none Spring populations of Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) on barley, at Lincoln and elsewhere in Canterbury, New Zealand, declined markedly during 1984-87. Spring suction-trap catches of M. dirhodum also declined at the same time. Numbers of immigrant M. dirhodum on young barley, and maximum aphid population density on the crop, were positively correlated with suction-trap catches of M. dirhodum during the preceding week or month, respectively. It is suggested that aphid population regulation by predators and parasitoids in the wider habitat of M. dirhodum could reduce the rate of immigration, and hence aphid population growth, in a given crop. Nutrition; Diet; Feeding; Carnivorous feeding; Predation; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Diseases and disorders; Diseases; Microbial diseases; Ecology; Population dynamics; Habitat; Terrestrial habitat; Man made habitat; Land and freshwater zones; Australasian region Aphidius ervi (Braconidae); Hemipteran hosts; Acyrthosiphon (Metopolophium) dirhodum, host population growth relationship; Prevalence; Population growth of Acyrthosiphon (Hemiptera) host related infection; Plant and vegetation habitats; Hordeum vulgare (barley); Cultivated land habitat; Hemipteran host population growth related prevalence; New Zealand; Canterbury; Hemipteran host population growth related parasitism, crop Acyrthosiphon (Metopolophium) dirhodum (Aphidoidea); Hymenopteran parasites; Aphidius ervi; Plant hosts; Hordeum vulgare (barley); Prevalence; Population growth related fungal & Aphidius (Hymenoptera) infection; Fungal diseases; Conidiobolus; Entomophthora & Erynia; Population growth related prevalence; Population changes; Predation, disease & parasitism relationships; Seasonal abundance; Crops; Predators; Insecta; Population growth related predation; Cultivated land habitat; New Zealand; Canterbury; Seasonal abundance & natural enemy populations, crop Acyrthosiphon (Metopolophium) vulgare (Aphidoidea); Predator prey interactions; Population growth related predation by Micromus tasmaniae (Neuroptera) Micromus tasmaniae (Neuroptera); Prey; Acyrthosiphon (Metopolophium) dirhodum (Hemiptera); Prey population growth related predation; Predator prey interactions; Population growth of Acyrthosiphon (Hemiptera) related predation; Plant and vegetation habitats; Hordeum vulgare (barley); Cultivated land habitat; New Zealand; Canterbury; Hemipteran prey population growth related predation, crop