Novel approaches for the management of cabbage root fly

Delia radicum L. (Diptera: Anthomyiidae), the cabbage root fly, is a specialist root-feeding insect pest of Brassica crops. The impending withdrawal of chlorpyrifos, one of the main pesticides used against D. radicum, opens new opportunities to research alternative pest management strategies. Manipulating host plant location cues to influence D. radicum adult and larval behaviour, along with induced plant defence responses, offer potential integrative crop protection solutions (Chapter 1). This thesis aimed to identify the semiochemistry underpinning D. radicum larval host plant location, and to investigate whether plant defence induction treatments (methyl jasmonate [MeJA], D-Fructose) and a herbivore induced volatile (dimethyl disulfide [DMDS]) affect D. radicum larval performance and adult oviposition preference. In choice-test bioassays, larvae were inconsistent in their responses to root exudates collected from Brassica host plants (Chapter 2). A combined bioassay and EthoVision® video-tracking approach was developed to record and analyse larval movements in response to volatiles emitted from host and non-host plant roots (Chapter 3). Larvae were significantly attracted to host plant root volatiles. Olfactory stimuli from roots of the non-host plant onion (Allium cepa L. ‘Ailsa Craig’), which share overlapping, yet distinctive volatile profiles to that of Brassica plants, also elicited positive taxis. By analysing the volatile metabolome of broccoli (B. oleracea L. convar. botrytis L. Alef. var. cymosa Duchesne ‘Parthenon’) and onion roots using solid phase micro extraction-gas chromatography-mass spectrometry (SPME-GCMS), a suite of candidate volatile orientation cues were identified. A SPME-based method was developed to non-invasively collect root volatiles in situ from glasshouse- and field-grown broccoli plants pre- and post-D. radicum infestation (Chapters 4, 5 and 6). GC-MS analyses revealed that sulfur compounds, showing characteristic temporal emission patterns, were the principal volatiles released by roots in response to damage. This new method, which has potential for wide application in chemical ecology research, allows the study of volatiles in the soil in situ that are critical for interactions between trophic levels. In EthoVision® bioassays, a major volatile constituent of broccoli roots, DMDS, was attractive to larvae, but toxic at the highest dose tested (Chapter 3). Glasshouse and on-farm experiments using broccoli were conducted to evaluate the efficacy of MeJA, D-Fructose and DMDS against D. radicum compared to commercially available crop protection products (chlorpyrifos [Dursban® WG], spinosad [Tracer®], Steinernema feltiae Filipjev [Nematoda: Steinernematidae] [Entonem] and garlic granules [ECOguard®]). MeJA and garlic reduced larval performance under glasshouse conditions whereas D-Fructose and DMDS did not at the concentrations tested (Chapter 7). In field studies, MeJA combined with reduced rate chlorpyrifos, spinosad, and S. feltiae all showed partial efficacy for controlling D. radicum larvae. Inherent field site, weather and D. radicum population density variability highlighted that glasshouse results cannot always be reproduced in more complex field environments (Chapter 8). Further research is needed into formulation, mode of application and timing to improve efficacy of promising treatments that may help in future integrated pest management (IPM) for this key pest in the absence of existing pesticides