Diversity by design: Exploring trap crops and companion plants to control Phyllotreta cruciferae, the crucifer flea beetle, in broccoli

The crucifer flea beetle (CFB), Phyllotreta cruciferae Goeze (Coleoptera: Chrysomelidae: Alticinae) is a major pest of Brassica crops throughout North America. We explored alternative control methods including trap crops and companion plants to protect broccoli ( Brassica oleracea var. italica) from pest attack. Greater species diversity influences ecosystem function and we applied this to trap cropping. We examined whether diverse trap-crop plantings were more effective than any single species at drawing CFB away from broccoli. Overall, our results revealed that trap crop species not particularly effective when planted alone, nonetheless provided substantial plant protection when planted in multi-species polyculture. We also examined the use of combining companion plants with trap crops to enhance control of CFB in broccoli. Companion plants are interplantings of one marketable crop (the companion) within another (the protection target), in order to visually or chemically mask cues used by herbivores to locate the protection target. We attempted to further reduce the attractiveness of broccoli by interplanting it with the companion crops: bunching green onion (Allium fistulosum x cepa), Yukon gold potato (Solanum tuberosum), dill (Anethum graveolens) or Golden Guardian marigold (Tagetes patula). Altogether, our experiments revealed no evidence that companion-planting complements trap cropping to improve flea beetle control in broccoli. Finally, we examined if insecticide applications and broccoli proximity to the trap crop would further improve pest suppression. We assessed the effect of trap crop distance, by planting broccoli at three distances away from the trap crop: near (59 cm), middle (3.5 m) and far (7 m). To test effects of CFB removal, we randomly selected half of the trap crop plots and applied a pyrethroid insecticide twice during the season. We found that trap crop, regardless of treatment, protected broccoli planted at various distances without changing apparent densities of CFB on broccoli. Overall, our results suggest that CFB frequently move between the trap crop and broccoli, but when given a choice will do most of their feeding on the trap crop. However, we found that companion plants may provide no additional protection at controlling the CFB in broccoli.