Iridomyrmex humilis (Mayr) and some other ants, through preference, efficient foraging and recruitment, quickly removed artificially exposed batches of insect eggs including those of the serious pest of eucalyptus, Phoracantha semipunctata (Fabricius). This supports evidence of the importance of some ants as predators of the many insects that lay freely exposed eggs. However, P. semipunctata normally lays eggs in peeling bark crevices where the approximately 60% laid within gaps less than ca. 0.65 mm wide could not be reached even by small ants such as I. humilis and Pheidole pallidula (Nyland-er). In tree trunks, which can become heavily infested after felling, egg predation by ants seems unimportant compared with the later large mortality of competing larvae, so, egg predation cannot account for the observed striking positive correlation between presence of I. humilis and lack of Phoracantha semipunctata damage to standing trees. Egg predation and other effects of I. humilis may be more important on living trees which are attacked initially by relatively few P. semipunctata. This needs more detailed investigation as do the overall egg predatory roles of ant species in biological control.