Asynchrony between host plant susceptibility and peak flight activity of the swede midge, Contarinia nasturtii, may reduce feeding damage in organic cole crop production

The swede midge, Contarinia nasturtii, is an invasive pest insect, attacking the developing meristem of cruciferous vegetables such as broccoli, cabbage and cauliflower.  Host plants are most susceptible to larval feeding damage prior to the formation of floral buds.  With 4-5 overlapping generations per year in southern Ontario, swede midge are active in the field as adults and larvae throughout the growing season.  Flight activity of the adult population is characterized by a series of distinct peaks in May and June, followed by lower numbers of active individuals in July, August and September.  Avoiding peak flight activity by manipulating planting dates minimizes the overlap between host plant susceptibility and high populations of adult swede midge, potentially reducing feeding damage to the developing meristem.  Field trials conducted in 2012 and 2013 at Elora, ON, compared swede midge larval feeding damage to cabbage and broccoli based on 6 different planting dates (May 3, May 24, June 15, July 6, July 27 and August 15), with 2013 trials including fast-developing (Bay Meadows) and slow-developing (Windsor) broccoli cultivars as an additional factor.  Results suggest that swede midge damage is lower for early cabbage planting dates and late broccoli planting dates.  Lower damage levels typically exceed acceptable levels for marketability purposes, underlining the need for a combination of management tactics for effective swede midge control.