Cabbage root fly passes through three generations a year in the south of England. The first generation peaks in late April to early May having overwintered in diapause. Recently, populations have been discovered in which the emergence of first generation flies was protracted, lasting until mid-July. The project aimed to investigate the incidence of late emergence in Devon and to examine the ways in which late emergers differed from early emergers in their progress through diapause. A new emergence trap was developed and used in conjunction with yellow water traps to monitor the emergence of cabbage root fly populations at locations of contrasting agricultural practice. Late emergence was widespread in Devon, occurring later (up to September), and in a greater proportion of some populations than any previously reported. The phenomenon appears to be a response to the planting of brassicas in June, as flies emerging at the normal time may not be able to locate a host crop. Emergence in a substantial proportion of one population was delayed for a year. This has not been been previously reported in cabbage root fly and represents a 'sit and wait' strategy associated with short-lived habitats, occurring in patches often separated by considerable distances but which frequently reappear in approximately the same location. A gas chromatography technique was developed which is capable of individually monitoring the respiration rates of large numbers of pupae. The results suggested that the temperature optimum for diapause development may r1se in late emergers, possibly above the threshold for postdiapause development. There was considerable 1ntrapopulat1on variation in temperature responses. Overall, the results suggest that the level of variation in temperature response and emergence times between and within populations will require careful local investigation with continuous reassessment of selection pressures, for an accurate prediction of cabbage root fly emergence.

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.

Le développement d’une agriculture moins dépendante des pesticides repose en partie sur la maximisation des services écosystémiques. L’objectif principal de cette thèse était de quantifier le service de régulation des insectes ravageurs par leurs ennemis naturels, en conditions expérimentales et en situation de production. Le travail a porté sur les brassicacées légumières soumises aux attaques de ravageurs aériens et souterrains (pucerons et mouche du chou). Nous avons évalué leur nuisibilité pour quantifier les dégâts et dommages qu’ils peuvent générer. Nos résultats confirment la moindre nuisibilité des pucerons par rapport à celle de la mouche du chou. Pour celle-ci, la nuisibilité sur légume-fleur s’exprime non seulement par la mortalité des plants mais aussi par un effet sur leur développement.Nous avons également montré que les prédateurs épigés réduisaient significativement les populations des deux ravageurs, ce qui, pour la mouche du chou, conduit à une baisse conséquente de la mortalité des plants. Enfin, simplifier le travail du sol au printemps n’a pas permis de favoriser les prédateurs hivernant dans la parcelle, ni d’augmenter le service de régulation à court terme. Cependant les prédateurs précoces sont déterminants et doivent être favorisés par des pratiques intra- et péri-parcellaires ciblées. Les connaissances acquises dans ce travail démontrent le potentiel de la lutte biologique par conservation pour une gestion agro-écologique efficace des insectes ravageurs. / Developping farming systems that use less pesticides is partly based on the intensification of ecosystem services. The main aim of this thesis was to quantify the insect pest regulation service provided by their natural enemies, both in experimental plots and in farmers fields. We worked on brassicaceous vegetables that are confronted to pest attacks on above- and belowground organs (aphids and the cabbage root fly). We assessed their harmfulness, in order to quantify the injuries and damage these pests can inflict to plants. Our results confirm that aphids are less harmful than the cabbage root fly. On flower vegetables, the latter not only leads to plant mortality but can also slow plant development.We also showed that ground dwelling predators significantly reduce the populations of both pests, leading, for the cabbage root fly, to a strong decrease in plant mortality rate. Finally, simplifying soil tillage in the spring did not favor ground dwelling predators overwintering inside the field and neither did it increase natural regulation service in the short term. However, early ground dwelling predators are fundamental and must be favored by tailored practices both inside and around the fields. The knowledge gained in the present thesis show the potential of conservation biological control for the agroecological management of insect pest populations.

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