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Acaudinum centaureae

Dark knapweed root aphid

On 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 the length of 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 and bear 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. The aphids 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.

 

Biology

We have only found this species 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 picture below shows an adult Acaudinum centaureae aptera on the roots.

The green-brown immatures (see picture below) can be found feeding on the leaf bases, usually covered by the ant tenting.

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).

 

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.

Acknowledgements

We have made provisional 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).

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