Dispersal of the Onion Fly (Diptera: Anthomyiidae) and Larval Damage in Rotated Onion Fields

Onion flies, Delia antique (Meigen), trapped in water pan traps in nonhost vegetation throughout a 740-ha muck, were captured in large numbers up to 1.5 km from overwintering sites in May and June during the first flight. Few flies were captured in nonhost areas during the second flight in July. Similar, high levels of onion maggot (OM) damage were observed in 10 small onion plots planted in organic soils in the 740-ha muck at locations varying from 0.4 to 1.5 km distant from OM overwintering sites. Damage in untreated plots in commercial fields not planted to onions the previous year was related to distance from overwintering sites (fields planted to onions the previous year), and declined with increasing distance. Lowest damage levels, comparable to damage measured in chlorpyrifos-treated research plots, were observed in fields located in small, isolated mucks that had been planted to another crop the previous growing season.