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Small yarrow aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution
Adult viviparous apterae of Coloradoa achilleae are small oval aphids, coloured pale green to greyish green or reddish (see pictures below). The tips of the antennae and the tarsi are dark. The antennal terminal process is 1.1-1.4 times longer than the base of the sixth abdominal segment. The siphunculi are 1.3-1.8 times the length of the cauda. The siphunculi are near cylindrical, or with a weakly swollen apical part, and a well developed flange. The cauda is obtuse (=not pointed), slightly constricted at the base and with 5 hairs.
Alate Coloradoa achilleae viviparae (not shown here) have pale marginal sclerites, rather pale siphunculi and a dark or pale cauda. The micrographs below show an adult Coloradoa achilleae aptera, dorsal and ventral views.
The clarified slide mounts of adult viviparous female Coloradoa achilleae : wingless, and winged are shown below.
Coloradoa achilleae is a small, inconspicuous species that can be found feeding on yarrow (Achillea millefolium) especially on leaves at the base of the stem. Their overwintering eggs hatch in late spring, but there is no information available on the population trends of this species through the year. Sexual forms develop in autumn, with eggs laid at the base of the host plant. The small yarrow aphid is found in Britain, and throughout Europe into Russia, as well as having been introduced into USA.
Biology & Ecology
Coloradoa achilleae is easily overlooked, although once one knows where to look, it is not too difficult to find. We first found it when looking at the (much more conspicuous) colonies of Uroleucon achilleae, a species which also favours the lower leaves of yarrow. The dark red Uroleucon aphids were interspersed with pale yellow/green Coloradoa aphids. Subsequent finds were made by shaking yarrow into a tray (note: 'thrashing' vegetation for aphids tends to just result in damaged specimens).
Coloradoa achilleae generally feed (and hide) at the internodes of the yarrow leaves.
Numbers, presumably, build up in summer - possibly peaking in late summer when several of the yarrow aphids reach their maximum numbers. However, there have (to our knowledge) been no studies looking at the population trends of this species.
The hind tibiae of the ovipara are only a little thickened, with 5-30 rather large scent plaques on the basal half. In other respects the ovipara is similar to the adult vivipara. The male (see picture below) is orange yellow with black antennae.
Halbert et al. (2000) describes how Coloradoa achilleae was first found in America on a wild Achillea millefolium in North Carolina. Since several Achillea species have become popular ornamental plants, it was considered likely that these inconspicuous aphids would be distributed widely in the near future. The author was puzzled as to how Eurasian Coloradoa species, which are host specific to various weeds, keep getting introduced into North America.
Other aphids on same host:
Coloradoa achilleae has been recorded from 7 Achillea species (Achillea asplenifolia, Achillea collina, Achillea crithmifolia, Achillea ligustica, Achillea millefolium, Achillea nobilis, Achillea setacea).
Blackman & Eastop list 46 species of aphid as feeding on yarrow (Achillea millefolium) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 26 as occurring in Britain (Show British list).