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Waxy tall bluebell aphidOn this page: Identification & Distribution Other aphids on the same host
Identification & Distribution
Adult apterae of Sitobion niwanistum (see first picture below) are pale green, dusted with white wax powder - but accumulation of wax between the segments may result in a 'striped' appearance. The blackish eyes are especially conspicuous. Antennae are pale, except for extreme tip of the fifth segment and all of sixth, both of which are dusky or dark, as are the apices of the tibiae and tarsi. The siphunculi are pale with dusky tips, and the cauda is pale. The antennal terminal process is about 5.2 times as long as the base of antennal segment VI. Antennal segment III bears 1-2 secondary rhinaria. Antennal tubercles are especially well developed. The hairs on the antennae are sparse and fine, usually shorter than the width of the segment. The rostrum reaches to between the second and third pairs of coxae. The apical rostral segment (RIV+V) is 0.5-0.6 times the second hind tarsal segment (HTII). Marginal tubercles are present on the pronotum. The siphunculi are cylindrical, only a little wider at base than at apex, with indistinct subapical reticulation over less than 0.1 of total length (cf. Macrosiphum mertensiae and Macrosiphum euphorbiae, which have siphunculi tapering from the base, with at least 4-5 rows of polygonal reticulation). The cauda is short, broad, somewhat spatulate, not constricted, and bears 8-10 hairs. The body length of an adult aptera is 2.8-3 mm. The second picture below shows two fundatrices of Sitobion niwanistum, along with some newly deposited nymphs.
Note: Sitobion niwanistum was first known as Adactynus niwanista, then as Macrosiphum niwanistum, before being assigned to genus Sitobion.
Images copyright Andrew Jensen under a creative commons licence.
Alatae of Sitobion niwanistum (not pictured) have the head and thorax brown, and the abdomen light green, with the lateral areas and two indistinct dorsal bands dusky. Like the aptera, they are dusted with white wax powder. Antennal segments IV-VI, the tips of the antenna and of the siphunculi, the tips of tibiae and entire tarsi are dusky. The wing veins are slightly fuscous bordered.
Image copyright Walter Siegmund under a creative commons licence.
Sitobion niwanistum feeds on the undersides of leaves of tall bluebell (Mertensia paniculata) (see picture above). It is monoecious holocyclic, with oviparae and apterous males in late August-September. Jensen found the aphid abundant in the forest in the Blue Mountains near the border of Washington and Oregon. Most specimens were on the lower leaves of the plant, and dropped very readily from the plant when disturbed. The waxy tall bluebell aphid is found in northwestern USA, across to Ohio in the Great Lakes Region - a distribution mirroring that of its host.
Other aphids on the same host
Sitobion niwanistum has only been recorded on 1 Mertensia species (Mertensia paniculata).
Blackman & Eastop list 2 species of aphid as feeding on tall bluebell (Mertensia paniculata) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 1 as occurring in Britain (Show British list).