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Identification & Distribution

In spring Brachycaudus prunifex induce curled leaf pseudogalls on new growth of blackthorn (Prunus spinosa, see first picture below) (cf. Brachycaudus helichrysi, which produce very similar pseudogalls on blackthorn). Most adult Brachycaudus prunifex apterae are shiny blackish-brown (see second picture below). They have separate cross bars on thoracic segments, but the cross bars on their abdomen are usually fused to give a solid patch on abdominal tergites II-V (cf. Brachycaudus prunicola, which often have the dorsal cross bands not completely fused intersegmentally and/or with a spinal gap).

Some individuals of Brachycaudus prunifex may instead be reddish or greenish (see first picture below) with reduced dark pigmentation on the dorsum. Brachycaudus prunifex can only be satisfactorily separated from Brachycaudus prunicola by examining the length of antennal or body hairs: The longest hair on the inner side of antennal segment II is 0.8-1.6 times the basal diameter of antennal segment III (cf Brachycaudus prunicola which has longest hair on the inner side of antennal segment II 0.4-1.0 times that diameter). The dark siphunculi are quite short, and of similar length to the dark cauda. The body length of adult apterae is 1.4-2.4 mm.

Alate Brachycaudus prunifex (see second picture above) are reddish- or greenish-brown, with a large dark mid-dorsal patch. They have 22-41 secondary rhinaria on antennal segment III, 2-13 on segment IV and usually none on segment V.

The clarified slide mounts below are of adult viviparous female Brachycaudus prunifex : wingless, and winged.

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

Brachycaudus prunifex does not host alternate but lives all-year-round on blackthorn (Prunus spinosa). It had been suggested that the species host-alternates to Tragopogon, but this has not been substantiated. Sexuales are most likely produced in autumn, but so far only males have been found. Brachycaudus prunifex is found in England, Wales, and Ireland, where the closely related Brachycaudus prunicola does not occur. Brachycaudus prunifex has also been found in northern France. In the rest of Europe its niche is occupied by Brachycaudus prunicola.

 

Biology & Ecology

Habitat

We found Brachycaudus prunifex for the first time in May 2020, in the midst of the coronavirus pandemic. We are not of course suggesting that the two events are linked in any way, only that travel restrictions resulted in an even more intensive investigation than usual of any aphid populations we could access. This resulted in repeated examinations of the galls on blackthorn bushes which had previously only yielded abundant colonies of Brachycaudus helichrysi.

A few bushes had intensively ant-tended, reddish pseudogalls (see picture above) filled with large numbers of, mainly immature, Brachycaudus prunifex.

In young colonies most of the aphids were on the inside of the curled leaves (see two pictures above of aphids inside the galls).

In galls containing large colonies, some aphids spilled out of the gall on to the leading shoot and other leaves (see picture above). The adult apterae were usually easy to pick out, given their black sclerotized dorsa (see picture below).

Although it was relatively early in the year, alatae were fairly common. Many of the immatures in the galls were fourth instar immature alatae (see first picture below), and there were also a number of adult alatae present (see second picture below).

Ant attendance

The galls of Brachycaudus prunifex are open galls (unlike for example the closed galls of Pemphigus populi) - and allow easy access to the aphids for tending ants.

Aside from any protection from predators, the ants provide a waste disposal system for honeydew produced in the galls.

For their part, the aphids provide their attendant ants with a dependable supply of high-energy carbohydrate-rich (albeit protein-poor) food in the form of honeydew.

Natural enemies

The high frequency of ant attendance undoubtedly serves to reduce the number and types of predators attacking Brachycaudus prunifex in its galls. But despite the ant presence, we did find large numbers of syrphid eggs laid around the outsides of the galls (see picture below).

We also found several nearly-mature syrphid larvae within the galls. These dorsally-ventrally flattened larvae were translucent with broad yellow chevrons.

These syrphid larvae most resembled those of Leucozona laternaria (see picture and description in Rotheray, 1984) but the larval margin seems too serrated for that species, which is anyway normally associated with umbellifers.

The syrphid larvae (and egg deposition) seemed unaffected by the presence of numerous attending ants. Perhaps, as we discussed for syrphid larvae predating Aphis viburniphila, these larvae are using chemical mimicry to disguise themselves and hence circumvent attack by otherwise aggressive ants. Lohman et al. (2006)discussed the mechanisms by which this may be achieved.

 

Other aphids on the same host

Brachycaudus prunifex mainly occurs on sloe (=blackthorn, Prunus spinosa) but has been recorded once on cherry plum (Prunus cerasifera).

Blackman & Eastop list 13 species of aphid as feeding on blackthorn (Prunus spinosa) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 9 as occurring in Britain (Show British list).

Acknowledgements

Our particular thanks to Roger Blackman for images of his clarified slide mounts.

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).

Useful weblinks

References

  • Lohman, D.J. et al. (2006). Convergence of chemical mimicry in a guild of aphid predators. Ecological Entomology 31(1), 41-51. Full text

  • Rotheray, G.E. (1984). Colour guide to hoverfly larvae (Diptera, Syrphidae) in Britain and Europe. Dipterists Digest No. 9. Derek Whiteley, Sheffield.