Evaluation of Trap Cropping for the Control of Diamondback Moth

Chapter 1: This quantitative meta-analysis investigates the relationship between oviposition preference, larval mortality on trap crops, and trap crop success by examining two systems that have a large amount of data available for use in meta-analysis. We investigate trap cropping systems used for control of the diamondback moth (Plutella xylostella) and those used in the push-pull system in Kenya for control of stemborers (Chilo partellus), a key pest of maize. Our results suggest that survival on a trap crop relative to the cash crop is a more important indicator than oviposition preference for the trap crop relative to the cash crop for success of trap cropping systems. In the push-pull system, relative survival on the trap crop grass Brachiaria was lower than survival on Napier grass, oviposition preference for Napier grass was greater than for Brachiaria, and trap cropping systems with Brachiaria had lower damage to maize than Napier systems. For diamondback moth, relative to cabbage, yellow rocket (Barbarea vulgaris) was a less suitable host than Indian mustard (Brassica juncea) and collards (Brassica oleracea), and caused the greatest reduction of diamondback moth larval infestation on cabbage, although it did not attract the greatest oviposition by female moths. Previous research has primarily focused on oviposition preference to predict trap crop success. Our findings that survival on the trap crop is also a key component of success of trap cropping systems could help guide future trap cropping research. Chapter 2: In Chapter 1, we demonstrate that oviposition preference for, and larval mortality on trap crops are important factors for the success of trap cropping systems. Here, we evaluated Indian mustard (Brassica juncea) and yellow rocket (Barbarea vulgaris) as trap crops for protecting broccoli (Brassica oleracea) against diamondback moth (Plutella xylostella) in a typical commercial setting in Yuma, Arizona over two cropping periods. During the fall of 2018 and spring of 2019, we used a randomized block design experiment to compare broccoli infestations in plots transplanted with 100% broccoli, 10% or 20% yellow rocket, and 10% Indian mustard. In field choice tests, both yellow rocket and Indian mustard were highly preferred for oviposition over broccoli. Furthermore, the number of larvae was significantly lower on yellow rocket and Indian mustard relative to broccoli, suggesting higher larval mortality on the trap crops relative to broccoli. Nevertheless, during the fall and spring growing season, no significant reduction in the number of larvae on broccoli plants or proportion of broccoli crowns infested at harvest occurred in plots with trap crops relative to plots with 100% broccoli. Our results show that using these trap crops in the density and planting pattern we tested is ineffective for controlling diamondback moth in commercially grown broccoli in Yuma