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Myzocallis walshiiBlack-bordered oak aphidOn this page: Identification & Distribution Biology & Ecology Life cycle Natural enemies Other aphids on the same host Damage & ControlIdentification & DistributionAll adult viviparae are alate. Immature Myzocallis walshii (see first picture below) bear numerous long capitate hairs, and lack pigmentation on the nymphal head, wing pads and femora (cf. Myzocallis exultans, whose immatures have some pigmentation). Myzocallis walshii immatures generally bear little or no pigment on their dorsum, but nymphs from some locations have prominent dorsal, lateral, and sublateral abdominal patches. Adult alate Myzocallis walshii (see second picture below) have the costal margin of the forewing usually with a continuous band of pigment extending beyond the pterostigma to the tip of the wing (it may be reduced to dusky patches at the tips of wing veins, Rs and the apical branch of medial vein). The thorax is rather pale except that the lateral margins of the pronotum have conspicuous dark longitudinal stripes, continuing on the mesonotum to the base of the forewing. The antennal terminal process is 1.7 to 2.9 times the length of the base of antennal segment VI (cf. Myzocallis longiunguis, whose terminal process is more than three times the length of the base of antennal VI; and Myzocallis multisetis, whose terminal process is usually less than l.5 times the length of the base of antennal VI). The apical rostral segment (RIV+V) is 0.62-0.92 times the length of the second hind tarsal segment (HT II).
Both images above by permission, copyright Claude Pilon, all rights reserved. The alatae of Myzocallis walshii generally lack pigment on their femora (cf. Myzocallis exultans, whose alates have pigmented femora). They bear two setae on each side of the prothorax (cf. Myzocallis longiunguis, which bears 1 seta on each side of the prothorax; and cf. Myzocallis multisetis, which bears three or more setae on each side of the prothorax). The marginal tubercles on abdominal tergites II-IV are generally unpigmented except in some late season alatae. The fore tibiae of alate Myzocallis walshii are much darker than their middle and hind tibiae, and the tibiae bear darker pigmentation at the tips than proximally (cf. Myzocallis granovskyi, which has all its tibiae black, and lives on the upper leaf surface). The siphunculi are short and smooth without an apical flange, and the cauda is knobbed. The body length of adult Myzocallis walshii alatae is 1.6-2.0 mm.
Image above by permission, copyright Claude Pilon, all rights reserved. Myzocallis walshii lives on the leaf underside (see picture above) of various species in the red oak group (Quercus section Lobatae). It does not host alternate. Sexual forms develop in autumn, and the species overwinters as eggs. Several authors (e.g. Havelka & Stary, 2007) have commented on the large quantity of honeydew produced, which soon contaminates the upper surfaces of leaves below the aphid colonies. Despite this, these aphids are not ant attended. Myzocallis walshii is common throughout eastern North America and was introduced to the west coast, where it has been found on the native live oak (Quercus agrifolia) and on planted red oak in Oregon, Washington and British Columbia. In 1989 it was found on Quercus rubra planted in France, and has since spread to most of Europe. In Britain, Baker (2009) recorded it on Quercus rubra in Derbyshire, Cardiff and Barry. Biology & EcologyLife cycleThe life cycle of Myzocallis walshii in Europe was described by Havelka & Stary (2007). Eggs laid on oak hatch in spring and give rise to the first of several parthenogenetic generations in which alate viviparous females give rise to nymphs that develop to more alate viviparae. There are obvious differences in the aphid morphs between the seasons. A light-coloured spring form occurs early in the season, when the colonies are large and often cover the whole lower surface of leaves. Later in the season, the dark late summer form occurred (from about August) at the same sites and on the same trees, but the colonies are less dense, and less numerous on the older leaves. In late autumn the sexual generation begins with the production of apterous oviparous females and alate males. When mature and mated, these oviparae lay from 4-6 eggs per female in cracks and crevices among the bark, shortly before the leaves begin to fall. Natural enemiesIn Europe native parasitoids have adapted to the alien species. On the Iberian Peninsula two braconids (Trioxys pallidus and Trioxys tenuicaudus) have been reared from Myzocallis walshii (Pons et al., 2006). These and two others braconids (Praon flavinode and Trioxys curvicaudus) have been recorded as Myzocallis walshii parasitoids (Havelka & Stary, 2007). Modic (2010) found the predatory mirid bug Deraeocoris lutescens apparently predating Myzocallis walshii in Slovenia. This mirid is a polyphagous predator widely distributed in Europe and commonly found on oak, hazel, pear and apple trees where it feeds on aphids, small caterpillars, mite and insect eggs. Other aphids on the same hostMyzocallis walshii occurs on 8 species of Quercus (Quercus agrifolia, Quercus alba, Quercus bicolor, Quercus coccinea, Quercus imbricaria, Quercus palustris, Quercus rubra, Quercus velutina). Apart from Quercus alba and Quercus bicolor, these are all in the 'red oak' group (Section Lobatae).
Damage and controlMyzocallis walshii is native to eastern North America, but has proved to be highly invasive in Europe since it was first introduced. Its main host plant is the American red oak (Quercus rubra), a species used as an ornamental tree in urban green areas. In Spain Pons & Lumbierres (2010) describe how large numbers of Myzocallis walshii on American red oak produce a great amount of honeydew causing aesthetic and 'comfort' damage. They found a positive correlation between aphid abundance and damage levels, even when the aphid density was low. Natural enemies, mostly coccinellids and parasitoids, were present, although their abundance was relatively low.
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