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Elm-currant aphidOn this page: Identification & Distribution Biology & Ecology: Life cycle Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution:
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 whose immature alatae are light green).
The adult winged viviparae of Eriosoma ulmi are dark green to bluish grey (see second picture above) with dark cross bands on the abdomen (see first micrograph below). The antennae are about half the length of the body. There are no secondary rhinaria on the fifth antennal segment (see second micrograph below) (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.
Biology & Ecology:
Urban et al. (2003) gives a detailed account of the life cycle of Eriosoma ulmi. Development begins in spring in the period of elm budbreak. Larvae hatch from eggs wintering in fissures of the bark of stems and large branches. After a short rest, larvae travel towards buds and settle on the underside of unfolding leaves.
Due to secretions of salivary glands injected into young leaf tissues, the lateral edge (more rarely apex) of a leaf blade rolls up downwards, thus gradually forming a leaf-roll or pseudogall (see two pictures below). The deformed part of the leaf becomes yellowish or whitish green.
Alvarez et al. (2013) describes the histology of the pseudogalls. Galls induced by Eriosoma ulmi resemble the leaf structure of Ulmus in certain aspects, but in most aspects they do not, showing only cellular hypertrophy. Galls are established by one or occasionally several young fundatrices. Once mature, each wax-covered fundatrix (see picture below) produces larvae for a period of 2 to 4 weeks.
The colour of young nymphs seems rather variable ranging from grey to green to yellow (see pictures above and below).
By the time they have reached the fourth instar and showing the developing wings, most are reddish-brown (see first picture below) or yellowish-brown (see second picture below). Those in the second picture below are the same aphids as the yellow nymphs in the picture above.
These develop to migrant alatae (see picture below) by late May or June.
These alates leave the galls and migrate to the root collar of currant bushes, mainly redcurrant (Ribes rubra) or blackcurrant (Ribes niger). There they produce larvae which feed on thin roots of a diameter of about 1 mm. The resulting aphids live freely (not in galls) on roots under a thick protective layer of wax fibres. In the course of about 3 months during summer and early autumn they undergo a total of 7 generations on roots of the secondary hosts (Janiszewska-Cichoka, 1971). The last generation comprises winged females, and in September and October these return to elm and produce larvae of the last generation in fissures of the bark of stems. These larvae moult 2 to 3 times and mature into males and females. After mating each female lays just one egg into a bark fissure, and then dies, its body protecting the egg.
We have not encountered any parasitoids active in Eriosoma ulmi galls, but we have found predatory anthocorid bugs (see picture below).
During outbreaks of Eriosoma ulmi on the elm Ulmus glabra in Moravia, Czech Republic, Urban et al. (2003) found that natural enemies killed 90% of aphids. Anthocoris confusus was the most effective control agent, killing aphids occurring in 80% of galls. Similarly Parker (1984) found that one species of anthocorid, Anthocoris gallarumulmi, was largely specific to gall aphids - both Eriosoma ulmi and Eriosoma patchiae, on elm.
The only other predators we have encountered are syrphid larvae. Urban et al. (2003) found that larvae of Syrphidae, mainly Syrphus ribesii, killed the aphids in 4% of galls.
The only other predator in his study was birds, which killed aphids in 6% of galls. Tanasijtshuk. et al. (1976) described a new species of predatory Chamaemyiidae, Leucopsis magnicerca sp.n., from leaf galls caused on Ulmus by Eriosoma ulmi.
Other aphids on same host:
Eriosoma ulmi, like others of their genus, are not specific as to which species of elm (Ulmus ) they use as their primary host.
Blackman & Eastop list about 75 species of aphids as feeding on elms worldwide, and provides formal identification keys for aphids on Ulmus
When it comes to their secondary host, Ribes species, Eriosoma ulmi occurs on only a small proportion of the 150 known species: Ribes aciculare, Ribes aureum, Ribes divaricatum, Ribes janczewskii, Ribes meyeri, Ribes nigrum, Ribes oxyacanthoides, Ribes rubrum, Ribes sanguineum and Ribes uva-crispa.
Damage and control
Urban et al. (2003) considers Eriosoma ulmi a serious pest of both ornamental elms and currant bushes. Elm are damaged by the sooty moulds which develop on the honeydew, and galled parts of the leaf soon die. Affected currant bushes become stunted and yield drops. Chemical control on elm is feasible during the period when the fundatrices are hatching, but is unlikely to be economically justifiable.