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Spiny witchhazel gall aphidOn this page: Identification & Distribution Biology & Ecology Ant attendance Other aphids on the same host
Identification & Distribution
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 above) 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 first picture below) by inducing reddish-brown to crimson-coloured swellings of the leaf lamina between the veins.
Images above by permission, copyright Claude Pilon, all rights reserved.
When feeding in the pseudogalls on the undersides of birch leaves, the apterae of Hamamelistes spinosus are dark brownish red to purple with abdominal tufts of white wax (see second picture above). Immatures are reddish-brown (see third 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.
Biology & Ecology
On the secondary host birch, Hamamelistes spinosus lives in open pseudogalls and is commonly ant-attended (see picture below).
Image above by permission, copyright Claude Pilon, all rights reserved.
Hamamelistes spinosus was not formerly thought to be ant-attended on the primary host as they live within 'closed' galls, although ants were known to attend the aphids remaining in the gall after it splits open to allow alates to depart (see 'ant attendance on Hamamelis'). However, recent evidence indicates that ants do attend the (apparently) closed galls and consume an attractive substance in the vicinity of the gall (see picture below).
Image above by permission, copyright Mary Anne Borge, all rights reserved.
Further observations of ants attending apparently closed galls are given on Papa Ray's Adventure Channel (see second half of video).
The source of the attractant is at present unclear. It could be:
The latter situation has been reported for a number of species of gall-inducing wasps (Cynipidae) in the northern hemisphere, whose galls are frequently covered with ants feeding on 'honeydew' secreted by the plant (Washburn, 1984; Nicholls et al., 2017). Aranda-Rickert (2017) showed the same effect for a species of cynipid in the southern hemisphere whose galls produced large amounts of sucrose-rich, nectar-like secretions. It was presumed the sugary exudates were a mizture of phloem and xylem sap. No typical extra-floral nectary and subnectary parenchymatic tissues or secretory trichomes could be observed, but there was a dense vascularization with phloem elements reaching the gall periphery. No such vascularization has yet been reported in aphid galls, but the possibility should clearly be examined.
Other aphids on the same host
Hamamelistes spinosus is recorded from just one species of witch-hazel (Hamamelis virginiana).
Blackman & Eastop list 3 species of aphid as feeding on common witch-hazel (Hamamelis virginiana) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 15 as occurring in Britain (Show British list).
Hamamelistes spinosus is recorded from 8 species of Birch (Betula albosinensis, Betula alleghaniensis, Betula lenta, Betula nigra, Betula papyrifera, Betula pendula, Betula populifolia, Betula pumila).