1 Population trends of Hypsipyla grandella and their relationship with abiotic (humidity, precipitation and temperature) and biotic factors (availability of mahogany shoots and natural mortality agents), as well as damage patterns, were studied for 16 months in Turrialba, Costa Rica. 2 Hypsipyla grandella was active in the field all year round, and its population density was influenced by temperature, availability of new shoots and natural mortality agents. 3 It was possible to predict four population peaks, which appeared approximately every 1881 degree-days. 4 Even though H. grandella larvae mainly attack shoots, they also fed on the bark of the bole, especially when green shoots were scarce. 5 Four parasitoid species were recorded (Bracon c. chontalensis, Brachymeria c. conica, Apanteles sp. and Dolichogenidea sp.) but their impact on H. grandella abundance was negligible. Ecology; Habitat; Terrestrial habitat; Land zones; Neotropical region; Central America Hypsipyla grandella (Pyralidae); Population dynamics; Forest and woodland; Costa Rica; Turrialba; Population trends, mahogany stand no species none