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A new species for Britain?
Stomaphis near wojciechowskii (Pale giant oak aphid)
Dransfield, R.D., Hodgson, J.F. & Brightwell, R.
Uploaded 19 July 2015. Updated 12 July 2015On this page: Description Biology & Ecology: Habitat Ant attendance Other aphids on the same host
A very rare (or entirely new) species, of giant aphid has been found by Julian Hodgson on the trunks of oaks at Monks Wood National Nature Reserve in Cambridgeshire, England. This aphid belongs to the genus Stomaphis, the species of which are characterized by their large size (up to 7 mm in body length) and an extraordinarily long rostrum (up to twice the body length, or more in nymphs) - which enables them to probe through the bark crevices and feed deep inside oak and other trees. Stomaphis are nearly always closely-attended by ants which feed on their honeydew and help protect the aphids from predators and parasitoids.
Only a few Stomaphis species occur in Britain, and all are rare or very rare. The giant oak aphid (the world's largest aphid, Stomaphis quercus ) occurs on oak and birch. The giant maple aphid (Stomaphis graffii) feeds on the trunks of maple and sycamore. Stomaphis longirostris, which feeds on willow and poplar, has also been reported but is unconfirmed (Baker, 2015 ).
Given that the Cambridgeshire Stomaphis was on the trunks of oak trees (Quercus robur), it might be expected to be Stomaphis quercus. However, several points cast doubt on this:
Stomaphis wojciechowskii has never previously been observed in Britain, and is well outside its currently predicted geographic range (Depa et al., 2017 ). However, its concealed life style has resulted in it only recently being discovered in other European countries.
Both images copyright Julian Hodgson, all rights reserved.
Given that the paired spinal sclerotic plates are wider than long, we may conclude that the first image (above) shows a fundatrix (Depa, pers. comm.). It is elongate-oval and pale in colour. Since it is feeding, the rostrum is buried in the tree bark. When not feeding the long rostrum is carried under the body and can be seen protruding a full body length behind the insect in the second picture above - which is probably a nymph. Measurements of live specimens suggest the rostrum is slightly more than twice the body length, and that the antennae are 0.32-0.37 times the body length. It has numerous densely placed, erect hairs on the body, antennae and leg, with an obvious row of darker spinal plates. The dark grey siphuncular cones are rather small.
Image copyright Julian Hodgson, all rights reserved
This alate (found in June 2018) has rather short, narrow wings with brown-bordered veins. Alate Stomaphis wojciechowskii had not been described prior to 2018, but Depa pers. comm. also found several in June this year. Among closely ant-tended species winged forms tend to be rare or unknown. In some cases, as in Trama troglodytes, few winged forms are found because the ants bite off their wings. The rarity of alatae may partially explain why no Stomaphis has ever been recorded by Rothamsted Insect Survey suction-traps (Bell et al., 2015, Appendix S2) and why no Stomaphis are recorded on NBN Gateway (on 11 July 2018).
A further reason to suspect the Cambridgeshire Stomaphis may be Stomaphis wojciechowskii, rather than Stomaphis quercus, is it was not attended by the jet black ant Lasius fuliginosus but by the brown ant Lasius brunneus. The latter ant species nearly always attends Stomaphis wojciechowskii, whereas Stomaphis quercus are attended by Lasius fuliginosus. - There is however one known exception: Sardinian populations of Stomaphis quercus (confirmed by COI barcoding), are pale and are attended by Lasius brunneus (see Loi et al., 2012 and Depa et al., 2017 ).
Identifying this Cambridgeshire Stomaphis species is clearly a priority, and two pathways have been followed:
Hopefully the morphometric and molecular comparisons will show whether the Cambridgeshire population is Stomaphis wojciechowskii or Stomaphis quercus, or something else.
Biology & Ecology:
Julian found the first Stomaphis specimens walking openly on the trunks of oak trees during daylight in May 2018. This behaviour subsequently proved to be exceptional. Similarly, although individual aphids have been found feeding at the base of deep, open bark crevices, this also appears to be exceptional. However, aphids can often be found walking up and down bark crevices at night; usually escorted by Lasius brunneus, though sometimes unescorted. Most aphids were found hidden from view beneath sections of bark crevices covered over either by bark or other material including moss, lichen fragments, or 'soil' deposited along the ant trails.
A favoured location for these aphids was at the upper terminations of bark crevices, where the bark has grown over the crevice to form a small chamber which is open only at the bottom end and is guarded by ants. Within these crevices small groups of aphids live and feed. These hidden groups typically contain about 5 or 6 aphids (with a range of 2 - 11), comprising 1 - 2 large adults (possibly fundatrices), 2 - 3 smaller adults and 1 - 2 nymphs. The image below shows such a typical group, photographed after the covering flakes of bark had been removed.
Image copyright Julian Hodgson, all rights reserved
All such groups, found to date, were located from about 0.55 to 3 meters above ground. The upper limit given here is necessarily limited by the height of the observer, or how high he could easily climb, but the ant trails also continue much higher up the trees.
The Cambridgeshire Stomaphis aphids are attended exclusively by the brown ant (Lasius brunneus). There are good populations of this species on the oak trees. Activity of the brown ant is mainly nocturnal, so the best time to search for aphids being attended by these ants is at night. The ants in the two pictures below are engaged in feeding from the honeydew, stroking the aphid with their antennae to encourage honeydew production, and guarding the aphids from potential predators.
Both images copyright Julian Hodgson, all rights reserved
If the aphids hidden in the crevices are exposed to light, the ants usually immediately begin escorting the aphids down the crevices to a place of safety within the moss-covered base of the trunks. The ants will occasionally pick up the smallest individuals (probably mostly nymphs) in their jaws and literally run down the crevice, sometimes falling in the process in their haste to remove the aphids. Similar behaviour has been observed with root aphids, in particular Smynthurodes betae.
Other aphids on same host:
Blackman & Eastop list 34 species of aphid as feeding on common or pedunculate oak (Quercus robur) worldwide, and provide formal identification keys.
Of those aphid species, Baker (2015) lists 14 as occurring in Britain: Hoplocallis picta, Lachnus longirostris, Lachnus roboris, Moritziella corticalis, Myzocallis boerneri, Myzocallis castanicola, Phylloxera glabra, Stomaphis quercus, Thelaxes dryophila, Thelaxes suberi, Tuberculatus annulatus, Tuberculatus borealis, Tuberculatus neglectus and Tuberculatus querceus.
Blackman & Eastop list Stomaphis species as feeding on 12 species of oak (Quercus) worldwide. Among those, Stomaphis japonica and Stomaphis quercus have been observed on the most species, followed by Stomaphis bratislavensis, Stomaphis wojciechowskii [and possibly Stomaphis longirostris & Stomaphis quercisucta].
Recent work (Depa et al., 2017 ) indicates Stomaphis wojciechowskii has a wide host range: including other Quercus species, Alnus glutinosa, Juglans regia, Salix species (unidentified) and Tilia cordata.