Biological control strategies against the cabbage root fly Delia radicum

Contemporary intensified agroecosystems are contributing to a reduction in natural enemy (NE) populations. In conservation biological control (CBC), NEs are favoured by providing supporting resources. CBC could be complemented with inoculation biological control (IBC) to enhance the effect of NEs. However, increased NE species richness may result in positive (e.g. niche complementarity [NC]) or negative (e.g. intraguild predation [IGP]) effects on pest suppression through species interactions. The research reported in this thesis investigated the potential of combining CBC with IBC as a strategy for cabbage root fly, Delia radicum, control. A three-year field experiment explored the combined effect of floral resources and perennial shelter habitats (i.e., conservation strips [CS]) on NEs of D. radicum. CS increased the abundance of hymenopteran parasitoids, but did not increase parasitism by either of the two dominant parasitoid species, Trybliographa rapae and Aleochara bipustulata. The entomopathogenic fungi Metarhizium brunneum and Beauveria bassiana were found to be pathogenic to both D. radicum and T. rapae, thus representing an IGP risk to T. rapae. However, the parasitoid laid more eggs in healthy than in M. brunneum infected D. radicum larvae in choice assays. Host plant habitats harbouring high densities of M. brunneum were avoided when given a choice, but host density was more important for attraction than fungal presence, indicating a trade-off between IGP risk and reproductive success. Despite the IGP risk, field cage studies showed that combining T. rapae and M. brunneum, at both low and high fungal densities, reduced D. radicum population levels more than when either NE acted alone. Selective ovipositioning in healthy larvae presumably led to this resource partitioning and resulting NC. However, fungal presence, particularly at high density, reduced the number of emerged F1 T. rapae. Consequently, combining CBC of T. rapae with IBC using a relatively low density of M. brunneum is recommended to ensure stable, long-term D. radicum control