Colonization and gall formation by the woolly apple aphid (Eriosoma lanigerum Hausm.) on selected apple rootstocks

Colonization and gall formation by the woolly apple aphid, Eriosoma lanigerum (Hausmann), was compared on susceptible seedling apple trees, Malus domestica Borkhausen, and resistant Malling-Merton 106 rootstocks in two greenhouse studies. In one study, seedling apple trees were planted in root observation cages in a commercial soil mix used by local wholesale nurseries. Trees were infested at four aerial densities: 0, 5, 10 and 20 first stage woolly apple aphid nymphs. There were no significant differences between treatments in time required for first aerial colony, first root colony, fusion of aerial colonies or in depth of root colonization. Root colonization occurred most frequently 15 cm below soil level. Root galls were formed on three trees four months after initial aerial infestation. In the second study, gall formation induced by the woolly apple aphid was compared on seedling and MM 106 rootstocks infested at three densities: 0, 2, and 20 first stage nymphs, in root cages. Root segments from the trees were removed monthly and prepared for microscopic examination. Gall formation occurred within three months after infestation on both rootstocks. No histological or morphological differences between gall tissues in the two rootstocks were observed.