Pemphigus populiglobuli

In spring the fundatrices of Pemphigus populiglobuli induce galls at the base of cottonwood leaves (usually Populus angustifolia, Populus balsamifera or Populus trichocarpa). The galls (see first picture below) are green, often tinged with red and/or yellow when mature. They are globular, conical or bullet-shaped, and are formed by swelling of the base of the leaf lamina and thickening of the petiole. The exit hole from the gall (see second picture below) is a slit on the underside between leaf and petiole (cf. Pemphigus populicaulis which has a small round hole on the underside between leaf and petiole as the exit hole). The slit in the gall is rarely equal to one-half the circumference of the gall and is not visible from above. The lip-like borders of the opening are frequently quite rough. Maxson & Knowlton (1929) note that galls located at some distance from the base of the leaf and resembling those produced by Pemphigus populivenae sometimes occur, especially on heavily infested trees.

The English name for Pemphigus populiglobuli 'poplar bullet gall aphid' was chosen by Fitch (1859), who described the aphid thus, because the gall was of a similar size to a bullet (presumably a musket ball).

Both images above copyright James Bailey under a Creative Commons License.

The Pemphigus populiglobuli fundatrix has wax glands present on the entire body, and is coloured yellowish or green. The immature offspring of the fundatrix (see first picture below) are pale yellow. They all develop to emigrant alatae (see second picture below). The alate Pemphigus populiglobuli is lightly dusted over with a whitish powder, which gives it a bluish tinge. The head and thorax are dark, almost black, and the abdomen is greenish-yellow. There is a broad dusky band along the posterior margin of the pterostigma, and the wing veins are slightly dusky to brownish. The venation is normal for the genus. Antennae are 6-segmented. The antennal terminal process is short, but is usually longer than 0.03 mm and more than 1.5 times its basal width (cf. Pemphigus populicaulis, which has the terminal process usually shorter than 0.03 mm, and less than 1.5 times its basal width). Annular secondary rhinaria are distributed 6-13 on segment III, 2-4 on segment IV, and 2-4 on segment V, and 3-5 on segment VI. The body length of the alate Pemphigus populiglobuli is 1.5-2.3 mm.