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Variegated oak aphidOn this page: Identification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:Apterae are shining blackish brown with a body length of 2.5-5.5 mm. The antennae are quite short - for apterae 0.4-0.5 times the body length. There are two conical tubercles on the front of the mesosternum. The siphuncular cones are large and dark. The dorsum has only a few short hairs. Alates have the forewing membrane pigmented except for four clear patches (hence the name 'variegated').
Variegated oak aphids are found on twigs and small branches of oak (Quercus sp.) and sometimes sweet chestnut (Castanea sativa). The 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 first image shows the shiny brown apterous vivipara with several nymphs. The second image shows an alate female Lachnus roboris feeding on a young branch. Later in the year aphids may also colonize the cups of the acorns.
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:
Lachnus roboris is nearly always attended by ants usually by wood ants (Formica rufa). 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 ants against predators.
The aphids also have another defense strategy against predators. - they kick out with the hind legs.
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 shown in the second image, which darken over time. Lubiarz (2008) recorded that over 500 eggs could be laid on one twig, with 19-71 eggs per 1cm of shoot.
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.