Reporting
Crop rotation is the first

Crop rotation is the first line of defense to avoid contaminated soil this year. Eliminating weeds will aid in the management of leafminers. Row covers can also be used to exclude flies if placed over the crop before flies are active. Begin scouting susceptible crops in mid-May.  Scout undersides of leaves for eggs and treat when they are first observed in order to target larvae as they hatch. 
During most years, you may need sprays to prevent injury. Apply insecticides according to label directions. There are effective treatments available for both conventional and organic growers, however, regulations differ by state so we suggest you speak to your cooperative extension county office: http://extension.psu.edu/counties