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Aphis cornuta (Newly described species)
Horned oxtongue aphidOn this page: Identification & Distribution Biology & Ecology: Discovery Sexual forms Other aphids on the same host
Identification & Distribution:
The (newly described) horned oxtongue aphid - Aphis cornuta - lives in ant-tented colonies at the stem base of prickly oxtongue (Picris echioides, see first picture below). Adult Aphis cornuta apterae are bright yellow to yellow-green with dark siphunculi and dusky cauda (see second picture below). There are sometimes pale brownish-green sclerotic areas laterally on the thoracic and first abdominal tergites, brownish-green cross-bands on abdominal tergites 7 and 8, and small brownish-green triangular patches posterior to the bases of the siphunculi. The antennal terminal process is 3.13-4.13 times the length of the base of segment 6 (cf. Aphis picridis where the antennal terminal process is 2.0-2.8 times longer than the base). The rostrum is long - extending back well beyond the bases of the hind coxae, 0.38-0.57 times the body length (cf. Aphis picridis in which the rostrum is also quite long at 0.35-0.46 times the body length). The apical rostral segment (R IV+V) is 1.40-1.78 times longer than second segment of hind tarsus (HTII). Aphis cornuta aptereae have exceptionally large marginal tubercles on the prothorax with a basal diameter usually > 50 μm (cf. Aphis ochropus which has the basal diameter of its prothoracic tubercles < 50 μm. Blackman, pers. comm.) - hence the English name of horned oxtongue aphid. There are also quite large marginal tubercles on abdominal tergites 1 and 7, and also often smaller ones on abdominal tergite 6 (cf. Aphis picridis which only has small marginal tubercles on tergites 1 and 7). The siphunculi are 1.91-2.82 times longer than the cauda (cf. Aphis picridis which has the siphunculi 1.6-1.9 times longer than the cauda). The cauda is tongue-shaped with a slight midway constriction and bears 6-8 hairs.
The Aphis cornuta alate (see third picture above) has a shiny dark head and thorax and a yellow-green abdomen. Dorsal abdominal sclerotisation is limited to small marginal sclerites, small triangular postsiphuncular sclerites and narrow cross bands on abdominal tergites 7 and 8 as in the apterae, and a small sclerite of irregular shape between the siphunculi. The siphunculi are dark and the cauda is dusky. Immatures are pale yellow. Early instars have dark tips to mainly pale siphunculi, whilst fourth instars have mainly dark siphunculi which are paler towards the bases.
The micrographs below show an Aphis cornuta aptera in isopropanol (dorsal and lateral) and an alate (dorsal).
The micrographs below show a clarified mount of an Aphis cornuta aptera, with an enlargement of the head and prothorax showing the very large prothoracic tubercles.
Micrographs of clarified mount copyright Roger Blackman, all rights reserved.
The horned oxtongue aphid is only recorded from southern England. It was first found in 2015 in ant-attended colonies at the stem base of prickly oxtongue plants (Picris echioides) growing at the roadside on the Clifton-Shefford bypass in Bedfordshire. In 2018 it was found again at the same location and at another site in Bedfordshire, Etonbury Woods. The most recent finding was in July 2019, at Rye Harbour in East Sussex.
Biology & Ecology
We found these particular aphids in a somewhat unprepossessing habitat - namely a Bedfordshire roadside-verge nature reserve, beside a lay-by, in early September 2015.
The grass had been recently cut (earlier than it should have been, given its nature reserve status), but despite that a large colony of the aphid survived on a small oxtongue plant, Picris (=Helminthotheca) echioides (see picture below).
We initially thought they might be Aphis nasturtii, but their siphunculi are dark, their siphunculi/cauda length is >2, and the rostrum extends beyond the hind coxae. Given the features above, and that the body colour of adult apterae (in life) is bright yellow to yellow-green (first image below), plus the alatae are green (second image below), we provisionally identified them as Aphis picridis - which has not been recorded in UK before. Perhaps the xeric roadside conditions were enabling otherwise rare species to flourish - as appears to be the case for Myrmica schenckioides, a new ant species to the Netherlands (Boer & Noordijk, 2005).
On the advice of Dr. Roger Blackman we sent some alcohol-preserved specimens to the Natural History museum, from which three adult specimens were retrieved and converted to clarified mounts (see above). Roger noted there were significant differences between our specimens and the published descriptions of Aphis picridis.
All the indications were that this was a previously undescribed species, new to science - but there was a problem, we had far too few specimens (only three) for it to be described as a new species. We were unable to visit the site until September 2018, when we again found the aphid on a single bristly oxtongue plant. This enabled us to collect sufficient specimens for adequate descriptions to be made - see Blackman et al. (2019).
A few weeks later in mid-October 2018 our colleague and host in Bedfordshire, Alan Outen together with Clare Wardle visited Etonbury Woods in Bedfordshire. There they soon found several Picris echioides which had been ant-tented around their bases. Excavation of one of the plants revealed a colony of Aphis cornuta which was of especial interest since it contained the sexual forms, namely the apterous males and oviparae.
The apterous male (see first and second pictures below) has antennae that are 0.77-0.92 of body length. Secondary rhinaria are small and numerous with 27-32 on antennal segment III, 20-28 on segment IV and 4-5 on segment V.
The ovipara (see third picture above) is similar to the apterous vivipara in most respects, but has shorter antennae (0.56-0.71× body length) and shorter siphunculi (1.46-1.58× cauda). An immature ovipara is shown below.
Other aphids on same host
Blackman & Eastop list 16 species of aphid as feeding on bristly oxtongue (Picris echioides = Helminthotheca echioides) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 15 as occurring in Britain: (Show British list).