Monitoring of populations indicated that seed chalcids emerge a month earlier in southern Ontario than they do in the north. The later dates of emergence in northern Ontario were not surprising as degree-days accumulate less rapidly at higher latitudes than at lower latitudes. Populations of Bruchophagus platypterus increased progressively through the summer and reached a peak in about mid August in both southern and northern Ontario. The highest populations coincided with an abundance of trefoil seed pods in the field. Thus, the emergence of B. platypterus was synchronised with trefoil pod-set.
Trefoil is an indeterminate bloomer and flowers continuously for nearly seven weeks.
In southern Ontario, this pattern of flowering gave rise to a succession of pods on which one complete, and a partial second generation of B. platypterus developed on the trefoil growing on roadsides and in abandoned fields. The regrowth ensues soon after on which a generation of B. platypterus develops, although only a few of these become adults and emerge before fields are harvested for seed. They rest over winter and emerge the following spring. In northern Ontario, the short growing season precludes cutting of the first growth of trefoil for hay. However, the phenology of trefoil was similar to that recorded on regrowth trefoil in southern Ontario.