The field biology of subterranean Eriosoma lanigerum (Hausmann) was studied on two farms in the Elgin apple growing area from October 1995 to January 1999. There were two peaks of activity in subterranean E. lanigerum populations, one during early summer and one during autumn. Nitrogen levels in the roots also peaked at these times of the year. All developmental stages were recorded on the roots of apple trees throughout the year. Peak numbers of first instar aphids were recorded during spring. Embryos were present in all instars throughout the year. More embryos were recorded in fourth instar and adult aphids than in the other developmental stages. Peak numbers of embryos were also recorded in the final two developmental stages (fourth instar and adults) during spring. Nitrogen levels were higher in healthy roots and in roots adjacent to galls than in galls.