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Italian alder aphidIdentification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution: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 apterae is 2.3-3.2 mm. Winged forms (see second picture below) 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 (see first picture below) 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 which can be seen in the second picture below.
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 (Jansen & Warner, 2002 ).
Biology & Ecology
Life cycleCrypturaphis grassi lives on Italian alder (Alnus cordata) (see picture of leaves below), a tree native to the southern Apennine Mountains of Italy and parts of Corsica.
The tree (together with its aphid) has been introduced to various European and other countries including Britain, where it has become naturalised. Italian alder is commonly grown in urban sites such as supermarket carparks, where our photos were taken. Crypturaphis grassi overwinters in the egg stage laid the previous autumn on twigs and small branches. The eggs hatch in early spring, and nymphs can be found on the trees from March onwards (see pictures below).
By late April, large numbers of apterous adults and nymphs can be found (see picture below), especially on the undersides of the leaves, but with a few on the upper sides.
We have mainly found alatae of Crypturaphis grassi (see picture below) in July-August.
Sexual forms develop in autumn. The ovipara is apterous and quite different in appearance from the viviparous aptera being orange-brown in colour with transverse dark abdominal stripes (see picture below). The male is alate (not pictured).
Colour & crypsisThe viviparous apterae of Crypturaphis grassi vary in colour from a rather translucent pale yellowish-green to a deeper yellow through to yellowish orange (see picture below of large colony on leaf underside). Older individuals tend to be more strongly coloured. The colour of individuals is a good example of background pattern matching where the animal's colours are a haphazard sample of the background - in this case the leaf of Italian alder.
Although most individuals are found on the underside of leaves, Luker (2011) noted that more mature individuals seem to favour the upper side of the leaves particularly at the top of the petiole at the base of the midrib. This position appears to optimise the background pattern matching (see picture below), and also reduces intraspecific competition with younger members of the colony.
As well as background pattern matching, Crypturaphis makes good use of disruptive coloration (Cuthill et al. 2005 ).
The dark spots effectively break up the outline of the insect (see first picture above). The true outline if the insect is more apparent in the second picture above which shows a yellow-orange specimen of Crypturaphis grassi in alcohol.
Natural enemiesVery few natural enemies have been recorded for Crypturaphis grassi. We have found larvae of Harmonia axyridis on leaves occupied by Crypturaphis grassi (see picture below), although we have not actually observed them predating this aphid.
Although not obviously waxy, this aphid species does have prominent wax glands on the underside. Perhaps wax is only produced if the aphid is attacked.
Other aphids on same host:Blackman & Eastop list 2 species of aphid as feeding on Italian alder (Alnus cordata) worldwide, and provide formal identification keys. Baker (2015) lists both aphid species as occurring in Britain: Crypturaphis grassii and Pterocallis alni.