Pemphigus spyrothecae (= spirothecae ) Passerini 1856.

Feeding by the stem mother causes the leaf petiole to twist into spirals (usually three) that surround the aphid. The spirals swell, with their seams becoming tightly pressed together to form a hollow, spiral- shaped gall that completely surrounds the stem mother and her offspring (Fig. 15). Galls mature in late August or early September and are initially green, but often become reddish. Alatae exit the mature gall via a series of small pores that form along the seams of the spirals. Pemphigus spyrothecae does not have a secondary host and, as reported for P. bursarius and suspected for P. monophagus , has a morphologically distinct subset of first instars that form a soldier caste. Experimental studies show that galls containing 50 soldier and 50 non-soldier aphids versus 100 non-soldier aphids can be 10 times less likely to be attacked by predators under field conditions (Foster and Rhoden 1998).

Populus nigra L. var. italica is the primary host (Alleyne and Morrison 1977). This aphid was first reported in North America from material collected in 1972–1973 in Quebec, where it was extremely common (Alleyne and Morrison 1974). Blackman and Eastop (1994) report its introduction into western Canada. In addition, high densities were observed by the author in 2003 at Kelowna, British Columbia.

Pemphigus spirothecae
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Host: Lombardy poplar, Populus nigra var italica; Populus balsamifera subsp trichocarpa [tristis]

Feeding activity of the stem mother causes the petiole to twist, forming an ever-tightening spiral.

Pemphigus spirothecae

Galls on Lombardy poplar (Populus nigra var italica) formed by both twisting and swelling of the petiole

This aphid produces galls in early spring on the petioles of Lombardy poplars and has long been known to occur on poplars in Europe (Connold 1901; Theobald 1929; Stroyan, pers. comm.). No previous record of this aphid exists in North America. The feeding activity of the stem mother causes the petiole to twist into a spiral. The spiralling continues until the stem mother is completely surrounded by petiole tissue (Fig. 1). Within this spiral the stem mother reproduces and the gall swells, producing a large cavity in which the aphid progeny develop. Unlike P. bursarius galls which mature in early summer, galls of P. spirothecae mature in late August and early September. This is primarily attributable to the monoecious life cycle of P. spirothecae which produces one migrant form, the sexupara, within the mature gall. This form corresponds to that of P. bursarius produced on the roots of alternate hosts.

As galls of P. spirothecae mature they change from green to pale red and a variable number of tiny pores appear along the seams (Fig. 3). Much larger openings may develop on this region of the gall, facilitating mass release. At leaf-fall, large numbers of apterae and alates are still found within the galls. Galls of P. spirothecae aphids were extremely abundant on Lombardy poplars in regions surveyed.