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0.7-0.9 mm.
Apterous: tiny aphid, yellow to orangey yellow, piriform with antenna very short
Alate: very small, head and thorax dark and abdomen yellow to orangey yellow, antenna short with just 3 segments, absence of cornicles and wings with simple venation.

These two species are part of a complex of species. They are close morphologically and are indistinguishable.

Monoecious holocyclic

Quercus spp. (oak)

Phylloxera glabra is monoecious holocyclic on Quercus robur, pedunculate oak.
Phylloxera quercus is also found on evergreen species (Quercus coccifera, kermes oak, Q. ilex, holm oak) but also on deciduous ones (Quercus petraea, sessile oak, Q. pubescens, white oak, Q. robur) in the summer.
Phylloxera glabra overwinters as eggs in cracks in the oak-tree bark. In spring, after the eggs have hatched, the larvae gather on the underside of young leaves. In the summer a series of generations follows. In autumn the sexual adults appear and the oviparae lay their eggs following fertilization.

Infestations cause yellow to brown spots to form on leaves. Large-scale attacks provoke leaves to fall prematurely.