Genus Sitobion

Genus Sitobion [Macrosiphini]

Sitobion are medium-sized green to dull brownish-green or reddish-brown aphids, with blackish antennae. Their adult viviparae may be winged or wingless. They typically have an intersegmental sclerotic pattern, but some species may have a more or less completely brownish sclerotic dorsal surface. The siphunculi are rather long, blackish and sclerotic, with the apical part reticulated and a small, but distinct flange. The cauda is pale, elongate and finger-shaped from half to nine-tenths as long as siphunculi.

Sitobion is a large genus of over 80 species worldwide. A few species alternate between Rosaceae and grasses, but the majority of species remain on grasses (Poaceae) all year. On grasses they may or may not have a sexual stage. Even within one species, clones may produce males and egg laying females in the autumn - or only produce parthenogenetic females. Sitobion aphids are not attended by ants. At least one species (Sitobion avenae) is a serious pest of cereals.

 

Sitobion avenae (English grain aphid)

The adult aptera of Sitobion avenae (see first picture below) is medium-sized and spindle-shaped. It shows colour polymorphism with green and brown forms predominating (immatures are green or red.) The antennae are black and somewhat shorter than the body. The legs are yellow but with the tips of femora, tarsi and tibiae dark. The siphunculi are cylindrical and black and somewhat longer (1.1-1.5 ×) than the pale pointed cauda (cf. Sitobion fragariae which also occurs on grasses, but has its siphunculi 1.7-2.7 × the length of the more rounded cauda). The body length of the aptera is 1.3-3.3 mm. long.

The alate (see second picture above) is 1.6-2.9 mm. long and also occurs in two colour forms - green and brown. It has distinct dark intersegmental markings on the upper surface of the abdomen.

There is no host alternation and the species spends its entire year on cereals and grasses. It occurs on all cereals including rice and maize and can develop on most grasses (Poaceae), as well as on some rushes (Juncaceae) and sedges (Cyperaceae). The majority of the population is anholocyclic (produces only asexual morphs) and overwinters as nymphs or apterae on grasses or winter cereals, but a small proportion of the population is holocyclic (alternates parthenogenetic with sexual reproduction) and overwinters as eggs which hatch in March. Sitobion avenae is widespread throughout the world, with a preference for temperate climates. It occurs widely in Europe, North Africa, the Middle East and Asia.

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Sitobion fragariae (Blackberry-grass aphid)

The Sitobion fragariae aptera is dirty yellowish green (see first picture below), with small brown intersegmental sclerites on the abdominal dorsum. The antennae are about the same length as the body, with the basal segments paler than the rest. The siphunculi are usually entirely black, although they may have paler bases on the primary host. They are 1.8-2.7 times longer than the pale pointed cauda (on blackberry cf. Macrosiphum funestum, which has the siphunculi 2.5-3.5 times the length of the cauda; on grass cf. Sitobion avenae, which has the siphunculi 1.1-1.5 times the length of the cauda). The body length of Sitobion fragariae apterae is 1.6-3.0 mm long.

The alates (see second picture above) have a pattern of dorsal dark intersegmental markings that is usually more extensive than in Sitobion avenae.

The blackberry - grass aphid host alternates from blackberry (Rubus fruticosus agg.) and occasionally other Rosaceae to grasses (Poaceae) especially Holcus spp. and some sedges (Carex spp). Sitobion fragariae eggs hatch in spring and the young nymphs feed on the breaking buds. Colonies build up and in summer alates migrate to cereals and grasses. A return migration takes place in autumn.

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Acknowledgements Whilst we make every effort to ensure that identifications are correct, we cannot absolutely warranty their accuracy. We have mostly made identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

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References  Blackman, R.L. & Eastop, V.F. (2006). Aphids on the world's herbaceous plants and shrubs. Vols 1 and 2. John Wiley & Sons.  Heie, O.E. (1980-1995). The Aphidoidea, Hemiptera, of Fennoscandia and Denmark. (Fauna Entomologica Scandinavica) E.J. Brill, London  Stroyan, H. L. G. (1952). The identification of aphids of economic importance. Plant Pathology 1, 9-14, 42-48, 92-99, 123-129.

 

Identification requests David Fenwick, 10 June 14 Have an interesting aphid for you, what appears to be a Sitobion. It was found on Wall Barley, Hordeum murinum, on a small pier at Newlyn today. 10.06.14. SW 46490 28497. Juveniles quite orange in colour. Few adults, mostly juveniles on each inflorescence, grass also looked like it was going over, becoming dry and purplish. Hope I have the right genus. Image(s) copyright www.aphotofauna.com all rights reserved.     ... Think I have it, Sitobion avenae! A variable little beastie! Bob, InfluentialPoints: I know it can be rather variable, but I'm not convinced it is Sitobian avenae. Even allowing for foreshortening due to perspective, the siphunculi are too long relative to the cauda. (S. avenae siphunculi are no more than 1.4 times the length of the cauda.) Given the host and the dark siphunculi, it is much more likely to be Sitobion fragariae, whose siphunculi are at least twice the length of the cauda.