Abstract
Within the Brassicaceae, wild as well as crop species are challenged by specialist herbivores including cabbage white butterflies (Pieris spp.). The wild crucifer Brassica nigra responds to oviposition by Pieris butterflies by the synergistic expression of two egg-killing traits. Genotypes that express a hypersensitive response (HR)-like necrosis (direct egg-killing) also emit oviposition-induced plant volatiles (OIPVs) attracting Trichogramma egg parasitoids (indirect egg-killing). This so-called double defense line can result in high butterfly egg mortalities. It remains unknown whether this strategy is unique to B. nigra or more common in Brassica species. To test this, we examined the response of different Trichogramma evanescens lines to OIPVs emitted by B. nigra and three close relatives (Brassica napus, Brassica rapa, and Brassica oleracea). Furthermore, we evaluated whether HR-like necrosis played a role in the attraction toward plant volatiles. Our results show a specificity in wasp attraction to different plant species. Three out of four plant species attracted a specific T. evanescens strain, including the crops B. rapa and B. napus. Parasitoid attraction was positively affected by presence of HR-like necrosis in one plant species. Our findings imply that, despite being a true generalist in terms of host range, T. evanescens shows intraspecific variation during host searching, which should be taken into account when selecting parasitoid lines for biocontrol of certain crops. Finally, we conclude that also crop plants within the Brassicaceae family possess egg-killing traits and can exert the double-defense line which may enable effective selection of egg-killing defense traits by cabbage breeders.
Highlights
Eggs of herbivorous insects deposited on leaves, stems and branches are often the start of herbivory
Recent studies on the Brassicaceae family have shown that several species can develop a hypersensitive response (HR)-like leaf necrosis against the eggs of specialist Pieridae butterflies, mainly species of lineage II that includes many of the common cabbage crops (Griese et al, 2021)
All species tested were capable of expressing HR-like necrosis in response to P. brassicae eggs, the occurrence and severity of HR-like necrosis differed between species: all B. nigra plants tested responded with HR-like necrosis to egg deposition, while almost none of the B. oleracea plants did
Summary
Eggs of herbivorous insects deposited on leaves, stems and branches are often the start of herbivory. Plants have evolved to recognize egg deposition as a warning signal of herbivory and developed an array of defenses to reduce egg survival. Such direct egg-killing traits include wound tissue growth, formation of neoplasm or necrotic tissue, and production of ovicidal substances, and have been reported in several plant families after oviposition of diverse herbivorous insect species (Hilker and Fatouros, 2015, 2016; Fatouros et al, 2016). B. nigra plants were shown to use a so-called double defense line: plants that express HRlike necrosis emit Trichogramma-attracting OIPVs, together leading to total egg mortalities up to 80% (Fatouros et al, 2015). In addition to the presence of HR-like necrosis, attraction of Trichogramma parasitoids to egg-infested B. nigra plants depends on the time elapsed since oviposition (either 24 h and/or 72 h), the identity of the herbivore, the place of origin of Trichogramma parasitoids, and the presence of other herbivores (Fatouros et al, 2012, 2014; Cusumano et al, 2015; Ponzio et al, 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
