Where colonies of E. lanigerum feed on twigs and roots the tree forms galls, which are small initially but increase in size over time and are most damaging when they are formed among the roots. Continued feeding in the roots can destroy the roots and consequently reduce the growth of the tree or kill it. The aphids are attracted to the sunken sites caused by the perennial canker fungus Cryptosporiopsis perennans and the galls are then the most frequent places on the tree for the fungus to re-infect it. The galls can also allow infection by Gloeosporium sp. The galls are more sensitive to cold weather than the surrounding tissue and burst at -18 °C(0 °F), the resulting damage providing the fungus with an entry site for reinfection. As the aphids feed they produce a honeydew which drips onto the fruit allowing the growth of sooty mould which has an impact on the saleability of the fruit. The honeydew can also create unpleasant conditions for orchard workers to work in due to high levels of sticky honeydew excreted. The honeydew can also cover the leaves allowing the mould to grow there too. The aphids can also infest the apples, especially on varieties with an open calyx when the aphis can feed on the apple core.
