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Aphids on alder

Blackman & Eastop list about 50 species of aphids  as feeding on alders worldwide, and provides formal identification keys for aphids on Alnus. Of those, Baker (2015)  lists 6 aphid species on common alder (Alnus glutinosa) and/or Italian alder (Alnus cordata) as occurring in Britain: Clethrobius comes,  Crypturaphis grassi,  Glyphina betulae,  Pterocallis alni,  Pterocallis maculata  and Stomaphis quercus. 

Bell et al. (2015)  (Appendix S2) have also published an "annotated checklist of aphids present in the UK". We discuss some of the reasons for the differences between Baker's and Bell's lists in our rare aphids page. 

The species below are those we have found most frequently, listed in rough order of abundance. Assistance on identifying the two species of alder is given below. 

 

Pterocallis alni (Common alder aphid)

Both the winged and wingless adult viviparae of Pterocallis alni are yellowish white to yellowish green, sometimes (rarely?) with darker green markings. Their antennae are ringed with black. The hind femur has a characteristic black spot near its apex. The tarsi are black. The body length of Pterocallis alni apterae is 1.3-2.0 mm.

 

This species is found almost exclusively on common alder (Alnus glutinosa) in Europe, living dispersed on undersides of leaves and only rarely attended by ants. Sexual forms occur in September-November. It is common and widespread in Britain and the rest of Europe, east to Turkey, Iran and Lebanon. It has been introduced to North America, New Zealand, Australia and Chile.

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Pterocallis maculata (Green-barred alder aphid)

Adult apterae of Pterocallis maculata are yellowish-green or green, with a pattern of diffuse dark green dorsal cross bands, giving an overall blotchy appearance (see first picture below). The antennae of Pterocallis maculata are short, only 0.6-0.8 times the body length, and have black apices to the antennal segments. The siphunculi of Pterocallis maculata are dark, at least at the tip. There is a black spot near the apex of the hind femur and the tarsi are black. The body length of the aptera is 1.4-2.1 mm. The alate (see second picture below) has a greenish abdomen with dark green markings.

 

Second image copyright Alan Outen,  all rights reserved.

Each abdominal segment bear 5 pairs of pigmented capitate hairs situated on very pale and inconspicuous round sclerites. The third antennal segment has more than 2 long hairs, and some hairs on the fourth and fifth antennal segment are longer than the basal diameters of their respective segments.

Pterocallis maculata lives on the undersides of the leaves of alder (Alnus glutinosa and other species) in colonies along the veins. It is nearly always attended by ants. Apterous males and oviparae occur in September-October. The green-barred alder aphid is quite rare in Britain, but is widely distributed in Europe, east to Turkey and Iran.

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Crypturaphis grassi (Italian alder aphid)

Crypturaphis grassi apterae are brownish and dorso-ventrally flattened. The body length of apterae is 2.3-3.2 mm. Winged forms have a black head and thoracic lobes, a paler prothorax, and an ill-defined and variably developed brown patch across abdominal tergites 5-6. The body length of Crypturaphis grassi alates is 2.2-3.0 mm. Immature stages are green.

 

Italian alder aphids live dispersed along veins on both upper and lower leaf surfaces of Italian alder (Alnus cordata), and are apparently specific to this host. Oviparae and males occur in October-November, but aphids may overwinter viviparously. Crypturaphis grassi is native to southern Italy and Corsica, but was first recorded in the UK in 1998, and has since spread widely in southern England and Wales

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Species of alder

We cover two species of alder, European alder (Alnus glutinosa) and Italian alder (Alnus cordata). European alder is a medium-sized tree that does best in moist soils. It has short-stalked rounded leaves which are wedge shaped at the base with a slightly toothed margin (below first).

 

Italian alder is a medium sized tree that does well in dry soils. It has alternate heart-shaped glossy green leaves (above second).

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).

Useful weblinks 

References

  •  Blackman, R.L. & Eastop, V.F. Aphids on the world's plants An online identification and information guide. Full text