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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. |
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