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Genus Rhopalosiphum

Cherry - grass aphids

Species Overview: Genus Rhopalosiphum  Rhopalosiphum nymphaeae  Rhopalosiphum oxyacanthae  Rhopalosiphum padi 

Genus Rhopalosiphum

These are small to medium-sized aphids with a body that is broadly oval to rather elongate in shape. The dorsal body cuticle is membranous with pigmented patterns confined to small intersegmental muscle sclerites and variably present bands across the pronotum and abdominal tergite 8. Marginal tubercles are normally present on the pronotum and on abdominal segments 1 and 7. The siphunculi are variable in shape - cylindrical, tapering, vasiform or slightly swollen, always with an apical expansion and a strong flange. The cauda is shorter than the siphunculi and bluntly tapering or tongue shaped.

There are about 14 species in the Rhopalosiphum genus. Most species are holocyclic over some part of their distribution but many have anholocyclic populations. Rhopalosiphum species usually host alternate between plum (Prunus spp.) or apple (Pyroideae) as primary hosts and grasses (Poaceae) or sedges (Cyperaceae) as secondary hosts. Rhopalosiphum species may be ant attended and some are important crop pests.

 

Rhopalosiphum nymphaeae (Water lily aphid)

Apterae of Rhopalosiphum nymphaeae are more or less shiny reddish-brown to dark olive (brownish on the primary host), and are dusted with a light grey wax, especially on the head, thorax and legs of immatures. The dorsal cuticle has reticulation formed by regularly shaped roundish bead-like spinules. The siphunculi are more than twice the length of the cauda and are swollen on the distal half. The antennae are about 0.6 times the body length. The terminal process of antennal segment VI is prominently long and about 3-4 times the length of the base of that segment. The cauda is elongate and slender. The body length of Rhopalosiphum nymphaeae aptera is 1.6-2.6 mm.

 

On the primary host the apical rostral segment is more than 0.14 mm long and is 1-1.1 times as long as the second hind tarsal segment. On the secondary host the apical rostral segment is 0.15-0.2 mm long and is 1.15-1.45 times as long as the second hind tarsal segment. Rhopalosiphum nymphaeae alatae (see second picture above) are shining brown, sometimes with white dorsal wax markings.

In spring the water lily aphid feeds on various Prunus species (such as Prunus spinosa) where it feeds on leaf petioles and fruit stalks curling the leaves. In early summer Rhopalosiphum nymphaeae alatae migrate to the secondary hosts comprising a large variety of water plants, including Nymphaea (water lilies), Potamogeton (pondweeds), and Sparganium (arrowheads). Their distribution is almost cosmopolitan.

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Rhopalosiphum oxyacanthae (= insertum) (Apple-grass aphid)

Rhopalosiphum oxyacanthae apterae (see first picture below) on apple are small light green to yellow-green aphids that are elongate-oval in shape. They have fairly well-marked dark green stripes down the centre of the back and along each side. The frontal head tubercles are low, with the median frontal tubercle about the same height as the antennal tubercles. The siphunculi are quite short - about one tenth as long as the body - and pale with dusky tips. The 5-segmented antennae are about a third the length of the body. The body length of the adult aptera on the primary host is 2.1-2.6 mm. Winged Rhopalosiphum oxyacanthae viviparae (see second picture below) have blackish head, thorax and siphunculi and a green abdomen with some brown plates and pigmentation.

 

The apple-grass aphid aptera on its secondary host (grass roots) has a slightly different appearance. It is pale green, yellowish green or bluish green. The antennae are normally 5-segmented and much shorter than half the body length. The siphunculi are brown, 0.11-0.13 times the body length and nearly twice the length of the cauda. The body length of the apterous adult on the secondary host is 1.1-1.9 mm.

The apple-grass aphid host alternates between apple and related species (Rosaceae: Cotoneaster, Crataegus, Malus, Pyrus, Sorbus) and the roots of various Grasses (Poaceae). It has a sexual stage in its life cycle with eggs laid on apple. The first generation in March induces curling perpendicular to the mid-rib of the young leaves of the primary host. Apple-grass aphids may be attended by ants. Winged forms migrate in late May-June to the underground parts of various grasses, but colonies may persist into summer on primary hosts. Rhopalosiphum oxyacanthae is found in Europe and Japan.

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Rhopalosiphum padi (Bird cherry - oat aphid)

Adult Rhopalosiphum padi apterae on their secondary host (grasses) (see first picture below) are pale green to dark green, brown or nearly black, with a rust-red suffusion around the base of each siphunculus. On their primary host (bird cherry) adult Rhopalosiphum padi apterae have a coating of mealy wax (see second picture below) and live in a rolled leaf gall. The terminal process of the sixth antennal segment of the aptera is 3.1-5.2 times as long as the base of that segment. The apical ends of the siphunculi are slightly swollen and end with a strong flange preceded by a distinct constriction. The cauda is rather pale and shorter than the siphunculi. The body length of Rhopalosiphum padi apterae is 1.2-2.4 mm.

 

The alate vivipara of Rhopalosiphum padi is green, with a rusty red suffusion around the siphuncular bases (see first picture above).

The bird cherry - oat aphid host alternates between bird cherry (Prunus padus) as the primary host and various grasses (Poaceae) as the secondary host. Some populations reproduce parthenogenetically all year on grasses. Rhopalosiphum padi is the principal vector of barley yellow dwarf virus, and has a cosmopolitan distribution.

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Acknowledgements

We have made provisional 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).

 

Our thanks to Paul Piron for noting that, since Rhopalosiphum nymphaeae do not have siphunculi as swollen as in the image shown, our original image for Rhopalosiphum nymphaeae (shown below) was misidentified: It is in fact Rhopalosiphoninus calthae  which also feeds on water plants.

Useful weblinks 

References

  •  Stroyan, H.L.G. (1984). Aphids - Pterocommatinae and Aphidinae (Aphidini). Handbooks for the identification of British insects. 2 (6). Royal Entomological Society of London.