Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Oregano leaf-curl aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution
Aphis origani is one of a group of species related to Aphis frangulae, but living monoeciously on Labiatae (see also Aphis passeriniana on Salvia and Aphis lamiorum on Lamium purpureum). Aphis origani lives on the stems and leaf-undersides of oregano (Origanum vulgare) causing a characteristic downward leaf-curl where they are often tented over with earth by attending ants (see first picture below). Adult apterae of Aphis origani vary in colour from greenish-yellow to dark mottled green (see second and third pictures below) with no waxing. Their antennal terminal process is 1.8-2.6 times the length of the base of antennal segment VI. The apical rostral segment (RIV+V) is 1.08-1.15 times the length of the second hind tarsal segment (HTII). The dorsal sclerotic pattern is variable but, at most, there may be marginal and postsiphuncular sclerites and bands on tergites VII-VIII. Hairs on the hind tibia vary from slightly shorter to slightly longer than the least-width of the tibia. The siphunculi are wholly dark and the cauda is dusky (cf. Aphis nasturtii, which have pale siphunculi with dark apices and a pale cauda). The siphunculi are 1.05-1.45 times as long as the cauda (cf. Aphis gossypii, whose siphunculi are 1.3-2.5 times the caudal length). The body length of adult apterae is 1.0-1.8 mm. Immature Aphis origani apterae are wax-free, and similarly coloured to their adults.
Unlike other Aphis origani morphs, immature alatae have discrete white pleural wax spots (see first picture below). Mature alatae (see second picture below) are typical of aphids of the 'frangulae' group being unwaxed, with dark intersegmental sclerites present in a few specimens, but spinal sclerites are rarely present on tergites I-V. The alatae have 2-8 secondary rhinaria on antennal segment III.
The images below show an adult Aphis origani aptera and an alate in isopropyl acohol.
Downward leaf-curl and ant-tenting are good indicators of the presence of Aphis origani which form colonies up the stems and flowers, and on the leaf undersides of their main host, oregano or marjoram (Origanum vulgare). It has also been recorded on several other members of the Lamiaceae, but Blackman cautions that this could be due to confusion with other species. The aphids are usually (if not always) attended by ants. Sexual forms have apparently not yet been described, but oviparae and males would be expected to occur in autumn. The oregano leaf-curl aphid is found in Europe excluding Scandinavia, and in Morocco, Central Asia and Pakistan. Aphis origani is found in Britain but Stroyan (1984) only knew of records from the Clifton Gorge in Avon and from Kent, and suggested the species may need a more continental climate than Britain possesses. We can now confirm its presence in East Sussex.
Biology & Ecology
There may be an interspecific association between the gall midge Blastomyia origani and the aphid Aphis origani, both of which gall oregano. The gall produced by the gall midge is shown in the first picture below. The larvae of the gall midge transform the shoot tip or lateral buds into oval tufts of leaves up to 15 mm long and 10 mm broad (Roskam & Carbonelle, 2015). The leaves around the gall are broadened, thickened and arched, and often have long white hairs on the upper side.
The aphid galls are quite different, and are formed by downward leaf curl part way up the stem (see second picture above). The stem and leaf-undersides are then covered with earth particles by attending ants. Note however that the growing tip of the plant shown with an aphid gall has also been galled by Blastomyia midges. In fact all the aphid galls we have found (admittedly a very small sample) have been on plants also galled by Blastomyia. This perhaps explains the degree of confusion in the past between the gall caused by Aphis origani and the gall caused by a gall midge Blastomyia origani (see Redfern, British Plant Gall Society).
Aphis origani feed both on the undersides of the inwardly-curled leaves (see colony in picture below) and under the earth-covered stems.
Other aphids on the same host
Aphis origani occurs on 2 species of oregano (Origanum) (Origanum × paniculatum, Origanum vulgare). It has also been recorded on two species of basil (Clinopodium nepeta, Clinopodium vulgare), two species of mint (Mentha arvensis, Mentha longifolia), one species of Phlomis (Phlomis tuberosa), one species of Satureja (Satureja montana) and one species of Sideritis (Sideritis romana).
Blackman & Eastop list 9 species of aphid as feeding on oregano (Origanum vulgare) worldwide, and provide formal identification keys (Show World list). Of those, Baker (2015) lists all 9 as occurring in Britain (Show British list).