SmallAgromyza frontellafemales (800 µg) females. Experiments testing the efficacy of aqueous abdomen extracts from large females with different histories of feeding activity and from females of different sizes, together with comparisons of both feeding and marking behavior of large and small females, provided strong evidence that this difference is due to the quantity of pheromone deposited because: (i) ODP production byA. frontellais directly related to feeding activity, (ii) small females feed and mark significantly less than large ones, and (iii) the acceptance or rejection of marked leaflets follows a dose-dependent response. The possible ecological implications of such differences in pheromone production are discussed.