Abstract: There are few examples of naturally controlled pest populations in glasshouses. Evaluating the occurrence of these systems may provide information about conditions that can promote natural enemy densities helpful to glasshouse growers. We evaluated the incidence of apparently naturally controlled populations of the algal-feeding shore fly Scatella tenuicosta Collin (Diptera: Ephydridae) over 8weeks, in eight glasshouses in England. Across glasshouses, shore fly numbers were negatively correlated with the numbers of its parasitoid Aphaereta debilitata Morley (Hymenoptera: Braconidae), suggesting that A. debilitata may control shore flies naturally. Counterintuitively, the amount of algae present was negatively correlated with shore fly numbers, but positively correlated with numbers of the parasitoid. We therefore suggest that if growers want to encourage natural control of shore fly through A. debilitata then, contrary to conventional wisdom, algal growth should be encouraged within a glasshouse. This conservation approach to biological control could then be part of an integrated pest management programme for shore flies. This interrelationship between the pest, its food, and its natural enemy also highlights the importance of considering all trophic levels within a biological control system, even where the pest is not a direct pest of the crop. [ABSTRACT FROM AUTHOR]