Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Variegated beech aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host Damage & Control
Identification & Distribution:
Apterae of Lachnus pallipes are shining dark reddish to blackish brown. The antennae are 0.4-0.5 times the body length. The abdominal dorsum is densely haired (cf. Lachnus roboris
which only has a few short hairs on the dorsum). The middle parts of the tibiae are pale (cf. Lachnus roboris which has the hind tibiae pale at the base, but progressively darker over most of their length). The siphuncular plates are relatively pale. The body length of the adult Lachnus pallipes aptera is 3.0-5.0 mm.
Variegated beech aphids are found on two-year-old or older branches and stems of beech (Fagus sylvatica). They may also feed in ant shelters on roots of beech in summer. Recent DNA evidence suggests that a very similar species Lachnus longirostris, which occurs on oak, is conspecific with Lachnus pallipes. The variegated beech aphid is found over most of Europe south to Bulgaria and east to Russia and (apparently) the Far East.
Biology & Ecology:
The overwintering eggs of Lachnus pallipes hatch in early summer and develop as relatively small but very dense colonies. The image below shows a mature colony in late summer. The aphids are nearly always attended by ants - usually wood ants (Formica species). Lachnus aphids have a number of structural and behavioural modifications to facilitate this relationship.
Firstly note the dense setae around the back end of the aphid. These form a basket (a 'trophobiotic organ') that holds the honeydew droplet until it is imbibed by ants (Holldobbler & Wilson, 1990 ). Another structural adaptation is the relatively short wings of the alate so they are well clear of the honeydew. Also all these aphids are holding their hind legs in the air.
Kloft (1960) suggested that that the leg lifting response in Lachnus simulates the appearance of a sister worker to an ant. The rear of the aphid's abdomen represents the head of a worker offering food, the siphunculi represent the opened mandibles, the cauda represents the ants' extruded labium and the waving of the legs imitates the antennal movements of the ant. This behaviour is thought to have evolved from the non-specific defensive kicking behaviour used by many aphid species.
These images show what happens once an ant arrives at a variegated beech aphid. The first image shows the ant 'antennating' the aphid - in other words stroking it with its antennae. Then the ant moves round and takes the developing droplet from the aphid's anus. Sometimes the ant will consume the honeydew directly but more usually it practices trophallaxis or sharing of food (Holldobbler & Wilson, 1990 ). The forager ant's crop has a tight constriction at the posterior end formed by the proventriculus. This segregates the personal supply of food - which will be allowed through to the midgut where it is digested - from the communal supply in the crop - which is regurgitated to its nestmates as shown in this image.
In this way the honeydew from the aphids is distributed throughout the ant colony. Such adaptations are most developed in advanced ant species like Formica, but other ant species also tend Lachnus pallipes. Quinet (1997) found that the Jet Black Ant (Lasius fuliginosus) established permanent trunk trails to colonies of this aphid when food sources were scarce.
As well as 'milking' the aphids for honeydew, ants actively defend the aphids against natural enemies. These images show Southern Wood Ants in full defensive posture. Probably as a result, it is rare to find any predators active within a colony.
The variegated beech aphid has a sexual stage in its life cycle, and males and oviparae develop in October. Egg laying takes place in autumn after mating.
Other aphids on same host:
Blackman & Eastop list 12 or 13 species of aphid as feeding on beeches (Fagus species) worldwide, and provide formal identification keys.
Damage and control
The aphid certainly causes feeding damage by rupture of the tree cambium, but since beech is seldom grown commercially (other than for ornamental purposes) it is unlikely that control of this rather uncommon aphid would ever be considered.
The relative scarcity of Lachnus pallipes (at least in the UK) probably results from beech being concentrated on limestone soils, whereas wood ants prefer more sandy areas. Only where beech are growing in suitable wood ant (or jet black ant) habitat is one likely to come across this aphid.