Field studies were carried out to assess diel activity patterns of the balsam gall midge, Paradiplosis tumifex Gagné (Diptera: Cecidomyiidae) in 2012, and its inquiline, Dasineura balsamicola (Lintner) (Diptera: Cecidomyiidae) in 2013, in a young balsam fir (Abies balsamea (Linnaeus) Miller) stand in New Brunswick, Canada. Both gallmaker and inquiline are most active during the afternoon/evening hours (17:00–22:00 hours). Male gallmaker activity was largely confined to the space below the crown and typically involved short periods of flight of <30 seconds. Calling and mating by the gallmaker occurred at ground level and were followed by dispersal of females to the vegetative crown. Female gallmakers were typically observed in the tree crown beginning in early afternoon, with peak oviposition occurring between 20:00 and 21:00 hours. Female inquilines displayed similar activity patterns, although no calling or mating were observed. Moreover, inquiline flight and foraging for oviposition sites were more active than the gallmaker, with shorter rest periods and more buds visited than the gallmaker. Our results indicate that population monitoring should focus on female gallmakers as they fly during the evening. Also, before any treatment application, care should be taken to accurately identify the insects to ensure that the inquiline is not inadvertently killed.