Effects of nitrogen fertilization on insect pests, their parasitoids, plant diseases and volatile organic compounds in Brassica napus

Nitrogen (N) availability is a key factor influencing the yield of Brassica napus L. Thus, mineral fertilization is widely used to improve the quality and quantity of seeds. In this study, we conducted field experiments to determine the impact of nitrogen fertilization on B. napus pests, their parasitoids and plant diseases. The results showed that N treatment had an impact on the abundance of pollen beetles (Meligethes aeneus Fab.) and cabbage seed weevils (Ceutorhynchus obstrictus Marsh.) as well as dark spot disease (Alternaria brassicae (Berk.) Sacc.). Since pest abundance was not correlated with the flower and silique numbers, the feeding and oviposition sites, plant smell bouquets were analysed to determine potentially attractive or repellent volatile organic compounds. We detected 19 different compounds among which acetic acid and several lipoxygenase pathway products were emitted at higher levels from N-treated plants. Emission of a few other terpenoid compounds was correlated with the pest abundance in field conditions. Abundance of parasitoids of both pests was related to the host availability rather than to the fertilization treatment. Therefore, we suggest that plant chemical cues play a minor role in localization of hosts in close proximity to parasitoid. Dark spot disease levels decreased with increasing N availability, possibly reflecting enhanced emissions of acetic acid, a known antifungal volatile. This study demonstrates the effects of N fertilization on bud and flower volatile bouquets, which might play a role in B. napus insect pest host selection and in resistance to fungal plant diseases. Further studies are necessary to investigate the behavioural responses of insects to the changed volatile bouquets.

► The abundance of insect pests in Brassica napus fields is affected by N fertilization.
► Semiochemical stimuli may be more important cues in host plant search than visual.
► Several VOCs emitted by B. napus were correlated with insect abundance.
► Plants with lower N fertilization levels were more infected with Alternaria brassicae.
► The disease scores were in negative correlation with the emission of acetic acid.