Woolly apple aphid (WAA), Eriosoma lanigerum, is an important pest of apples, occurring throughout the apple-growing regions of the world. Entomopathogenic nematodes (EPNs) of the families Steinernematidae and Heterorhabditidae have generated intensive interest as inundative biocontrol agents of pest insects. Considering resistance of WAA against certain insecticides and the inability of parasitoids to control the aphid successfully early in the season, EPNs offer an alternative biocontrol option. The susceptibility of WAA to seven indigenous EPN species was tested in laboratory bioassays in which Steinernema yirgalemense and Heterorhabditis zealandica were identified as being the most effective against the subterranean adult females of the WAA, with infection rates of 39%and 28 %, respectively. The effect of E. lanigerum size showed that the last instar is most susceptible to infection, whereas none of the crawlers were infected. Increasing the exposure time of the aphids and nematode concentration had no positive effect on the WAA infection rate. On evaluating the ability of infective juveniles of S. yirgalemense and H. zealandica to tolerate exposure to imidacloprid, both nematode species were found to be compatible at the recommended dosage, with no significant nematode mortality having occurred. Neither did imidacloprid affect the ability of the nematodes to infect their hosts after 24 h exposure. The associated bacteria, isolated from 12 EPN isolates, showed a visual inhibitory growth effect, when they were exposed to the haemolymph of WAA. From these results, it is concluded that, although S. yirgalemense showed some infection of WAA, its low mortality and inability to complete its life cycle within the host, limits its effectiveness as a biological control agent against the subterranean stages of WAA in apple orchards.