Abstract:In southern Norway, the grassFestuca pratensissupported the emerging overwintered population of the agromyzidChromatomyia fusculauntil the appearance of the primary host plant, spring barley (Hordeum vulgare). Oats (Avena sativa), rye (Secale cereale) and the grass species,Dactylis glomerataandAlopecurus paratensiswere alternative secondary hosts.C. fusculafemales preferred the upper surface of the two youngest barley leaves for feeding and oviposition. This behaviour is advantageous for the farmers when applying an insecticide spray because the distribution of a contact insecticide on the upper two leaves is the easiest possibility for plant protection practice. The earliest (1st and 2nd) leaves of barley died quickly due to unusually warm and dry weather conditions. Consequently, larval survival on these leaves was low. Leaf numbers 3–5 supported the greatest population of successful larvae. Overwintered caged, gravid females ofC. fusculamade an average of 3–4 punctures and laid one egg per h