The balsam gall midge Paradiplosis tumifex Gagné (Diptera: Cecidomyiidae) is a major pest for the Christmas tree industry. This galler is frequently associated with the inquiline Dasineura balsamicola (Lintner) (Diptera: Cecidomyiidae), which is involved in the dynamics of the galler. Despite their importance, seasonal ecology of both midges under the climatic conditions prevailing in eastern Canada is still poorly understood. More importantly, nothing has yet been done to fully assess the impact of temperature on these insects, at key events such as adult emergence and larval overwintering. Here we followed P. tumifex and D. balsamicola spring phenology in the field, as well as their survival during winter diapause under simulated climatic scenarios in the laboratory. We observed spring asynchrony between fir host trees and P. tumifex in the first year of study, but under prevailing epidemic conditions, we observed no impact on summer abundance. We clarified available knowledge on their ecology, showing that overwintering habitats and strategies differ between the galler and its inquiline, which should alter pest control strategies. Experimental overwintering data suggest that diapausing conditions affect these species differentially and could potentially impact the spring sex ratio of their midges, which tends to be strongly female biased.