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Oriental mugwort aphidOn this page: Identification & Distribution Biology & Ecology Discovery in Britain Life cycle Other aphids on the same host
Identification & Distribution
Of the Tuberocephalus species whose life cycles are known, most are thought to host alternate from Prunus primary hosts where they produce galls in spring (see below). The galls of Tuberocephalus tsengi have yet to be found, and this species may be anholocyclic. If not, then judging by the primary hosts of related species, it is most likely to be an oriental cherry species.
Adult apterae of Tuberocephalus tsengi on the secondary host (Artemisia, see two pictures below) are yellowish green with dark red eyes, black tips to the antennae, legs and siphunculi - and a pale cauda (cf. Myzus persicae, which does not have black tips to the antennae). The antennal tubercles are well developed, being rugose (= wrinkled) thumb-like processes about as long as their basal widths, projecting inwards and forwards and bearing thick hairs that are about as long as the basal diameter of antennal segment III.
The inner sides of antennal segment I are swollen and rugose but do not project forward beyond the base of antennal segment II. The 6-segmented antennae are 0.5-0.65 times the body length, and the terminal process is 1.67-2.33 times the length of the base of antennal segment VI (cf. Tuberocephalus sasakii, which has 5-segmented antennae, 0.65-0.85 the body length, with the terminal process 2.1-2.8 times the length of the base of that segment). The terminal process is much shorter than antennal segment III (cf. Myzus persicae, which has the terminal process usually longer than segment III). The apical rostral segment (RIV+V) is 1.4-1.9 times longer than the second hind tarsal segment (HTII) (cf. Tuberocephalus sasakii, which has RIV+V 1.0-1.3 times the length of HTII). The siphunculi have a large flange and are 1.85-2.46 × longer than the cauda (cf. Tuberocephalus sasakii, which has siphunculi with a moderate apical flange and 2.18-3.83 × longer than the cauda), The cauda is finger-like and tapering, with 4-7 hairs. The body length of adult Tuberocephalus tsengi apterae is 1.21-1.74 mm.
The alate Tuberocephalus tsengi (see first picture above) has a black head and thorax, dark appendages and siphunculi and a light green abdomen with dark markings. The antennal tubercles are lower than in the aptera, converging apically. Antennal segment I is much thicker than II, with the inner side gibbous and rugose. The terminal process is about 2.2 times longer than the base of antennal segment VI. Secondary rhinaria are distributed 21-37 on antennal segment III, 6-10 on segment IV and none on segment V. The dorsal abdomen has marginal sclerites and broad transverse sclerites on tergites II-VII, with some partially fused to form a central patch. The siphunculi of the Tuberocephalus tsengi alate are 2.1-2.5 times as long as the cauda.
The first two images below show a Tuberocephalus tsengi adult aptera in alcohol, and a close-up of the head tubercles. The third image is from a clarified mount.
Micrograph of clarified mount by permission of Roger Blackman, all rights reserved.
The images below show clarified mounts of an apterous viviparous aptera and a viviparous alate of Tuberocephalus tsengi.
Micrographs of clarified mounts by permission of Roger Blackman, all rights reserved.
Primary host generations, and the form of the galls that Tuberocephalus tsengi presumably produces on Prunus, are unknown. Their secondary hosts are Artemisia species - in UK they are only known from mugworts. Tuberocephalus tsengi occurs in China (original description as Myzus tsengi by Tao, 1963), Malaysia (BMNH collection) and Indonesia (Noordam, 2004), and probably in other countries of east and south-east Asia. Tuberocephalus tsengi also appears to be established in southern England on common mugwort (Artemisia vulgaris). Its confusion until recently with Tuberocephalus sasakii (the species were synonomized by Eastop & Hille Ris Lambers, 1976.) means both its distribution and life cycle are uncertain.
Biology & Ecology
Discovery in Britain
We first encountered Tuberocephalus tsengi in Rye Harbour Nature Reserve in East Sussex, England, in mid-May in 2018 when we found a large colony of an unknown species of aphid on the growing tip of a young mugwort (Artemisia vulgaris). We initially thought they were Myzus persicae, but examination under the microscope ruled this out because of various features of the antennae (see identification section above). Based on our photographs, Roger Blackman of the British Museum, Natural History suggested that it was a Tuberocephalus species. Subsequent examination of specimens sent to him showed they were closest to Tuberocephalus sasakii, a species not previously recorded outside eastern Asia. In Japan Tuberocephalus sasakii host alternates between cherry trees and Artemisia spp., but Tuberocephalus sasakii has also been reported from China, Korea, Malaysia, Indonesia and eastern Russia living anholocyclically on its secondary host (Sorin & Remaudière, 1998).
Closer examination revealed clear differences between ours and the Japanese specimens of Tuberocephalus sasakii from Artemisia in the Natural History Museum collection. The features of the aphid found in Britain instead agreed well with those of another aphid, Myzus tsengi, described from Artemisia species in Sichuan, China by Tao (1963). This species was previously synonomized with Tuberocephalus sasakii, but further investigation by Blackman et al. (2020) using the new British material and specimens from several East Asian countries resulted in him removing Tuberocephalus tsengi (Tao) from synonymy with Tuberocephalus sasakii and recognising it as a good species present in East Asia and now in Britain.
Repeated searches in 2019 failed to produce any more, but on June 10th in 2020 another colony of Tuberocephalus tsengi was found close to where they were first observed in Rye Harbour Nature Reserve. As in 2018, the aphids covered the growing tip of a mugwort (Artemisia vulgaris). On June 23rd in 2020 a further colony of Tuberocephalus tsengi was found on mugwort in Rye Harbour Village.
On primary host
Tuberocephalus tsengi most likely undergoes the full holocycle in its native China, inducing a gall on its primary host. The identity of the primary host and the form (or forms) of the gall of Tuberocephalus tsengi are so far unknown, but other Tuberocephalus species use various Prunus species as their primary host. The pictures below show the galls of two such species, the first being Tuberocephalus sasakii and the second being Tuberocephalus tianmushanensis.
Whether the galls of Tuberocephalus tsengi resemble these, or are even on cherry (Prunus), remains to be seen.
On secondary host
Tuberocephalus tsengi was most likely introduced to Britain as eggs on an ornamental cherry, as occurred with Tuberocephalus tianmushanensis in France (Remaudière & Sorin , 1993) - if so, Tuberocephalus tsengi has moved to a secondary host, mugwort, a common weed. It may be continuing to host alternate, but our finding of aphids on this secondary host in exactly the same site two years later suggests it may be overwintering parthenogenetically on that secondary host. Tuberocephalus sasakii is also thought to be anholocyclic in West Bengal as it common there on Artemisia, but is not known to occur in West Bengal on Prunus (Ghosh, 1973).
The second find of the aphid in Britain in 2020 was a little later in the year than that in 2018 (June rather than May) and, unlike in 2018, many of the aphids were developing to alatae (see two pictures below).
The nymphs were reared through to adult alatae on a cut head of Artemisia vulgaris put in a plastic box with a tight fitting lid - the standard method used for rearing third and fourth instar nymphs through to adulthood.
Other aphids on the same host
Tuberocephalus species have very different forms on their primary and secondary hosts. Unlike Tuberocephalus sasakii, whose primary host forms are known and whose galls have been recorded from 8 Prunus species, those of Tuberocephalus tsengi are unknown (unless their primary host forms have also been confused with those of Tuberocephalus sasakii).
Ten Tuberocephalus species have been recorded from their primary, Prunus, host(s) but seven of those speces are only known from that host. Blackman et al. (2020) discounted 5 of those as likely primary host forms of Tuberocephalus tsengi. Of the remainder, one species, Tuberocephalus momonis (from Japan, China, Taiwan, Korea and Mongolia) roll the edges of the leaves of Prunus persica (peach) and turn them red. Since their colonies persist into summer and autumn there may be no host alternation (albeit not all host-alternating aphids wholly vacate their primary hosts). The other species, Tuberocephalus usubai (from Japan), is only known as emigrant alatae from Prunus buergeriana (= Chinese bird cherry, Japanese dog cherry) - but any gall is unknown.
The most likely route of introduction of Tuberocephalus tsengi to Britain is on imported plants, either as overwintering eggs on its primary host, or as viviparae on a secondary host. Whilst Prunus buergeriana is not popular, some varieties of Prunus persica are recommended for British gardens - or Tuberocephalus tsengi could have arrived on a contaminant or 'weed' species.
Tuberocephalus tsengi occurs on 2 species of Artemisia (Artemisia carvifolia, Artemisia vulgaris) - but, given its past confusion with Tuberocephalus sasakii, it may have been recorded from others ('Tuberocephalus sasakii' has been found on Artemisia capillaris, feddei, indica, princeps, stolonifera, vulgaris).