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Dark knapweed root aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution
Acaudinum centaureae live at the stem base of greater knapweed (Centaurea scabiosa) where they are tented over with earth by ants (see first picture below). Adult Acaudinum centaureae apterae are shiny blackish green to black (see second picture below). The head and the antennae are dark apart from the base of antennal segment III which is reddish brown. The antennal terminal process is 5.0-6.9 times as long as the base of antennal segment VI. The apical rostral segment is 1.4-1.5 times the length of the second hind tarsal segment. The abdominal dorsum is membranous apart from a sclerite or cross band on tergite VIII and intersegmental muscle sclerites (see first micrograph below). The siphunculi are dark, slightly curved near the base and bearing transverse rows of very fine spicules. They are more than ten times longer than the very short broad, rounded cauda. The body length of adult Acaudinum centaureae apterae is 1.7-2.5 mm.
The alate Acaudinum centaureae (not pictured) has marginal sclerites on tergites II-IV. Its antennal terminal process is 4.5-6.0 times the length of the base of antennal segment VI, and there are 28-40 secondary rhinaria on antennal segment III. Immature aphids are brown or greenish-brown (see third picture above).
The micrographs below show an adult aptera of Acaudinum centaureae in alcohol, dorsal and ventral.
The dark knapweed root aphid feeds at the base of the leaf stalks and on the stem base and roots of the greater knapweed (Centaurea scabiosa) where it is attended by ants, often Lasius niger (see picture below). It is a characteristic species of xerothermic (= hot dry) grasslands. There is no host alternation. Acaudinum centaureae produce sexual forms in autumn, and the species overwinters in the egg stage. Acaudinum centaureae is found over most of Europe, including Britain, east to Russia.
We have only found Acaudinum centaureae in one location - on chalk grassland at Birling Gap, by Beachy Head, on the East Sussex coast. The habitat preference for chalk grassland is largely determined by the host plant, greater knapweed (Centaurea scabiosa) which is mainly found on calcareous soils. The flowerheads are purple with the outer florets much longer than the inner (see first picture below) (cf. common knapweed, Centaurea nigra, which has the florets all equal in length).
Greater knapweed is also distinguished by having deeply dissected leaves (see second picture above) which form a clump at the base.
The overwintering eggs of Acaudinum centaureae which were laid on the stems of greater knapweed hatch in spring. Young immatures (see picture below) are green or brownish green, and can be found feeding on the leaf bases, usually covered by the ant tenting.
By the third or fourth instar (see picture below) the immatures are brown, and have longer siphunculi curved near the base.
The picture below shows an adult Acaudinum centaureae aptera on the roots. There are then several generations of parthenogentic reproduction through the summer, with dispersive alatae produced in mid-summer.
In September/October apterous oviparae and alate males develop. After mating the oviparae lay the overwintering eggs on the host plant stems.
All the colonies of Acaudinum centaureae that we have so far found have been sharing the host with another root aphid, namely Protrama radicis, with both species attended by ants, usually Lasius niger (see picture below).
One rather drastic way the ants use to stop any Acaudinum leaving the colony they are attending is to chew the wings off any alatae that develop. An example of this can be seen in the picture below).
Other aphids on same host
Acaudinum centaureae has been recorded from 9 species of Centaurea, although in Britain it has only been recorded from Centaurea scabiosa.