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Genus Symydobius [Calaphidini]
Symydobius are medium to large, rather shiny dark brown aphids. Both winged and wingless adult viviparous females occur and (unusually) they have a similar pattern of sclerotization. Siphunculi (when present) are small and truncate. Their antennae are dark, or have the basal half of the 4th and 5th segments conspicuously paler. Males are wingless, and oviparae have the posterior abdominal segments extended into an ovipositor-like structure.
The Symydobius genus has seven species, which form colonies on the branches and twigs of birch and alder (Betulaceae). They have a sexual stage in their life cycle, but do not host alternate. They are always attended by ants.
Symydobius americanus (Dark birch aphid) North America
Adult apterae of Symydobius americanus (see first picture below) are a shiny dark brown to black in colour, and are alatoid in form as regards sclerotization. The antennae, which are as long as the body, have 2-3 white bands at the bases of segments IV-VI, and bear about 20 round secondary rhinaria distributed over the entire length of segment III (cf. Symydobius oblongus in Eurasia, which has secondary rhinaria only on the basal 0.5 of segment III). The antennal terminal process is 0.85-1.20 times as long as the base of antennal segment VI (cf. Symydobius intermedius in western USA, which has the terminal process less than 0.85 times the base of that segment). There are dark sclerotic crossbars on all the tergites. The pale siphunculi are short and truncate, somewhat broader at the base and with a flange. The cauda is broadly rounded, with no trace of a constriction.
The alate Symydobius americanus (see second picture above) is coloured similarly to the aptera. Antennal segment III bears about 25 rather small secondary rhinaria arranged in a row over the entire segment (cf. Symydobius oblongus in Eurasia, which has secondary rhinaria only on the basal 0.75 of segment III). The wing veins are heavily bordered with dark brown (cf. Symydobius oblongus in Eurasia, which has no dark bordering of the wing veins). There are dark sclerotic crossbars on each abdominal tergite. Young immatures (see first picture below) are reddish-brown with broader white bands on the antennae.
Symydobius americanus feeds on the stems and twigs of paper birch (Betula papyrifera). Sexual morphs occur in September-October. Like other Symydobius species, it is nearly always attended by ants. Symydobius americanus has a more northerly distribution than the only other Symydobius species in the USA (Symydobius intermedius), being found in the northern USA and Canada.
Symydobius oblongus (Shiny birch aphid) Europe, Asia
Apterae of Symydobius oblongus (see first picture below) are shiny dark brown with no wax covering. The antennae are brown apart from the basal parts of segments 4-6 which are conspicuously pale. They are slightly shorter than the length of the body with a terminal process that is shorter than the base of the last antennal segment. The dorsal abdominal pattern comprises a broad dark transverse bands and marginal sclerites on each tergite. The siphunculi are pale, short and truncate. The body length of apterae is 2.0-3.5 mm.
Alates of Symydobius oblongus (see second picture below) have brownish bordered wing veins and a similar dorsal abdominal pattern of sclerotization to the aptera. Oviparae have the dorsal sclerotic pattern reduced on segments behind the siphunculi. The males are apterous and like the viviparae, but smaller and more slender. Symydobius oblongus nymphs do not have the dorsal pigmented bands, except across tergite 8.
The shiny birch aphid is found on twigs, young stems and branches of both the silver birch (Betula pendula) and the downy birch (Betula pubescens). It is found both in birch woodland and on heaths with birch scrub. Sexual forms (oviparae and wingless males) occur in October-November. Symydobius oblongus is found throughout Europe and across Asia.