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Small downy birch aphidIdentification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution:Betulaphis quadrituberculata apterae are pale yellowish green, to pale yellow, to almost white. In autumn they may have patches of darker pigment. The antennae are shorter than the body (cf. Calaphis flava which is often found on the same leaves, and has antennae longer than the body). All dorsal body hairs of the apterae are usually long and capitate, although there are forms with hairs on abdominal tergites 1-4 much shorter (cf. Calaphis flava which has short, and inconspicuous, spinal and pleural hairs). The siphunculi are smooth truncate conical with a strongly flared apical rim. The cauda is broadly rounded and subtriangular, projecting slightly beyond the deeply cleft subanal plate. The body length of Betulaphis quadrituberculata apterae is 1.5-2.0 mm.
Alate Betulaphis quadrituberculata are broadly similar to the apterae, but the dorsal hairs are nearly all fine and acute, not capitate unless on abdominal tergite 8. Antennal segment III has a single row of 8-21 transverse, oval, distinctly fringed secondary rhinaria. Males are dark and apterous with rather long antennae. Oviparae have a pigmented dorsal sclerotic pattern, and have the posterior abdominal segments produced into an elongate ovipositor-like structure.
They are mainly found on the undersides of leaves of downy birch (Betula pubescens), but they also occur on silver birch (Betula pendula) and occasionally on grey alder (Alnus incana). Betulaphis quadrituberculata is widely distributed across Europe through Asia to China, and has been introduced to North America.
Biology & Ecology:
Betulaphis quadrituberculata overwinters in the egg stage on birch trees. The eggs hatch in spring and the young nymphs feed on the leaf undersides, developing into either winged or wingless fundatrices (Stroyan, 1977 ). Large numbers of mature alate fundatrices can be found on the tips of birch leaves in mid-May (see picture below).
In succeeding generations nearly all individuals are apterous and rather sedentary in habit. These aphids tend to be dispersed singly or in small groups, although they can reach high densities if conditions are suitable. Varty (1964) found a similar situation for Betulaphis quadrituberculata in New Brunswick, Canada, and concluded that Betulaphis quadrituberculata mainly disperses by the flight activity of fundatrices in spring.
Durak & Wojciechowski (2008) found that Betulaphis quadrituberculata was eudominant with Calaphis betulicola in mixed coniferous forest in June. Generally one peak of occurrence was noted in the middle of June. However, Betulaphis quadrituberculata continued to feed on the mature leaves of birch throughout the growing season, a relatively unusual practice for aphid species which often aestivate, or change host, as the condition of the host plant changes from young to mature and then to senescent leaves. The colour changes of the aphids reflects those changes.
By late summer they usually have darker green patches on a yellowish green background (see picture below).
In autumn they develop dark brown patches on a yellowish background (see picture below).
These seasonal colour changes appear to mimic changes in leaf colour over the course of the year, maximizing the effects of crypsis.
The egg hatches to a larva which passes through several instars as it consumes the aphid, before it pupates inside the remains of the aphid (known as an aphid 'mummy'). Some of the aphid mummies are shown in the picture below.
A single adult parasitoid emerges from each mummy several days later.
Stary & Havelka (2008) record parasitism of the related species Betulaphis brevipilosa by the braconid Aphidius aquilus in the Czech Republic. Baker (pers. comm.) also records Aphidius aquilus parasitizing Betulaphis quadrituberculata in Wales, so it seems likely that this is the parasitoid shown above.
Another important natural enemy of Betulaphis quadrituberculata is the midge larva Aphidoletes (see picture below).
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. Of those, only 18 species occur on silver birch (Betula pendula) and/or downy birch (Betula pubescens) in Europe.