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Mountain cornflower aphidOn this page: Identification & Distribution Biology & Ecology Discovery in Britain Confirmation of species identification Other aphids on the same host
Identification & Distribution
Adult apterae of Uroleucon montanivorum are spindle-shaped and coloured glossy reddish brown to black. The first picture below, of an adult aptera, is from Wales and represents a new species for Britain. The second picture below is of fourth instar Uroleucon montanivorum from central France. The antennae of the adult aptera are 1.2-1.4 times as long as the body, with their terminal process 6.9-7.5 times longer than the base of segment VI. Antennal segment III bears 40-58 secondary rhinaria, sited on the basal 0.6 of the segment; antennal segment IV is without rhinaria. Antennal hairs are about as long as the diameter of the base of antenna segment III. The apical rostral segment (RIV+V) is 1.6-1.9 times as long as the second hind tarsal segment (HTII) (cf. Uroleucon compositae, which has RIV+V 1.1-1.4 times the length of HTII, and Uroleucon aeneum, which has RIV+V 1.2-1.3 times the length of HTII). The hair-bearing sclerites of the abdominal tergites are discernible as transverse rows of black flecks. The anal and marginal sclerites are similarly black. Postsiphuncular sclerites are very distinctly developed, but antesiphuncular sclerites are lacking or rudimentary in Uroleucon montanivorum.
First image above copyright Alan Orange, all rights reserved.
The coxae of Uroleucon montanivorum apterae are black. Their femora have the basal 0.67-0.75 yellowish white, abruptly grading into matt-black distally (cf. Uroleucon compositae, whose mainly dark femora are only pale on the basal 0.2-0.3 of their length). The tibiae are dark basally and distally but otherwise rather pale (cf. Uroleucon jaceae, which has wholly dark tibiae). The siphunculi are black, tapering towards the apical end, and about twice as long as the cauda. The cauda is dark, sabre-shaped, slender, and with 20-29 hairs (cf. Uroleucon jaceicola, which has a pale yellow cauda with 9-18 hairs). The body length of adult Uroleucon montanivorum apterae is 3.6-4.4 mm.
Image above copyright Alan Orange, all rights reserved.
The alate Uroleucon montanivorum vivipara is similar to the aptera, but abdominal sclerotization is stronger. Their antennae are 1.2-1.5 times as long as the body. Antennal segment III has 59-80 secondary rhinaria distributed over the entire length. There are numerous spinal and pleural sclerites, the postsiphuncular sclerites are large, but ante-siphuncular sclerites are completely absent. The femora have the proximal 0.50-0.62 yellow-white, but the distal part is black. The tibiae are mainly light gray-yellow to brown-yellow, with the distal 0.20-0.25 brown-black.
The images below show clarified slide mounts of British specimens of an apterous viviparous aptera and a viviparous alate of aphids identified as Uroleucon montanivorum.
Images above copyright Alan Orange, all rights reserved.
Uroleucon montanivorum feeds on the apical leaves and shoots of the mountain cornflower (Centaurea montanum). Oviparae and alate males have been found from late summer to autumn. It has previously only been recorded from southern Germany, France and Switzerland, but has now been found present in a garden in Wales.
Biology & Ecology
Discovery in Britain
On 11 June 2020, Alan Orange found a colony of Uroleucon aphids on the stems of mature inflorescences, and on the undersides of young leaves of non-flowering shoots of mountain cornflower, Centaurea montana (front-right of image below) in a suburban garden in Llandaff, Cardiff, Glamorgan, Wales (ST1496.7775). Centaurea montana is widespread and common in the more southerly mountain ranges of Europe. It is rarer in the wild in the north, but is a popular garden plant.
First image above copyright Alan Orange, all rights reserved.
The Uroleucon aphids were frequent in the only patch (about 50 cm square) of their hostplant in the garden. These cornflowers were believed to have been in the garden for six years, but the aphids had not been noticed previously. Some adult apterae were present, with more numerous apterous and alatiform nymphs, and a small proportion of alatae. They were not ant attended, and the host showed no galling or distortion.
Uroleucon montanivorum is a large and conspicuous species, with long legs, and the body held somewhat free of the substratum. Individuals frequently flick the body rapidly sideways, then become still again. About 10 mature viviparae and two alates were preserved, and clarified mounts were made of an aptera and alate in Euparal. After some research, Alan decided these aphids were most likely Uroleucon montanivorum (and therefore a new addition to the UK species list), but was concerned the aptera seemed to have rather too few secondary rhinaria on antennal segment III.
Confirmation of species identification
Alan Orange sent us images of his clarified mounts, and his measurements - and, allowing for his concerns, he asked if we could confirm his identification.
The only Uroleucon recorded by Blackman as feeding on Centaurea montana is Uroleucon montanivorum, a fairly recently described species (1959) previously only known from France, Germany & Switzerland. We nevertheless also considered five other Uroleucon recorded on the Centaurea genus: Uroleucon compositae, Uroleucon jaceae, Uroleucon jaceicola, Uroleucon mongolicum, and Uroleucon pepperi.
Uroleucon jaceae and Uroleucon jaceicola are quite common on some Centaurea species in UK, but they were ruled out primarily on the basis of colour of the tibiae and cauda. The tibiae of the Welsh apterae are mainly pale yellowish brown, but darker just by their base and over the distal 25%. The colour of the tibiae appears to rule out the commonest Centaurea-feeding aphid - Uroleucon jaceae - which has black tibiae. The cauda of the Welsh apterae is dark. The dark cauda appears to rule out the only other Centaurea-feeding aphid found in Britain - Uroleucon jaceicola - which has a pale yellow cauda with too few hairs (8-18 vs. 23). There are no records of the other species occurring anywhere near Britain: Uroleucon compositae is known from tropical and subtropical climates and is relatively polyphagous. Uroleucon mongolicum is recorded from Serratula species in Mongolia, and Centaurea monantha in Korea. Uroleucon pepperi is an American species, feeding primarily from Cirsium species (also Centaurea dealbata) in western USA. None of these fitted the observed features.
We also considered the remote possibility this could be Uroleucon aeneum on an accidental or overflow host. Uroleucon aeneum often forms large colonies on the upper parts of thistles, but is closely related to Uroleucon jaceae, and 'vagrant' colonies are known from other genera: Arctium, Carthamus and Sonchus. We felt we could safely rule Uroleucon aeneum out, partly because these were obviously not small struggling colonies, but were thriving on a number of plants and their colony size and structure of looked very typical of Uroleucon montanivorum (see picture below).
In addition, Heie gives RIV+V as 1.2-1.3 x HTII for Uroleucon aeneum apterae, whereas the Welsh aptera was 2.0 × HTII. We then made detailed comparisons between clarified mounts of a British aptera and alate and the original descriptions of Uroleucon montanivorum given by Mosbacher (1959).
The characteristics detailed above are generally in good agreement with those listed for Uroleucon montanivorum by Mosbacher (1959), apart from the British aptera being somewhat smaller and having fewer secondary rhinaria. Some Uroleucon species such as Uroleucon jaceae are known to have unusually variable counts and distributions of secondary rhinaria, so it would not be surprising if Uroleucon montanivorum was variable in this respect. Moreover Uroleucon montanivorum in UK may have originated from very few specimens imported via the ornamental plant trade, or even a single clone. Given the limited interchange with European populations a UK population is also liable to genetic drift, so some deviation from the European population is unsurprising. Similarly deviant morphological features have been recorded from Stomaphis wojciechowskii and Cinara smolandiae. We therefore conclude that the Welsh individuals are indeed true Uroleucon montanivorum - whether it is a transient population or instead indicates that the species is established in UK remains to be seen.
Second UK record
The second observation was confirmed by us from specimens, images of live specimens and and their host-plant, kindly sent by Murdo Macdonald, HBRG Database Manager.
Microscopic examination of the preserved specimens showed that most of their characters fitted Uroleucon montanivorum, including the number and distribution of secondary rhinaria on ANT III, and the the colour of the femora. The one feature which did not fit was the ratio of the terminal process to the base of antennal segment 6, which was about 5-6 rather than 6.9-7.5.
We then realised that all the adults were oviparae, as shown by the presence of enlarged tibiae bearing scent plaques. In one way this was unfortunate, since there are no keys for oviparae, in another way this is quite interesting because it suggests they are reproducing sexually - assuming of course they are also producing males and viable eggs. We hoped this might explain the abnormally low PT:base but, referring to the original description (in German), U. montanivorum oviparae have a similar PT:base. All we can say is the characters don't fit any other Uroleucon species we know of. Since it is not uncommon for populations arising from a few introduced individuals to be atypical is it is highly probable that these Scottish aphids are U. montanivorum.
Given this second record, and that from Wales, we conclude this species is distributed across the country, albeit rather uncommon since its host is no longer a fashionable garden plant.
Other aphids on the same host
Uroleucon montanivorum has only been recorded on 1 species of cornflower Centaurea, the mountain or perennial cornflower (Centaurea montana) - and, possibly, Centaurea cyanus.
Blackman & Eastop only list 3 species of aphid as feeding on Centaurea montana (= the perennial cornflower, mountain cornflower, bachelor's button, montane knapweed, mountain bluet) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) now lists all 3 as occurring in Britain (Show British list).