Incidence of cabbage maggot (Diptera: Anthomyiidae) infestation and plant damage in seeded Brassica fields in California's central coast

•Feeding injury from Delia radicum was less during first 14-d after plant emergence.•Number of D. radicum eggs increased starting at 3-wks after plant emergence.•Thus, plants are vulnerable to D. radicum attack from 2-wks after emergence.

The temporal incidence of cabbage maggot, Delia radicum L. was investigated using three exclusion cage experiments, one each during spring, summer and fall, in broccoli fields as well as two surveys, one each in three broccoli fields, and sixteen turnip plantings in central coast of California. In the cage experiments, sets of broccoli plants were exposed to natural populations of D. radicum flies for ∼14-d periods after plant emergence throughout the growing season. For the surveys, soil, root samples, and yellow sticky traps were collected every week from broccoli fields to determine number of eggs, maggots, feeding-injury and adults. Only roots were sampled from turnip plantings to determine feeding-injury. In all three cage experiments, feeding injury from D. radicum maggot was less during the first 14-d than ∼15–28 d after plant emergence (DAE). In the summer and fall, feeding injury by D. radicum was less during 29–42 and 43–56 DAE than it was 15–28 DAE. In the survey of broccoli fields, a greater number of D. radicum eggs were detected starting the fourth week after planting (WAP). Similarly, an increase in number of D. radicum maggots and feeding injury was observed at fifth and sixth WAP, respectively. However, adults were abundant throughout the growing period. In the turnip survey, increase in injury from D. radicum feeding did not appear until the fifth WAP. Overall, these studies indicate that increased incidence of D. radicum was delayed by about two to three weeks after plant emergence. The implications of these results for timing of insecticide application for D. radicum in the central coast of California are discussed.