Soil insects present a significant challenge in organic vegetable production. Cabbage, seed corn, and onion maggots, addressed in this proposal, can severely reduce crop stands of cole crops and onions, and diminish the vigor and health of established plants. In San Benito County, these insects are responsible for substantially reducing the yields of onion, cabbage, Napa cabbage, and bok choi in organic production. We lack comprehensive information on the biology of these insects in California, making it difficult to predict when they will be active and pose a threat to crops. For Brussels sprouts, cabbage maggots pose the greatest risk up to four weeks after transplanting (1); after this period, plants can tolerate up to 60 tunnels in the root system without yield reduction. However, in other crops like cabbage and Napa cabbage, maggots can enter the heads, rendering them unsalable, even if the overall tonnage is not affected. Current cultural recommendations for organic crop production include: “avoid soils with high organic matter” and “do not plant in areas with fresh organic matter.” These recommendations are difficult for organic growers to implement, given their reliance on elevated levels of organic matter in the soil to provide adequate nutrition to their crops.

This project aimed to provide basic information on the activity of adult cabbage, seed corn, and onion maggots (the maggot complex) and to chart their emergence over one season. Data collected from spring 1993 through spring 1994 indicate distinct periods of high and low adult maggot populations. May and August 1993 showed the lowest maggot trap counts during the year. However, the remainder of the year showed significant adult maggot activity. The greatest maggot larvae activity on cabbage roots was observed during the fall, winter, and spring. This study provided baseline observations of adult maggot activity and indicated very small windows when adult maggots were inactive.