Brachycaudus bicolor

Identification & Distribution:

Brachycaudus bicolor apterae (see first picture below) are shining yellowish tinged with pink, to pale green, with separate cross bars on thoracic segments, a variably developed black patch situated dorsally on the abdomen and 2 or 3 black stripes at the tip. The hairs on abdominal tergite 8 are 30-113 μm long, much longer than the dorsal hairs on more anterior segments. Brachycaudus bicolor has conspicuous flat marginal tubercles present on all segments from the pronotum to abdominal tergite 7, and spinal tubercles on the pronotum and abdominal tergites 7 and 8 (see). Their siphunculi may be pale, dusky or quite dark. The body length of apterous Brachycaudus bicolor is 2.1-2.4 mm.

The alate (see second picture above) also has a variably developed black patch on the abdomen, and especially conspicuous marginal tubercles. She has 28-52 secondary rhinaria on antennal segment III, and 3-8 on segment IV. The two pictures below are micrographs of an apterous and an alate Brachycaudus bicolor in alcohol.

The clarified slide mounts below are of adult viviparous female Brachycaudus bicolor - wingless (lightly sclerotized & heavily sclerotized), and winged.

Micrographs of clarified mounts by permission of Roger Blackman, copyright AWP all rights reserved.

In Britain Brachycaudus bicolor is usually found in ant-attended colonies on root collars or at the bases of leaves of hound's tongue (Cynoglossum officinale). No sexual morphs have been recorded, and the species overwinters as parthenogenetic forms. Brachycaudus bicolor has been found in Britain, southern Europe, Egypt and parts of Asia.

Biology & Ecology:

The host plant of Brachycaudus bicolor is houndstongue (Cynoglossum officinale), a plant found in dry sandy areas, often near the coast. We have found it on several occasions on Winchelsea Beach in East Sussex. It has a basal rosette of floppy wavy crinkly leaves (see picture below) and the leaves have a thin scattering of fine white felty hairs.

The flowers (see picture below) are deep red, sometimes purple tinged, with 5 petals which rarely open fully.

The plant contains toxins (pyrrolizidine alkaloids) in its leaves and roots. In cattle large doses of these alkalkoids cause excessive thirst, diarrhoea and palsy of the hind legs, from which recovery is unlikely. Aphids often seqester toxic compounds in their food plant which makes them distasteful to predators. We might expect such aphids to develop aposematic coloration, but the cryptic behaviour of this aphid suggests otherwise.

One feature of the cryptic behaviour, especially of the adult apterae, was their rapid movement into shade as soon as they were brought out into the light for photography. The picture below shows three adult Brachycaudus bicolor with black dorsal shields. All three in the picture are fleeing for cover.

All the specimens we found were living with large numbers of pale green aphids, often with pinkish hues, which appeared to be their immatures, although we cannot rule out the presence of a few Brachycaudus helichrysi amongst them.

Although in Britain the usual host of Brachycaudus bicolor in Britain is hound's tongue, in Europe it is frequently found on a range of different genera in the Boraginaceae.

Other aphids on same host:

Blackman & Eastop list 7 species of aphid as feeding on hound's tongue (Cynoglossum officinale) worldwide, and provide formal identification keys.

Of those aphid species, Baker (2015) lists all 7 as occurring in Britain: Acyrthosiphon malvae, Aphis symphyti, Brachycaudus bicolor, Brachycaudus cardui, Brachycaudus helichrysi, Brachycaudus lateralis and Myzus persicae.

Acknowledgements Whilst we make every effort to ensure that identifications are correct, we cannot absolutely warranty their accuracy. We have mostly made 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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