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Genus Hamamelistes [Hormaphidini]
Hamamelistes alates are distinguished by having two oblique veins in the hind wing.
There are five species in the Hamamelistes genus, host-alternating from galls (or "pseudogalls") on witch hazel (Hamamelis species) to galls on birch (Betula species), or remaining on one or other host all year. In some Hamamelistes species the first instar nymphs have a role in defending the inhabitants of the galls.
Hamamelistes betulinus (Birch blister aphid)Hamamelistes betulinus wingless females are greenish or dark brown to black (see first picture below after wax removal). They are normally covered in white wax and live in yellowish blister-like pseudogalls on birch (see second picture below). They have short, 3- or 4-segmented antennae. In European populations, they lack siphuncular pores, although siphunculi are present in some generations in Japan. The body length of Hamamelistes betulinus apterae is about 1.5 mm.
Winged females have 5-segmented antennae and pigmented siphuncular pores. The body length of Hamamelistes betulinus alates is 1.3-2.0 mm.
In Japan, there is host alternation. The primary host is Hamamelis japonica, where sexual forms develop and eggs are laid on twigs and trunks. These hatch the following year and the developing fundatrices induce coral-like galls to develop from flower buds. Winged forms then migrate to birch. In Europe and northern Asia, the sexual stage on the primary host is lost, the aphid stays all year on birch. It feeds on the undersides of birch leaves, mainly silver birch (Betula pendula), causing pale yellowish blisters to develop on the upper surfaces. Hamamelistes betulinus overwinters as first instar larvae on the twigs.
Hamamelistes spinosus (Spiny witchhazel gall aphid)
Hamamelistes spinosus host alternates from its primary host, witch hazel (Hamamelis virginiana), where it lives in a gall, to its secondary host, birch (Betula species), where it lives in a pseudogall. Unusually for aphids, this cycle take two years to complete. The overwintering eggs on witch hazel hatch in the spring. The immature fundatrix feeds on the side of a developing flower bud which induces the bud to curve over and envelop the feeding aphid in a globular gall. The gall is pink at first, but turns green dotted with reddish-brown papillae (see first picture below). It develops characteristic curved spines up to 8mm long. The apterous fundatrices inside the spiny galls (not pictured) are almost globular, dark purplish brown and covered with wax meal. They have no siphunculi. The body length of apterous Hamamelistes spinosus fundatrices is about 2 mm.
All the offspring of the fundatrices develop into alatae (see second picture below) which is the morph that is used for species discrimination. The alatae of Hamamelistes spinosus have 5-segmented antennae and two oblique veins in the hind wing (cf. Hormaphis hamamelidis on Hamamelis, which has 3-segmented antennae and only one oblique vein in the hind wing). The two oblique veins on the hind wing are often incomplete, and well separated at their bases (cf. Hormaphis betulae alatae, which have hindwings with the 2 oblique veins complete and united or close together at their bases). Siphunculi are either absent or very inconspicuous (cf. Hamamelistes betulinus, alatae which have pigmented siphuncular pores).
The Hamamelistes spinosus alatae leave the galls and fly to their secondary host, birch, where they produce nymphs that feed for a while on the leaves and then settle on the stem. These develop to black, rugose coccid-like morphs (not pictured) which overwinter on the stems. The following spring the progeny of this morph move to the underside of young leaves where they produce pseudogalls (see third picture above) by inducing reddish-brown to crimson-coloured swellings of the leaf lamina between the veins.
The adult apterae in the pseudogalls are dark brownish red to purple with abdominal tufts of white wax; the immatures are reddish-brown (see fourth picture above). Aphids are numerous and have on their abdomens tufts of whitish wax. Early in May, the females produce the winged sexuparae which on reaching maturity in June fly to the witch hazel where they produce the sexuales. After mating, the female lays an egg which will give a fundatrix the following year. The eggs are flattened, three times as long (0.2 mm) as wide and covered with hairs that come from the abdomen of the female at the time of laying. This hair provides excellent camouflage to the egg because it resembles the pubescence of the stem. The spiny witchhazel gall aphid is found throughout North America, from Mexico to Canada.