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Aphids on walnut

Blackman & Eastop list about 20 species of aphids as feeding on walnut species worldwide, and provides formal identification keys for aphids on Juglans. About 20 species of aphids feed on the various species of walnut worldwide. The main aphid genera are Stomaphis, Panaphis, Chromaphis and Monelliopsis; the latter is exclusively North American.

Of the 8 aphid species Blackman & Eastop list as feeding on European walnut (Juglans regia) Baker (2015) lists 3 as occurring in Britain: Chromaphis juglandicola, Myzus persicae and Panaphis juglandis.

The species below are those we have found most frequently, listed in rough order of abundance. Note that walnut is prone to having accidentals attempting to feed on it, such as the common sycamore aphid (Drepanosiphum platanoidis). Assistance on identifying European walnut is given below.

 

Chromaphis juglandicola (Small walnut aphid)

Adult viviparae of Chromaphis juglandicola are all winged. Fourth instar nymphs (see aphid on left in first picture below) are yellow or yellowish white with paired brown spots on several of the abdominal tergites. The alates (see second picture below) are yellowish-white with pale brown thoracic lobes. In autumn the alatae have paired brown spots on abdominal tergites 4 and 5. The antennae have dark brown tips and are about half the body length. The terminal process is 0.2 times the length of the base of the last antennal segment. The siphunculi are conical and smooth. The body length of Chromaphis juglandicola alates is 1.2-2.3 mm.

The small walnut aphid lives scattered on the undersides of leaves of walnut (Juglans regia). In Europe and North America sexual forms develop in autumn, with eggs laid on the twigs and branches. Chromaphis juglandicola is found in Europe, Central Asia, India, Pakistan, China and North America.

 

Panaphis juglandis (Large walnut aphid)

The fourth-instar nymph (see first picture below) has transverse rows of brown patches on the dorsal abdomen. All Panaphis juglandis adult viviparae are alate. The alate (second picture below) is large and stout-bodied. It has a darkened head and thorax, and a yellow abdomen with broad dark bands across tergites III-VII, broken ones across tergites I-II and a small trapezoid sclerite on tergite VIII. The forewing veins are fuscous-bordered. The siphunculi are short and truncate. The body length of the Panaphis juglandis alate is 3.5-4.3 mm.

Panaphis juglandis live on the upper sides of leaves of European walnut (Juglans regia), in rows along the veins. They are sometimes ant-attended, but more often ants glean honeydew from the surrounding leaves. Sexual forms occur in September-October. The species is rarely found together with the small walnut aphid (Chromaphis juglandicola), apparently because Panaphis juglandis is adversely affected by the rain of honeydew from Chromaphis juglandicola. Panaphis juglandis occurs in Europe, and parts of central Asia, Pakistan and was introduced into the western USA.

 

Myzus persicae (Peach-potato aphid)

The apterae of Myzus persicae are generally yellowish-green (see first picture below) but vary from whitish or pale yellowish green to mid-green, rose-pink or red (see second picture below). They are often darker in cold conditions. The antennae are 0.7-1.0 times the body length, reaching to the siphunculi. The antennal terminal process is usually longer than antennal segment III, 0.90-1.49 times its length (cf. Myzus certus and Myzus dianthicola, both of which usually have the terminal process shorter than antennal segment III). Their siphunculi are slightly swollen towards the darkened tips (cf. Myzus ornatus and Myzus lythri, which both have tapering siphunculi). The siphunculi are 1.9-2.5 times the length of the rather pointed cauda, and at least 0.82 times longer than antennal segment III (cf. Myzus ascalonicus and Myzus cymbalariae, which both have siphunculi less than 0.81 times longer than antennal segment III). The body length of Myzus persicae apterae is 1.2-2.3 mm.

The alate Myzus persicae (see third picture above) has a solid pigmented area occupying the mid-abdominal dorsum from segments 3 to 6, as well as further bars on adjoining segments. Immatures have no visible cauda and shorter siphunculi but otherwise resemble the adult apterae. The colony structure of Myzus persicae on secondary hosts is usually dispersed (see second picture below) (cf. Myzus antirrhinii, which forms large dense colonies).

The peach-potato aphid does host alternate where its primary host - peach (Prunus persica) occurs. Eggs are laid on the primary host and spring colonies curl the young leaves of peach. However, most of the population overwinters as mobile stages on secondary hosts: herbaceous plants and brassicas. Myzus persicae is a major pest of its summer hosts, including potatoes, sugar beet, lettuce, brassicas and legumes, mainly because it transmits a number of important plant viruses. Whilst Myzus persicae is a polyphagous generalist.

 

Species of walnut

We cover only one species of walnut, the European walnut (Juglans regia), shown flowering below. The leaves are alternately arranged, 25-40 cm long each with 5-9 leaflets.

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).

Useful weblinks

References

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