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Bead-like cottonwood gall aphidOn this page: Identification & Distribution Other aphids on the same host
Identification & Distribution
Thecabius populimonilis induces rows of ovoid, bead-like galls on the leaves of poplar (Populus spp.). These galls are reddish green and develop from the upper side of the leaf lamina on both sides of the mid-rib.
Both images above by permission, copyright Claude Pilon, all rights reserved.
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 a large light cinnamon brown aptera (see picture below) or alate.
Image 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). The apical rostral segment (RIV+V) is about 0.08 mm long, and slightly shorter than antennal segment V. They often have spinal wax pore plates on all abdominal tergites. Their first tarsal segments usually all have 3-5 hairs, rarely only 2. There are no siphunculi or siphuncular pores.
First image above copyright (2010) Her Majesty the Queen in Right of Canada, Agriculture & Agri-Food Canada.
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). Pike et al. (2012) confirmed by DNA barcoding that colonies on willow roots (Salix ? dodgeana) in Washington are Thecabius populimonilis. 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 (Palmer 1952). Aoki et al. (1996b) has confirmed most of these observations; they found monoecious and heteroecious (>1 host spp.) life cycles at different locations, but their data on sex ratios indicate that some gene flow was probably occurring, rather than that two separate species being involved. Thecabius populimonilis is widely distributed in USA, western Canada and Mexico.
Other aphids on the same host
Thecabius populimonilis has been recorded from 4 species of poplar (Populus angustifolia, Populus balsamifera, Populus fremontii, Populus trichocarpa) and the roots of 1 species of willow (Salix dodgeana).