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Common oak thelaxidOn this page: Identification & Distribution Biology & Ecology Life cycle Population dynamics Ant attendance Other aphids on the same host Damage & Control
Identification & Distribution:
The wingless viviparae of Thelaxes dryophila (see first picture below) are oval, rather flattened, dark brownish-red to purplish grey with a paler spinal stripe. Their antennae, legs, siphunculi amd cauda are brownish. The antennae are 5-segmented and are slightly less than half as long as the body. Their terminal process is less than half the length of the base of the last antennal segment. Hairs on abdominal tergite 5 are spine-like (cf. Thelaxes suberi which has those hairs very thick and dagger-like, and occurs on oak species other than Quercus robur).
Winged females (see second picture above) have a black head and thorax with the antennae, legs, cauda and areas around the siphunculi dark. The abdomen has dorsal cross bands on the rear segments and dark marginal plates. Unlike most aphids, the wings of Thelaxes dryophila are folded horizontally, rather than tent-like, over the abdomen. The micrographs below show dorsal and ventral views of a Thelaxes dryophila aptera.
The common oak thelaxid does not host alternate, but remains all year on oak (Quercus spp.). Colonies at the tips of the shoots spread on to stems, leaf petioles and along mid ribs on the undersides of the leaves. They are also found on the developing acorns. Immature sexual forms are produced in early summer, but then aestivate until autumn when mating takes place, and eggs are laid. Thelaxes dryophila occurs in Europe, the Mediterranean region and south-west Asia.
Biology & Ecology:
Oak tends to come into leaf rather late in spring compared to many other trees. This problem is often circumvented by the fundatrix of Thelaxes dryophila feeding before budbreak just below a developing bud, beside the leaf-abscission scar.
The site is presumably chosen to maximise the flow of nutrients, but it is does expose the fundatrix to the depredations of predators. However, the fundatrix is often attended (and protected) by ants attracted by the flow of honeydew (see picture above).
The fundatrix may also be cryptically coloured (see picture above).
After bud break, the dark brown mature fundatrix moves to the expanding leaves and starts producing larvae (see picture below).
The younger larvae (see first picture above) are green with pale yellow longitudinal lines along the mid-line and laterally. By their fourth instar the larvae usually take on a pinkish hue (see pictures above and below) before moulting to a dark brownish-red to purplish-grey adult. The colony size then increases rapidly.
Alate sexuparae are produced as early as late May, and start to produce young males and females in June. These sexuales do not, however, develop, but go into aestivation for the summer. They can be found in close contact with the midrib, between the forks of veins or wherever shelter is available (see picture below of a group of young sexuales deposited by an alate).
The dorsal surface of sexuales is coated with wax and they are surrounded by a radial arrangement of long wax filaments (faintly visible in the picture above). They are not easily disturbed from their aestivation site (Polaszek, 1986). Thelaxes is the only aphid genus in which immature sexuales are known to aestivate.
It is not unusual to find alate females covering their offspring with their wings, offering a degree of protection from predators. Whether they only cover their own offspring, or indulge in crèching behaviour (covering the progeny of other females) is not known. Since females in the same colony are genetically identical, or nearly so, this seems very likely.
Lubiarz (2008) found that Thelaxes dryophila was the most numerous aphid on oak. It settled on the underside of leaves, leaf petioles, fruit peduncles and developing acorns (see picture below).
On leaves they were concentrated along the main veins (see picture below).
They were often attended by ants which protected the aphids from their natural enemies. Numbers peaked in June, but then declined and aphids had disappeared by late September.
Lubiarz (2009) looked at the community structure of phytophagous hemipterous insects on oak in natural and degraded landscapes in Lublin, Poland. Amongst the Aphidoidea and Phylloxeroidea, Thelaxes dryophila was a clear superdominant (comprising over 30% of the number of individuals). Even if all phytophagous hemipterous insects were considered, Thelaxes dryophila was superdominant or eudominant (comprising 20-30% of the number of individuals) in most of the sites examined.
Thelaxes dryophila is classed as a facultative rather than obligatory myrmecophile by Kindlmann et al. (2007). We have found colonies to be nearly always attended by ants, most often by the southern wood ant (Formica rufa, see picture below).
Thelaxes dryophila also appears to be quite fiercely protected against potential threats by Formica rufa (see picture below).
Other aphids on same host:
Thelaxes dryophila has been recorded from 15 Quercus species.
Blackman & Eastop list about 225 species of aphids as feeding on oaks worldwide, and provides formal identification keys for aphids on Quercus.