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Genus Tetraneura [Pemphigini]
Tetraneura are small aphids. The wingless forms have one-segmented tarsi and the winged forms have a simple, unbranched medial vein in the forewing. The 4th antennal segment is usually much shorter than the 5th. Wax gland plates may be present or absent.
There are about 30 species in the Tetraneura genus. At least seven of them host alternate between leaf galls on elm (Ulmaceae) and roots of grasses (Poaceae). The remainder are only described from either elm or the roots of grasses. They usually have a sexual stage in the life cycle. The fundatrices induce stalked, pouch-like red, yellow or green galls on the upper sides of leaves. Tetraneura are not attended by ants.
Tetraneura nigriabdominalis (Oriental grass root aphid) Asia, Europe, North America
The galls of Tetraneura nigriabdominalis on elm are rose-red and whitish green when immature, but rose-red with a greenish stalk when mature (see first picture below). They are stalked, elongate, spindle-or pouch-shaped, pubescent (hairy), not shiny, and often with a pointed apex (cf. galls of Tetraneura ulmi, which are stalked, shiny, but not pubescent; or cf. galls of Tetraneura caerulescens, which are stalked, globular and matt; or cf. galls of Tetraneura fusiformis, which are stalked, spindle-shaped and pubescent, but with numerous granular protuberances). The Tetraneura nigriabdominalis fundatrix (see second picture below) which stimulates production of the gall is greenish with a variable amount of wax dusting. The offspring of the fundatrix which develop within the gall (see third picture below) are light green and covered with wax tendrils. These develop to emigrant alatae.
Images above, copyright Claude Pilon, all rights reserved.
The emigrant alatae (not pictured) have a shiny black head and thoracic lobes and a brown abdomen. Antennal segment V is 1.7-3.0 times longer than segment VI (including the terminal process), and bears 5-14 secondary rhinaria (cf. Tetraneura ulmi, whose antennal segment V is 1.2-1.7 times longer than segment VI, and bears 3-8 secondary rhinaria). The primary rhinarium on antennal segment VI is not enlarged (cf. Tetraneura ulmi, which has the primary rhinarium on VI enlarged.) The apical rostral segment (R IV+V) is 0.47-0.70 times longer than the second hind tarsal segment (HTII) (cf. Tetraneura ulmi, which has RIV+V 0.66-0.84 times longer than HTII). The adult apterae of Tetraneura nigriabdominalis on the secondary host, grass roots, are rather small, greenish-white or brownish-white, plump and oval bodied, but not so globose as in most other Tetraneura species (cf. Tetraneura ulmi, which has the adult apterae orange yellow and globose; or cf. Tetraneura caerulescens, which has the adult apterae orange-brown to brown and secreting bluish flocculent wax).
Tetraneura nigriabdominalis forms galls on various elm species (e.g. Ulmus canescens, Ulmus japonica, Ulmus minor, Ulmus parvifolia, Ulmus procera and Ulmus pumila). It host alternates to the roots of grasses and cereals (Cynodon, Digitaria, Oryza, Saccharum, Setaria). Feeding by apterae of Tetraneura nigriabdominalis on the roots of some grasses causes a reddish purple discoloration of the leaves. It is found in the Far East (China, Japan, Korea) where it is thought to have originated, and is now also found in Georgia, Kazakhstan, southern and south-east Europe and the USA. In Britain Tetraneura nigriabdominalis has only been found in southern England as galls on bonsai elms imported from Japan, but may well establish in Britain given the continuing expansion of its range into northern Europe - presumably due to climate warming.
Tetraneura ulmi (elm-grass root aphid) Europe, Asia, North America
On the primary host, elm, Tetraneura ulmi develop within galls on the leaves. The galls are stalked, approximately bean-shaped, smooth and shiny, and coloured reddish-green and/or yellow (see first picture below). The Tetraneura ulmi fundatrix, which stimulates production of the gall, is light green with the head, thorax, antennae and legs dark and transverse bands of light wax across the abdomen and thorax (see second picture below). The offspring of the Tetraneura ulmi fundatrix develop within their gall to winged viviparous alates (not pictured here) which have a shiny black head, thorax, antennae and legs, and greyish black abdominal segments. The body length of Tetraneura ulmi alates is 1.8-2.6 mm.
The adult apterae on the secondary host, grass roots, are readily identified, being pale orange yellow, yellowish white or reddish (see third picture above). The head, prothorax and appendages are brown, and the body is (sometimes) lightly dusted with wax.
Tetraneura ulmi host alternates. The winged forms of the elm-grass root aphid emerge from elm galls (Ulmus spp.) in June-July to colonize roots of grasses (Poaceae). Populations without sexual forms occur commonly on secondary hosts. In September winged forms make a return migration to elm where they produce larvae which feed on the bark, and mature to apterous males and females. Fertilized females only lay one egg each. Tetraneura ulmi is found in Europe, across Asia to eastern Siberia, and has been introduced to North America.