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Genus Eriosoma [Eriosomatini]
Both winged and wingless Eriosoma aphids have rather conspicuous siphuncular pores with partially chitinized rims surrounded by a ring of hairs. Most species are wax-covered. The forewing of winged individuals usually has only one branch in the medial vein.
There are about 35 species most of which host alternate between galls on elm (Ulmaceae) and secondary hosts such as apple (Rosaceae) and currants (Grossulariaceae). They typically have a sexual stage in the life cycle. Eriosoma aphids are not attended by ants.
Eriosoma americanum (Woolly elm aphid) North America
The developing colonies of Eriosoma americanum on American elm (Ulmus americana) feed on the edges of young elm leaves. This causes them to roll downwards and form a pseudogall around the colony (see first picture below). The developing alatae in these rolled leaf galls are orange-pink and densely covered with grey wax (see second picture below).
Images above copyright Claude Pilon, all rights reserved.
Emigrant alatae (=spring migrants) emerging from galls in June (see third picture above) are dark green to blue-black with some wax wool, and rather dark wing veins. The terminal process of antennal segment VI is less than 0.5 times the length of the base of that segment. Antennal segment III is more than 0.8 times the head-width as measured across the eyes (cf. Eriosoma mimicum, which has segment III less than 0.8 times the head-width). Antennal segment III is a little shorter than antennal segments (IV+V+VI) (cf. Eriosoma crataegi, Eriosoma lanigerum & Eriosoma pyricola, which all have segment III as long as or longer than segments IV+V+VI). Antennal segment V has secondary rhinaria (cf. Eriosoma grossulariae and Eriosoma ulmi, which have no rhinaria on segment V). These alatae have rather conspicuous raised siphuncular pores, which have partially chitinized rims and are ringed by hairs. The body length of alatae is 2.0-2.5 mm. The alatae migrate to form colonies of pale pink, woolly apterae on the roots of their secondary hosts, especially Amelanchier laevis.
Eriosoma americanum host alternates between American elm (Ulmus americana) and the roots of saskatoon (also known as serviceberry, Amelanchier species). In August to October, the return migrants (sexuparae) to elm produce dwarf sexuales which lay overwintering eggs on bark. Woolly elm aphid occurs throughout the natural range of Ulmus americana in eastern North America, and in the west on planted elms.
Eriosoma lanigerum (Woolly apple aphid) Cosmopolitan where apples grown
On their secondary host (apple) Eriosoma lanigerum wingless females are purple, red or brown and are covered in thick white flocculent wax. This is produced by distinct wax glands on the head and along the thorax and abdomen. The six segmented antennae are 0.17-0.24 times the length of the body. The body length of Eriosoma lanigerum apterae is 1.2-2.6 mm.
Winged viviparous females (shown in the second picture above) have a brown-black head and thorax and a brown abdomen. Their antennae are about 0.4 times the length of the body. The siphunculi are reduced to a pair of rings on the posterior dorsum of the abdomen. The fourth instar alatiform nymph of Eriosoma lanigerum (shown in the picture below) is reddish brown with very small wax glands and consequently much less wax.
Wingless females of woolly apple aphids live in dense colonies on the roots, trunk or branches of the (secondary) host apple (Malus) where it is a serious pest, often causing deformation and cancer-like swellings of bark. Eriosoma lanigerum is also found on related species, such as hawthorn (Crataegus) and Cotoneaster. In most parts of the world, sexual forms have never been found on apple, and overwintering is in fissures on the lower part of the trunk and on the roots. sexuparae producing oviparae and males on apple have been reported in a few countries, with eggs laid on apple, but no resultant fundatrices have been found. In North America aphids of the Eriosoma lanigerum group (which includes several species closely-related to Eriosoma lanigerum, in USA) induce leaf-rosette galls on American elm (Ulmus americana). Some authorities classify such aphids as Eriosoma lanigerum. Others believe that Eriosoma lanigerum (in the strict sense) has lost its primary host and classify the (elm-feeding) host-alternating species as different species within the Eriosoma lanigerum species group.
Eriosoma lanuginosum (Elm balloon-gall aphid) Europe, Asia
In spring the Eriosoma lanuginosum fundatrix and her offspring develop in large, closed, bloated-leaf galls (see first picture below) on various elm (Ulmus) species. This gall is produced by extreme enlargement of the cells of the leaf parenchyma on one side of the mid-rib near its base. The gall tissue is light green at first and covered with fine white hairs, becoming brown as the gall matures. There are often clusters of galls near the ends of branches (see second picture below). The fundatrix which initiates the gall is blackish and wax powdered (not pictured), but the offspring of the fundatrix (known as fundatrigeniae) are wax-powdered, wingless and yellowish when immature.
First image copyright Sarah Gregg under a creative commons CC BY-NC-SA 2.0 licence
Alates produced in the second and third generations on elm are dark green to black and wax powdered (see first picture below). The body length of the adult Eriosoma lanuginosum alate is 2.1-3.1 mm. Apterae on the secondary hosts are pale yellow to reddish, with an adult body length of 2.0-2.7 mm.
The alates leave the galls on elm in late June-July and migrate to found colonies on fibrous rootlets of pear (Pyrus communis) or quince (Cydonia). The return migration to elm bark occurs in September. Eriosoma lanuginosum occurs throughout Europe, the Mediterranean area, and Asia east to Pakistan.
Eriosoma ulmi (Elm-currant aphid) Europe, Asia, Canada
In spring Eriosoma ulmi fundatrices develop in yellowish or whitish green galls on elm (see first picture below). The fundatrices and their apterous offspring are dark green and wax-covered. The six segmented antennae are 0.18-0.2 times the length of the body, with a terminal process that is a quarter of the length of the base of the last antennal segment. There are no siphunculi or siphuncular pores. Their alatiform offspring are brownish or dull green (cf. Eriosoma grossulariae which has light green immature alatae).
The adult winged viviparae of Eriosoma ulmi are dark green to bluish grey (see second picture above) with dark cross bands on the abdomen. The antennae are about half the length of the body. There are no secondary rhinaria on the fifth antennal segment (cf. Eriosoma crataegi & Eriosoma lanigerum which have 4-8 secondary rhinaria on the fifth antennal segment). Also the antennal terminal process is short and thick (cf. Eriosoma grossulariae which has the terminal process longer and thinner). The siphuncular pores are large and sited on low dark hairy cones.
The elm-currant aphid host alternates from the primary host elm (Ulmus spp.) to the secondary host, the roots of currant (Ribes). Eriosoma ulmi is found in Europe and much of Asia, eastward to Mongolia and China. It has recently been introduced into Canada.