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Identification & Distribution:Lachnus roboris apterae are shining blackish brown The antennae are quite short - for apterae 0.4-0.5 times the body length. The dorsum has only a few short hairs (cf. Lachnus longirostris & Lachnus pallipes which have the abdominal dorsum densely haired with long, fine-pointed hairs). There are two conical tubercles on the front of the mesosternum. The siphuncular cones are large and dark, with their maximum diameter exceeding the length of the second hind tarsal segment (cf. Lachnus pallipes & Lachnus longirostris which have smaller paler siphuncular cones with their maximum diameter less than the length of the second hind tarsal segment). The body length of an adult Lachnus roboris aptera is 2.5-5.5 mm.
Lachnus roboris alates have the forewing membrane pigmented except for four clear patches (hence the common name 'variegated' oak aphid). The clear area between Rs and media only extends half way to the media (cf. Lachnus longorostris where the the clear area between Rs and media extends to almost meet the media.)
Variegated oak aphids are found on twigs and small branches of oak (Quercus sp.) and sometimes sweet chestnut (Castanea sativa). Lachnus roboris do not host alternate. Apterous oviparae and large alate males occur in September-October and produce eggs which overwinter. They are present in Europe, the Mediterranean region and parts of the Middle East.
The taxonomy of Lachnus roboris is rather uncertain and Blackman & Eastop (1994) suggest that it is a complex of species with different karyotypes and host plants. In which case the species found on Castanea may be a different species. However, Dixon & Thieme (2007) record Lachnus roboris as feeding on either Quercus or Castanea. We have only found it on sweet chestnut (see picture below) when it is also present on nearby oak trees, which suggests chestnut may function in some way as an overflow host when numbers on oak reach very high levels.
Biology & Ecology:
The overwintering eggs of Lachnus roboris start hatching as early as March, long before bud break, with the young nymphs feeding on small twigs.
Populations then build up over spring and summmer reaching their peak in late summer and autumn.
Sexual forms are produced in October - the image below shows a winged male Lachnus roboris about to copulate with an oviparous female.
The fertilized female then lays the reddish brown eggs in dense aggregations, as shown below, which darken over time. Lubiarz (2008) recorded that over 500 eggs could be laid on one twig, with 19-71 eggs per 1 cm of shoot.
Lachnus roboris is nearly always attended by ants, usually by wood ants (Formica spp.). The ants consume the large amounts of honeydew produced by this species and in return (at least in the case of wood ants) will actively defend the aphids against predators.
Lachnus roboris also have another defense against predators - they kick out with the hind legs.
Blazhievskaya (1980) reports that the eggs are subject to attack by predators, particularly the coccinellid Adalia bipunctata. Out of 6281 eggs examined in autumn, 25% were destroyed by birds and coccinellids and 19% by fungal infections, while 56% appeared normal. However, only 30% actually hatched. We have also noted high rates of mortality of overwintering eggs.