitransversus but with protruding lips. It is greenish at first but turns brown as the season progresses. In fall it remains on the tree and thus may be easily seen during the winter or early spring.

Records of the distribution of P. populiramulorum Riley in Alberta are as follows: 1954, Grantham, Populus sargentii; 1956, Taber, Coaldale, P. sargentii.

Pemphigus populiramulorum Riley 1879 [Poplar twig gall aphid*].

Whereas galls of other Pemphigus species are formed from tissues of the leaf blade and/or petiole, galls of P. populiramulorum occur on twigs of the current year. Feeding on the young twigs causes swelling of the tissues that expand into a hollow globular structure that surrounds the stem mother and her offspring (Fig. 12). The exit opening is a narrow slit with slightly protruding lips. The location of galls on the woody tissues of the twig results in vacant galls frequently being retained on the tree during winter months. The point of attachment left by abscised galls is a characteristic flattened “button” region on the twig (Fig. 13). Galls in southern Alberta produced an average of 74 (range: 9–190) alatae, with gall exiting beginning in early to mid-July and being 90% complete by August 23 (Harper 1959 b). The primary host is P. deltoides (Palmer 1952; Harper 1959 a ; KF, pers. obs) . This aphid is described by Maxson and Knowlton (1929) as occurring on P. balsamifera , but this report has been questioned by Palmer (1952).

Pemphigus populiramorum
[Page 62]

Hosts: Populus fremontii, balsamifera subsp trichocarpa

The vertical slit, which distinguishes this species' galls, sometimes has slightly protruding lips.

Pemphigus populi-ramulorum Riley.

[Drawings of this gall appear on page 17 of the pdf]

While this species was described from material collected on Populus balsamifera it occurs commonly on Populus deltoides in localities outside the range of the balsam poplar. This species resembles quite closely both P. p.-caulis and P. p.- globuli.

This aphid is frequently encountered in Utah and is particularly interesting because of the gall it produces on the young twig bark of poplars, especially Populus balsamifera. Collections made at Leeds, both from twig galls and from galls below the bases of the leaves on the petiole, indicate that this one species produces two types of galls. This is a rather uncommon occurrence among gall-forming aphids. The openings to the leaf galls are often slightly beaked, and the mouth does not always open along the same axis, which gives variation in appearance to the openings of different galls.

In Utah this species has been collected from Benjamin, Benson, Farmington, Holliday, Hooper, Leeds, Provo, St. George, Santa Clara, and Zion National Park.