Which plant traits are most repellent as intercrops for pest control? Tests from simulated intercropping systems

Modern crop monocultures have long been thought to be more vulnerable to insect pest outbreaks because of the lack of crop diversity. There has been longstanding interest in diversifying cropping systems through intercropping, but a lack of a theoretical framework to guide intercrop plant selection has hampered progress. Although intercropped plants can repel insect specialists from their host plants, the key plant traits that contribute to repellency remain poorly understood. We examined a range of candidate plants intercropped with broccoli (Brassica oleraceae L. var. italica) to determine whether they were repellent against the swede midge, Contarinia nasturtii (Diptera: Cecidomyiidae), a specialist insect pest. In experimental mesocosms, we tested whether physical attributes (height or leaf area), vegetation type (herb, vegetable, or ground cover), and/or phylogenetic distance of the intercrop plant influenced C. nasturtii larval densities on broccoli. We found that C. nasturtii responded to the different treatments. Although we did not find an inverse correlation between phylogenetic distance and larval densities, the lack of the correlation was because the most closely related plant species were the most repellent. Intercrop plant height and leaf area did not influence larval densities, but intercrop vegetation type did. When groundcover plants were grown as intercrops, broccoli plants had the fewest number of larvae, while the highest number of larvae were found on B. oleracea plants planted in combination with herb intercrop plants. Our results show that phylogenetic distance and vegetation type can be important factors influencing the degree of repellency of intercrops to specialist insects.