Regional differences in flea beetle response to trap cropping in Northern Idaho and Southeastern Norway

Production of cole crops is constrained worldwide by the holarctic distribution of flea beetles in the genus Phyllotreta. Previous research indicates that cole crops grown in Northern Idaho and Southeastern Norway are challenged by this and other similar, crucifer-feeding pests.  In order to improve organic production of broccoli (Brassica oleracea var. Italica) in both regions, we evaluated different arrangements of a putative trap crop for their ability to protect broccoli from flea beetle colonization.  We conducted experiments in parallel between the University of Idaho and the Norwegian Institute of Environment and Agriculture to determine if the presence of a trap crop, or small changes in distance to the trap crop, bok choy (Brassica rapa pekinensis), altered the density of flea beetles on broccoli.  We found that both species of flea beetles (Phyllotreta undulata in Norway; Phyllotreta cruciferae in Idaho) strongly preferred the trap crop in the field.  In the case of P. undulata, this preference translated into reduced colonization of broccoli for the first two weeks of the growing season, but the same result was not observed for P. cruciferae in equivalent trap cropping arrangements.  In order to understand the basis of this difference in response, we quantify and compare the spatial aggregation of the two flea beetle species produced by 3 trap cropping arrangements (intercrops, adjacent blocks or distant blocks).  Such comparisons may help us identify types of host plant selection behavior in flea beetles that are amenable to control by trap cropping or other means of agricultural diversification.