Patterns of diapause frequency and emergence in swede midges of southern Ontario

The swede midge is an invasive pest of crucifers in North America and exhibits temporal plasticity in diapause; diapause frequencies change throughout the growing season and spring emergence is typically bimodal. Factors controlling the timing of swede midge diapause events are not well understood. Pre‐diapause larval swede midge populations were isolated within cages in the field and tracked for emergence over 3 years. Diapause frequency was inversely correlated with photoperiod and absolute maximum air temperature, however photoperiod did not influence emergence timing. Emergence from diapause occurred in two large peaks, in mid‐June and early July, with a third, smaller peak in late August. Emergence phenotypes may correspond to different diapause durations from 236 to 296 days, or post‐diapause development requiring between 516 and 1449 degree days. Approximately 2% of swede midges overwintered for 2 years. Early control efforts would be most effective if they targeted the diapausing cohort (i.e. adults emerging June to July) to prevent damage by subsequent generations. Although few individuals overwintered for 2 years, prolonged diapause should be considered when crop rotation is employed for swede midge management.