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Tansy leaf margin aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution:
Adult apterae of Coloradoa tanacetina are yellowish green or greenish yellow-brown, with the tips of the antennae and tarsi dark. The antennal terminal process is 1.4-2.0 times longer than the base of antennal segment 6. The last two fused segments of the rostrum (RIV+V) are 0.9-1.0 times longer than the second hind tarsal segment The longest hairs on abdominal tergite 8 are only 14-24 μm. The siphunculi are cylindrical and 1.3-2.2 times longer than the cauda. The length of the adult aptera of Coloradoa tanacetina is 1.1-2.0 mm.
Winged forms (not shown) have 9-15 secondary rhinaria on antennal segment 3, 5-11 on segment 4 and 1-9 on segment 5.
Coloradoa tanacetina feed in the indentations at the leaf margins of tansy (Tanacetum vulgare). Sexual forms of the aphid (pale green oviparae and very small orange-yellow males) occur in September and October. Coloradoa tanacetina are found across northern Europe, and have been introduced to USA.
Biology & Ecology:
We have only found Coloradoa tanacetina once - a dense population on an isolated patch of tansy growing on the beach at Seasalter near Whitstable, Kent.
As with most Coloradoa species, you are unlikely to notice this species unless you are specifically looking for it. The best way to detect the presence of a colony is to tap the leaves of tansy (Tanacetum vulgare) over your hand (see picture below), or (better) over a small tray. If the plant is infested, the aphids drop off readily.
A careful search of an (un-tapped) tansy leaf should then reveal the aphids feeding on the margins of the leaves, as shown in the picture below.
The leaf edge is perhaps chosen because penetration is easier at this point. Under high magnification, the flat surfaces of the leaves appear more shiny than the edges, suggesting some form of wax protection over most of the leaf.
Tansy leaves and flowers are toxic to humans if consumed in large quantities due to the presence of a volatile oil containing thujone, camphor and myrtenol. The tansy leaf margin aphid is clearly able to detoxify these toxins or sequester them where they cannot harm the aphid. The fact that the aphid is cryptically coloured suggests that the thujone is detoxified or excreted, rather than being stored and used to make the aphid distasteful to predators - as is the case with the aposematically-coloured crimson tansy aphid, Uroleucon tanaceti.
Unlike several other species on tansy, Coloradoa tanacetina is not attended by ants. Woodring et al. (2004) showed that the richness of the honeydew (rate of secretion x total concentration of sugars) and the presence of the attractive sugar melizitose were the critical factors determining the extent of ant attendance of the aphids feeding on tansy. Coloradoa tanacetina has a very low rate of secretion, so the richness of the honeydew was effectively zero for this species.
Other aphids on the same host
Despite the presence of toxins in tansy, many species of aphids can be found feeding on this plant, mostly specialising on tansy or closely related species - plus a few polyphagous species.
Blackman & Eastop list 23 species of aphid as feeding on Tanacetum vulgare worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 17 as occurring in Britain (Show British list).