Simultaneous use of winter cress (Brassicaceae) in trap cropping, conservation biological control, and pollinator conservation
Abstract: Winter cress, Barbarea vulgaris R. Br. (Brassicaceae), has been proposed as trap crops for the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae). In bloom, B. vulgaris could also be used to attract beneficial insects. In this study we tested the attractiveness of flowering B. vulgaris 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 B. vulgaris planted in the field border could affect densities of insect pests and beneficial coccinellids in adjacent cauliflower plants. Flowering B. vulgaris did not change the densities of insect pests found in adjacent cauliflower, except for Eurydema ornata L. (Hemiptera: Pentatomidae). Coccinellids were more abundant on flowering B. vulgaris than on cauliflower plants. Chrysomelids were also more abundant on flowering B. vulgaris than on cauliflower plants, while hemipteran and lepidopteran pests were more abundant on cauliflower than on B. vulgaris 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 B. vulgaris to P. xylostella, making it lose its effectiveness as a trap crop. Flowering B. vulgaris plants were visited by hoverflies, such as Sphaerophoria scripta L. (Diptera: Syrphidae), by pollen-feeding beettles, such as Psilothrix viridicoerulea Geoffroy (Coleoptera: Melyridae), and by mining bees of the genus Andrena (Hymenoptera: Andrenidae). Being biennial, B. vulgaris could be used as a trap crop for P. xylostella the first year, and to lower the populations of E. ornata, increase parasitism of P. xylostella, and attract pollinators when it flowers the second year.