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1.2-1.9 mm.
The species has a unique morphology featuring several archaic characters like round, scarcely visible cornicles, a small semi-circular cauda and a rounded anal plate. It also possesses two broad wax glands on the dorsum of the hind-part of the abdomen (post-cornicular urite 7) hence the name ‘woolly’.
Apterous: light green covered with whitish wax, antennae with five or six segments.
Alate: thorax black and abdomen light green; wings kept flat when at rest, on forewings medial vein with just one branch visible; antennae always composed of six segments without secondary rhinaria.

Monoecious holocyclic.
Nevertheless, during the winter this species can sustain its parthenogenetic populations and these can even become dominant in regions with mild winters.

Mainly Populus nigra, black poplar, P. deltoids, eastern cottonwood, P. x euramericana. The species has also been reported on P. suaveolens, Mongolian poplar, P. ciliata, Himalayan poplar,  P. maximowiczii, Japanese poplar, and  P. tremuloides, quaking aspen.
In natural conditions, this aphid develops in riparian forest environments colonized especially by P. nigra.
In poplar groves, the species prefers Euro-American hybrid poplars (P. x euramericana) which nonetheless manifest variable degrees of susceptibility depending on the genotypes.

P. passerinii develops in bark crevices on the trunk and at the base of branches of large poplars, preferring sites in the upper third of the tree. P. passerinii feeds to the detriment of the cortical parenchyma and does not provoke visible galls.

Agronomic impact

Infestations can be particularly devastating in a poplar grove.
P. passerinii appears to attack trees at rich, well drained sites. The invaded trees are over 6–7 years old (circumference nearly 90 cm).
The first signs of attack are difficult to detect seeing that the first colonies appear in the crevices of bark high in the upper third of the trunk (i.e. the area where lower branches are inserted).
When strong proliferations occur, colonies expand up to the lower part of the trunk, completely covering it with a whitish felt-like wax the insect secretes.

There is a range of varying consequences:

• loss of bark elasticity leading to characteristic ‘lizard skin’ cracking
• cortical necroses bordered by folds of scar tissue
• death of the lower branches
• absence of bud breaking and death in the year following the infestation (up to 70% of the trees in a given plot).

Predators

• Prostigmata (Trombiidae)
• Hemiptera (Anthocoridae, Miridae, Pentatomidae)
• Neuroptera (Chrysopidae, Hemerobiidae)
• Diptera (Chamaemyiidae, Chloropidae, Syrphidae)
• Coleoptera (Coccinellidae)