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Genus Thecabius [Pemphigini]
Thecabius are small to medium-sized aphids (the alate body length is usually 2 - 3 mm) which have short antennae and siphuncular pores. Dorsal wax gland plates are present on all segments, producing a dense covering of wax spicules.
There are about 17 Thecabius species, most of which host alternate and have a sexual stage in the life cycle. The primary host is poplar (Populus: Salicaceae) where the fundatrix induces a gall on the leaves, petioles or branches. The secondary host is the roots or stems of plants such as buttercups (Ranunculus: Ranunculaceae), Lysimachia (Myrsinaceae) and sallow (Salix). Thecabius are not attended by ants.
Thecabius affinis (Poplar-buttercup gall aphid) Europe, Asia, North America
The Thecabius affinis fundatrix is green or bluish green, covered in wax and lacks siphunculi, and inhabits a small gall of its own on poplar in spring formed by folding the edge of a leaf. The offspring of the fundatrix leave this gall and move to the midrib of a young leaf where they induce the lamina of the leaf to fold along the midrib towards the underside. The roof-like gall develops blisters and gradually turns reddish (see first picture below). The winged viviparous female of Thecabius affinis (see second picture below) that develops from the offspring of the fundatrix is greenish, covered with wax spicules and has siphuncular pores. The antennae are short, about half the body length. The terminal process is 0.25 times the length of the base of the last antennal segment. The body length of winged Thecabius affinis females is 2.2-3.1 mm.
The viviparous alates migrate to their secondary host - the stem bases and on the runners of buttercups (Ranunculus) - where their offspring develop to adult apterae (see third picture above) which are densely covered in wax spicules.
Thecabius affinis usually host alternates from its primary host of poplar (Populus) species to buttercup (Ranunculus spp.), although some populations remain all year on buttercup roots. On poplar the fundatrix inhabits a small gall of its own on poplar (Populus), but its offspring leave this gall and stimulate development of a larger gall where the outer, upper side of the leaf becomes blistered and reddish. The winged viviparous females that develop in the gall on poplar leave in late June-July to found waxy colonies at the stem bases and on the runners of buttercups. Alatae migrate back to the primary host in autumn. Thecabius affinis is found throughout Europe, most of Asia and in North America.
Thecabius populimonilis (Bead-like cottonwood gall aphid) North America
Thecabius populimonilis induces rows of ovoid, bead-like galls on the leaves of poplar (Populus spp.) (see first two pictures below). These galls are reddish green and develop from the upper side of the leaf lamina on both sides of the mid-rib. In spring each gall contains one developing Thecabius populimonilis fundatrix. This fundatrix is dull yellowish olive-green with a dusky brown head and covered with powdery wax. The first instar nymphs subsequently produced by the fundatrix leave the parental gall before inducing similar, solitary galls within which they develop to large light cinnamon brown apterae (see third picture below) or alatae.
Images above by permission, copyright Claude Pilon, all rights reserved.
Alate Thecabius populimonilis arising from those galls have 4-7 secondary rhinaria on antennal segment III, and 1-4 rhinaria on segment IV (except for fundatrigeniae, which have 5-19 on segment III, 3-7 on segment IV and 3-4 on segment V). Less than half the total antennal surface area is covered with rhinaria (cf. Thecabius lysimachiae, which has more than half the antenna covered with rhinaria). There are no siphunculi or siphuncular pores.
Some Thecabius populimonilis continue to reproduce on poplar through the summer. But in some locations large alatae (of body length 2.7-3.0 mm) are produced from solitary galls in June-July, which then host-alternate to the roots of willow (Salix). Alate sexuparae produced in the colonies on willow roots return to Populus in October where they produce sexuales. Populations that remain all summer on poplar produce smaller orange-coloured apterae in August to October. Their offspring develop to sexuparae in the gall with the parent, with up to 12 sexuparae being produced per gall. Thecabius populimonilis is widely distributed in USA, western Canada and Mexico.