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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 7 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, Stomaphis quercus and Stomaphis wojciechowskii.

The species below are those we have found most frequently, listed in rough order of abundance. Glyphina betulae and Clethrobius comes are more frequently found on birch, and Stomaphis wojciechowskii is more likely to be found on oak. 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. Their antennae are ringed with black. Antennal segment III has only 1- 2 conspicuous hairs, with none on segments IV & V (cf. Pterocallis maculata which has conspicuous hairs on antennal segments III-V). Dorsal hairs are pale (cf. Pterocallis maculata which has dark pigmented capitate hairs situated on pale round sclerites). The hind femur has a characteristic black spot near its apex. The tarsi are black. The siphunculi have dark tips. The body length of Pterocallis alni apterae is 1.3-2.0 mm.

The alate (see second picture above) has a greenish abdomen with dark green markings and dark siphunculi.

Pterocallis alni is found almost exclusively on common alder (Alnus glutinosa) in Europe, living dispersed on undersides of leaves and sometimes attended by ants. Sexual forms occur in September-November. The common alder aphid is common and widespread in Europe, east to Turkey, Iran and Lebanon and 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. Antennal segments III-V bear conspicuous hairs (see first micrograph below) (cf. Pterocallis alni which only has 1-2 conspicuous hairs on segment III). Each abdominal segment bear 5 pairs of dark pigmented capitate hairs (see second micrograph below) situated on pale round sclerites (cf. Pterocallis alni which has pale dorsal hairs). 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.

Second image copyright Alan Outen, all rights reserved.

The Pterocallis maculata alate (see second picture above) has a pale green abdomen and dark-tipped siphunculi.

Pterocallis maculata lives in colonies along the veins (cf. Pterocallis alni which is more or less scattered) on the undersides of the leaves of alder (Alnus glutinosa and other species). It is nearly always attended by ants (cf. Pterocallis alni which is never 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)

The rather translucent yellowish-green to yellowish-orange viviparous apterae of Crypturaphis grassi (see first picture below) are dorso-ventrally flattened with plate-like frontal and lateral projections. They have a double line of brown spots extending along the mid-line, as well as spots around the edge of the abdomen and on the head. The inconspicuous siphunculi are small and conical. The body length of adult Crypturaphis grassi apterae is 2.3-3.2 mm.

Winged forms (see second picture above) 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 Crypturaphis grassi are a very pale translucent yellow-green. Oviparae are similar in size and shape to the viviparous apterae, but are orange-brown in colour with transverse dark abdominal stripes, rather than spots. The oviparae have wax producing glands ventrally on either side of the cauda.

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. The northward shift in the distribution of this species has been attributed to climate change.

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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