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Phylloxera are small pear-shaped aphids (using 'aphids' in the loose sense, phylloxerids are not 'true' aphids). Both the sexual and parthenogenetic females are oviparous. The wings are held flat over the abdomen at rest. The ovipositor is vestigial or absent.
Phylloxera feed on plants in the walnut (Juglandaceae) and oak/chestnut (Fagaceae) families.
Phylloxera caryaecaulis (Hickory leaf stem gall aphid)
The eggs of Phylloxera caryaecaulis on hickory (Carya spp.) hatch in spring. Feeding by the newly-hatched immature fundatrices induces galls on young twigs, on petioles, or at bases of leaflet main veins, sometimes in clusters. The galls (see first picture below) are globular, pale yellowish green tinted with red before opening, but afterwards becoming leathery and black, and with a diameter of 5-25 mm. The adult fundatrix in the gall (visible amongst the immatures in the second picture below) is of a plump egg-shaped form, narrower posteriorly, dirty yellow in colour with dark legs and antennae. On reaching maturity the fundatrix reproduces parthenogenetically depositing up to a thousand eggs in the gall. The young nymphs that hatch from the eggs are white, shining and somewhat hyaline with pellucid white legs. More mature immatures with wing buds are light yellow or yellowish green with dusky legs and antennae. These mature to alate sexuparae (see third picture below).
Images above by permission, copyright Claude Pilon, all rights reserved.
The alatae have dusky wings, apparently distinctive dorsal prothoracic markings and a pair of orange or blackish spots on the abdomen. The body length of adult sexuparae is 0.8-1.8 mm. Small alatae produce mostly males, larger ones mostly sexual-females. After mating the females lay the overwintering eggs on hickory.
Phylloxera caryaecaulis has been recorded on pignut hickory (Carya glabra) and (from these observations) shagbark hickory (Carya ovata). There is no host alternation in this species of Phylloxera. It is found in eastern USA and Canada.
Phylloxera glabra (Oak leaf phylloxera)
Phylloxera glabra fundatrices feeding on oak leaves in spring causes the leaf edges to curl inwards. Feeding by later generations causes necrotic spots on the leaves (see first picture below). The yellowish orange adult apterae feed on the undersides of the leaves (see second picture below of adult with eggs). The dorsal processes of the aptera are undeveloped, and the tergum (=thickened dorsal plate) is nearly smooth (cf. Pylloxera quercus sp. group, including Phylloxera coccinea, which have club-shaped dorsal processes on the thorax). The primary rhinarium on the third antennal segment is subapical (=below or near the apex) with the terminal process developed. Abdominal tergites 2-5 have spiracles. The Phylloxera glabra adult body length is 0.7-0.85 mm.
The oak leaf phylloxera lives without host alternation on oak leaves (mainly Quercus robur). Female Phylloxera glabra surround themselves with concentric circles of their eggs (see picture above). After hatching, their nymphs wander off leaving gaps in the circle, or if disturbed, the adult female may leave. Phylloxera glabra is found in Europe, and has been introduced to New Zealand.