Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The unceasing wet weather we have had this summer has made some fall planted cole crop fields vulnerable to cabbage maggots, Delia radicum (CM). The flies that are attacking cole and radish fields now are most probably 4th generation CM. We usually see some damage from this late generation of flies in our area, but it is usually scattered around. This year it is much more pervasive throughout the area. This widespread damage is due to how wet it has been, which has a dramatic cooling effect on the soil, CM larvae are better able to survive in the wet, cooler soils vs the drier hotter soils that we normally have in the summer and early fall. Adult flies are most active from 10 am to 2 pm and are inactive at night, in strong winds and when temperatures are below 50°F or above 80°F. Female cabbage maggot flies seek out and lay eggs on the lower portions of stems of young host seedlings or in nearby cracks in the soil. Within a few days the eggs hatch and the tiny maggots burrow down to the roots or bulb (radish and turnip) and begin feeding. The maggots usually feed for 2 to 3 weeks before pupating in the soil. Most of these pupae will overwinter in the soil, so it is important to be sure to rotate any cole crops out of the fall planted area for spring planting. A Diazinon, Lorsban or Verimark soil application pre- or at-planting will help reduce CM problems. Using row-cover over the newly planted seed would also control CM. Once damage is found in radish or turnip bulbs there is no rescue treatment.
Figure 1. Turnip with feeding damage