none Interactions among tomato planting date, population growth of the potato aphid (PA), Macrosiphum euphorbiae (Thomas), and natural enemies were investigated. Studies showed that PA were influenced directly by planting date, and significantly higher aphid densities developed on young plants. Older plants supported low numbers of PA. Plant age also influenced population growth of PA indirectly through the interaction between green peach aphids (GPA), Myzus persicae (Sulzer), and natural enemies. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Competition; Land and freshwater zones; Nearctic region; North America; USA Diaeretiella rapae; Ephedrus californicus (Braconidae ); Hemipteran hosts; Macrosiphum euphorbiae; Host population growth on plant; California; Orange County; Hemipteran host population growth, influencing factors Macrosiphum euphorbiae (Aphidoidea); Hymenopteran parasites; Diaeretiella rapae & Ephedrus californicus; Plant hosts; Lycopersicon esculentum (tomato); Planting date effect on abundance, predators & parasites; Population dynamics; Growth; Seasonal abundance; Predators; Arachnida & Insecta; Interspecific competition; Myzus persicae; Plant host planting date relations; California; Orange County; Population growth, effect of plant host planting date, related factors