Can flowering Barbarea spp. (Brassicaceae) be used simultaneously as a trap crop and in conservation biological control?

Barbarea vulgaris R. Br. and B. verna (Mill.) Asch (Brassicaceae) have been proposed as trap crops for the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae). In bloom, these plants could also be used to attract beneficial insects. In this study, we tested the attractiveness of flowering Barbarea to P. xylostella and to two of its parasitoids, Diadegma insulare Cresson (Hymenoptera: Ichneumonidae) and Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae). We also tested whether the presence of flowering Barbarea planted in the field border could affect densities of 10 insect pests and two beneficial coccinellids in adjacent cauliflower plants. Flowering Barbarea did not change the densities of insects found in adjacent cauliflower, except for Eurydema ornata L. (Hemiptera: Pentatomidae), which densities on cauliflower were reduced by flowering B. vulgaris. Coccinellids were more abundant on flowering Barbarea than on cauliflower plants. Chrysomelids were also more abundant on flowering Barbarea than on cauliflower plants, while hemipteran and lepidopteran pests were often more abundant on cauliflower than on Barbarea plants. In plots with flowering B. vulgaris, P. xylostella pupae suffered 1.7 and 4.0 times more parasitism by D. insulare and D. collaris, respectively, than in plots without flowering B. vulgaris. Flowering reduced the attractiveness of Barbarea to P. xylostella, making it lose its effectiveness as a trap crop. However, being biennial, Barbarea could be used as a trap crop for P. xylostella the first year, and to lower the populations of E. ornata and increase parasitism of P. xylostella when it flowers the second year.