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Shiny birch aphidIdentification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution:Apterae of Symydobius oblongus (see first picture below) are shiny dark brown with no wax covering. The antennae are brown apart from the basal parts of segments 4-6 which are conspicuously pale. They are slightly shorter than the length of the body with a terminal process that is shorter than the base of the last antennal segment. The siphunculi are pale, short and truncate. The body length of apterae is 2.0-3.5 mm. Alates of Symydobius oblongus (see second picture below) have brownish bordered wing veins and a dorsal pattern of broad dark transverse bands on each tergite.
The dorsal abdominal pattern is more clearly visible in preserved specimens (see picture below). Symydobius oblongus apterae and alatae are very similar in this respect.
Oviparae have the dorsal sclerotic pattern reduced on segments behind the siphunculi. The males are apterous and like the viviparae, but smaller and more slender. Symydobius oblongus nymphs do not have the dorsal pigmented bands, except across tergite 8.
The shiny birch aphid is found on twigs, young stems and branches of both the silver birch (Betula pendula) and the downy birch (Betula pubescens). It is found both in birch woodland and on heaths with birch scrub. Sexual forms (oviparae and wingless males) occur in October-November. Symydobius oblongus is found throughout Europe and across Asia.
Biology & Ecology:
The eggs of Symydobius oblongus are laid in autumn on the twigs and small branches of birch, usually close to an axial bud. The first picture below shows yellow newly laid eggs, whilst the second shows the black hardened-off eggs.
The overwintering eggs hatch in March or April and the nymphs then feed on the younger twigs. The first picture below shows newly emerged first instar nymphs feeding on silver birch, whilst the second picture shows second and third instar nymphs on downy birch.
The nymphs often aggregate in clusters, possibly for defensive purposes and/or to take advantage of the 'sink effect' of feeding in a group. The first picture below shows a group of third and alatiform fourth instar nymphs, whilst the second picture below shows a newly moulted apterous adult.
The picture below shows a group of strongly marked mature apterae in mid-summer.
In autumn sexual forms arise. Oviparae (see picture below with eggs) differ from viviparae in having the posterior end of the abdomen behind the siphunculi produced into a rather flexible egg-laying structure, this being somewhat telescopic and armed at the end with pale chitinous teeth.
Symydobius oblongus is generally considered to be an obligate myrmecophile - in other words the aphid does not occur without its ant mutualist. As with most aphid species, Symydobius does not restrict its mutualistic relationship with ants to one species of ant. So far we have found them being attended by the southern wood ant (Formica rufa, see picture below), the Scottish wood ant (Formica aquilonia), the jet black ant (Lasius fuliginosus) and the black garden ant (Lasius niger). The most frequent attendees seems to be Formica species like the southern wood ant shown below.
The ant in the picture above is about to take the honeydew droplet excreted by one of the small nymphs. Douglas & Sudd (1980) showed that Symydobius oblongus can coordinate its defaecation with the presence of its attendant Formica lugubris, so that 86% of droplets are produced in the 14% of time when the aphis is attended. This coordination is the result of two processes of communication. The commonest is the response of the aphis to the arrival of an attendant, probably mediated by contact of the ant's antennae and palps with the aphis. Less frequently, aphids call ants, perhaps signalling their readiness to defaecate by movements of the abdomen which the ants detect visually.
Formica species do seem to be especially attracted to the honeydew produced by young nymphs, despite the fact that the quantity produced must be less than that produced by adult aphids. They are also especially attracted early in the year when there are presumably fewer food sources. This can result in massive aggregations of wood ants on the branches of birch (see picture below) as they try to get to the honeydew.
Several studies have demonstrated the positive impact of attendance by Formica ants on the population size of Symydobius:
There have been fewer studies looking at the performance of Symydobius when they are attended by Lasius niger (see picture below). Stadler & Dixon (2001) considered why aphids exhibit different degrees of myrmecophily, ranging from non-attendance to obligate myrmecophily. They compared the thistle aphid (Aphis fabae cirsiiacanthoidis ) which is facultatively associated with its ant population to Symydobius oblongus which is an obligate myrmecophile. In each case the aphids were tended by Lasius niger.
The consequences for these aphids of their different degrees of associations with ants were determined, in terms of costs and benefits to individuals and colonies in laboratory and field experiments. In the laboratory, individuals of the thistle aphid performed worse and those of Symydobius oblongus performed better when attended by ants than when unattended. In addition, the ants regulated the population size of Symydobius to an average of 50 to 70 individuals per birch sapling by removing aphids, but did not regulate the population size of the thistle aphid. Under field conditions, ant-attended colonies of both aphid species achieved higher peak numbers and lasted longer.
Novgorodova (2005) examined ant-aphid interactions in multispecies ant communities. Two aphid species on birch (Symydobius oblongus and Stomaphis quercus ) were the main source of carbohydrate food for a colony of Lasius fuliginosus (see picture of this ant species below).
Neither of these species were seen to be attacked by Lasius fuliginosus, but the ant did have a high percentage (about 37%) of other myrmecophiles species of aphids (Cinara pinea, Cinara boerneri and Cinara laricis ) in their prey. This was apparently because of a decrease in the attractiveness of the Cinara aphids later in the season. Even so the ants only killed unattended or damaged aphids.
There is very little in the literature about which (if any) aphid species Symydobius tends to associate with. We have found Symydobius living in very close association with Glyphina betulae
Glyphina is generally found on the young green part of the shoot whilst Symydobius occurs on the woody stems.
Symydobius aphids may sometimes be found stationary with their hind legs stretched out behind and on each side. This may occur after leg waving behaviour of the type used by Drepanosiphum platanoidis in response to predators (see Doring et al. (2008) and Russel (1972) ) or may be indicating to ants that it is ready to produce honeydew.
Unlike most species of aphids it is rare to find parasitized Symydobius mummies, apparently because of efficient protection by the attending ants. The picture below shows an aphid containing a developing parasitoid larva (the lower paler aphid of the three).
Symydobius oblongus does have its own specific parasitoid, Trioxys betulae, but it is only found if the aphids are not tended by ants. In Germany Volkl (1997) found around 3 mummies per colony in absence of ants, and about 0.25 mummies per colony when ants were present.
Coccinellid predators preying on Symydobius seem to be as rare as the parasitoids, but we have found syrphid larvae (see first picture below) and cecidomyiid larvae (see second picture below) preying on Symydobius.
The species of Syrphidae and Cecidomyiidae that we found were apparently ignored by the attending ants (see picture below).
Populations of Symydobius are, of course, usually tended for their honeydew by (otherwise) predatory ants. There are very few records of myrmecophiles being themselves predated by ants except late in the season when the aphids seem to loose their attractiveness to ants (Novgorodova, 2005 ). The only occasion we have observed is when a Lasius fuliginosus (apparently) predated Symydobius oblongus very late in the season (27th October).
The picture above shows an ant with a moribund Symydobius nymph in its jaws. Freshly laid eggs of Symydobius can be seen above and below the side shoot. We have also come across a good example of intraguild predation with a common crab spider (Xysticus cristatus) preying on the wood ants that were attending Symydobius.
In Belgium Portha & Detrain (2004) studied the local population dynamics of four aphid species coexisting on Betula pendula for two years. Symydobius oblongus was the least common species in both years, though tended by Lasius niger throughout the season. In 2001 it peaked in August, but in 2002 it peaked in June and had died out by August. It was thought that the heavy rains at the beginning of July 2002 washed this exposed aphid from the twig.
Other aphids on same host:
Blackman & Eastop list about 72 species of aphids as feeding on birches worldwide, and provides formal identification keys for aphids on Betula.