BR2CSFB - A toolkit for breeding resistance to adult and larval herbivory by the cabbage stem flea beetle

Although oilseed rape (OSR; Brassica napus) has traditionally been grown as the most profitable break crop, a loss of controls for cabbage stem flea beetle (CSFB; Psylliodes chrysocephala), has resulted in UK cropping area declining by 35% between 2012 and 2019. The National Farmers Union (NFU) estimate the removal of neonicotinoid seed treatments has cost farmers ~£94 million/year in lost opportunity and crop loss. Additional costs have been absorbed by the UK crushing industry because of the need to import OSR. Collectively, this poses a serious risk to the viability of the UK OSR industry and current farm crop rotation practices.

With the withdrawal of chemical controls, resistant cultivars are central to supporting Integrated Pest Management (IPM) strategies. However, unlike plant-pathogen interactions, our understanding of the interactions between host plants and chewing insects is limited. CSFBs are attracted to glucosinolates, chemicals used by Brassica species to deter non-specialist insect pests. Even if B. napus has defence or resistance mechanisms that deter CSFB feeding, the genetic control of such mechanisms and whether they can be exploited to breed for resistance has remained an open question. There are no known examples of resistance in B. napus and little knowledge of resistance mechanisms within our UK crop species. No resistant cultivars are currently available for any insect pest of OSR.

This proposal builds on preliminary data using controlled feeding studies and field trials, which shows that variation for reduced adult CSFB feeding is present within a diverse panel of B. napus. The panel, comprising historical varieties of winter and spring OSR, Chinese OSR, swede and kale, contains genetic diversity which is unlikely to be present in elite cultivars. This has enabled us to identify loci associated with CSFB feeding damage. Controlled larval infestation studies within this population have also identified variation in the numbers of emerging adult CSFB, demonstrating the presence of resistance to CSFB larvae. Together these observations indicate that some varieties of B. napus carry genes which can 1) deter adult CSFB feeding and 2) confer resistance against larval infestation or reduce larval fecundity. If identified, this variation can be exploited to breed OSR resistant to both damaging stages of CSFB.

This proposal developed by two research institutes (JIC and Rothamsted Research), seven major plant breeders and the OSR growers' Levy board (AHDB), aims to discover loci associated with adult CSFB feeding and larval resistance in B. napus. In parallel, we aim to develop an understanding of crop adaptations which affect OSR-CSFB interactions. This will be coupled with larval development studies, gene expression analysis and metabolite profiling to further elucidate key mechanisms by which Brassica plants identify and defend themselves against beetle attack. Key genes implicated in resistance and defence responses will be investigated using candidate gene studies in model plants, including candidate genes which underlie two loci implicated in the supporting data. Collectively, this knowledge, combined with germplasm and markers that will be used by participating breeders to integrate resistant alleles into commercial breeding pipelines, will facilitate the introduction of tolerant varieties into the UK OSR market.

Loss of chemical controls for cabbage stem flea beetle (CSFB; Psylliodes chrysocephala) and decline in OSR cropping area represent a serious risk to the viability of the UK OSR industry. Integrated pest management strategies offer an alternative to chemical control. Resistant cultivars are central to this approach.
Through controlled feeding studies and field trials we have identified variation for adult CSFB feeding within a DFFS of B. napus. Controlled larval infestation studies have also identified variation in the numbers of emerging adult CSFB. These data indicate that some lines of B. napus carry genes which 1) deter adult CSFB feeding and 2) confer antibiosis resistance against larval infestation. If identified, this variation can be exploited to breed OSR resistant to CSFB.
Here we describe a program to identify the genetic variation underlying these resistance traits and develop our understanding of interactions between plants and chewing beetle pests at adult and larval stages. Using Associative Transcriptomics, we have successfully identified variation at two loci associated with increased adult CSFB palatability which contain genes with known functions in plant defence signalling. This approach will be combined with biparental mapping and genome re-sequencing, to identify further loci associated with reduced adult palatability and larval antibiosis resistance. Using transcriptome time course analysis, larval development studies and metabolomic analysis, we will form an understanding of the underlying response of OSR to CSFB and the resulting effects on insect development, allowing us to begin to unravel the mechanisms underlying the observed CSFB resistance. Key genes implicated in resistance and defence responses will be investigated using candidate gene studies. These data will allow us to develop and test hypotheses which can be exploited in collaboration with our breeding partners for genetic plant protection against this key agricultural pest.