Galhas induzidas por Eriosoma lanigerum Hausmann (Hemiptera: Aphididae) em Malus domestica Borkh (Rosaceae)

Abstract

The trees of Malus domestica (Rosaceae) 'Eva' cultivar on 'M9' rootstock are susceptible to high infestation by the galling aphid Eriosoma lanigerum (Hemiptera: Aphididae). This aphid is considered a global pest, which induces galls on roots and stems of the apple trees, where it has an external position regarding plant tissues. Such a position is uncommon in galling insects, which usually inhabit a larval chamber within plant cells and tissues. Although most aphids feed directly on phloem cells, the woolly-apple-aphid can also ingest xylem content. The E. lanigerum feeding activity induces an unusual cambial activity, causing an asymmetrical cecidogenetic field in the secondary xylem radial system. The closer the aphid colonies are, the higher are the alterations in xylem structure, where abnormal vascular elements differentiate and vascular parenchyma cells are overproduced. The histochemical profile of the roots and stems of M. domestica in gall developmental sites are also altered by the E. lanigerum galling activity and is mainly associated with the maintenance of gall metabolism with greater accumulation of starch and phenolic compounds. The cecidogenetic field is less evident in the secondary xylem axial system, for the vessel elements in the stem portions below and above the galls are similar to those of the non-galled stem portions, contradicting the “gall constriction” hypothesis. We assume that the damage of gall induction to the crops relates to the secondary xylem profile of E. lanigerum galls and its influence on the water conductivity in M. domestica. This influence prioritizes water supply and accumulation to the gall developmental site, due to the large abnormal vessel elements and increased differentiation of parenchyma cells. Such peculiar anatomical traits favor the water status to gall development and the woolly-aphid hydration.