The oak leaf phylloxera (Phylloxera quercus) is an orange-coloured aphid that attacks just trees and shrubs of genus Quercus, mostly oaks, but also the perennial holm oak and kermes oak. Actually, this is not just one species, but several: Phylloxera quercus, P.quercina, P.coccinea and P.glabra, and perhaps some more amongst the ones that are described from Quercus in North of America. Likely, the species in the photos is Phylloxera quercus.
The oak leaf phylloxera moves on the underneath of oak leaves—or those of holm oaks or kermes oaks—where it affixes its beak and sucks the sap. 1 mm diameter, necrotic, orange spots crossing from the underneath to the top side develop at any point that the phylloxera has bitten. Since the phylloxera bites here and there, the affected leaf appears plenty of orange dots.
The life cycle of oak leaf phylloxera is complex, involving different stages and biological forms. One infested leaf may have at once the yellowish eggs, the orange nymphs which are usually the most numerous individuals, the wingless, orange, parthenogenetic females that exponentially increase the infestation, and the winged, darker sexual individuals. The 1st photo displays top and down sides of one oak (Quercus pubescens) leaf that is affected by phylloxera. The 4th photo shows the detail of the back of an infected leaf with eggs, nymphs and two winged adults.
Acrobat ants (Crematogaster scutellaris) often patrol the area protecting the oak leaf phylloxera individuals.
Phylloxera infestation may cause the fall of affected leaves and serious damage to the oak tree. Oak leaf phylloxera is more common on oak trees in urban parks and gardens than in that of the forests. The lack of aphid predators such as coccinellids (Coccinella septempunctata, Propylaea quatuordecimpunctata…) and other insects in urban areas explains why the incidence of phylloxera is higher on oaks in the city.
The oak leaf phylloxera relates to the well-known and feared grapevine phylloxera (Phylloxera vastatrix [= Viteus vitifoliae]) that fully devastated the vineyards of the Bages region at the end of the 19th century.
[photos Montserrat Porta and Jordi Badia]