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Variegated beech aphidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:Apterae of Lachnus pallipes are shining dark reddish to blackish brown except for siphuncular plates which are relatively pale. Body length is 3.0 - 5.0 mm. The abdominal dorsum is densely haired. The middle parts of the tibiae are pale and the antennae are 0.4 - 0.5 times the body length. Lachnus pallipes alatae have a pattern of forewing pigmentation similar to Lachnus roboris, but with a more extensive clear area between Rs and media.
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. A very similar species Lachnus longirostris occurs on oak (it is considered by some to be conspecific with Lachnus pallipes). Lachnus pallipes is found over most of Europe south to Bulgaria and east to Russia and (apparently) the Far East.
The first image shows the shining blackish-brown aptera. Note especially the hairy dorsum, the pale siphuncular plates and the pale middle parts of the tibiae. All of these characteristics can be used to distinguish both apterae and alates of Lachnus longirostris (= Lachnus pallipes ?) from Lachnus roboris on oak. In addition the wing patterning of alates is slightly different.
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 spp.). Lachnus aphids have a number of structural and behavioural modifications to facilitate this relationship.
Firstly note the dense setae (hairs) 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 end of the aphid. 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.
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.