Conservation biological control of insect pests in two horticultural crops.

Modern farming practices create inhospitable environments for natural enemies reducing their capacity to control pest insects. Conservation biological control (CBC) is a practice that aims to provide natural enemies with the resources they lack. This thesis investigated the potential of CBC for two important horticultural pests, the cabbage root fly Delia radicum and the lily leaf beetle Lilioceris lilii. The natural enemy complex of L. lilii larvae in Sweden is unknown, so a survey was conducted to collect information on the types of natural enemies present and the levels of parasitism. Four larval parasitoids of L. lilii were found, in varying relative abundance depending on the regions surveyed. Parasitism ranged between 21 and 57% depending on year and region. A three-year field experiment in the south-east of Sweden investigated CBC-based pest management for D. radicum that relied on within-field habitat manipulation practices. Nectar resources for the parasitoid Trybliographa rapae and perennial grass as overwintering habitat for epigeal predators were combined in conservation strips (CS). It was found that providing vegetative resources within the crop potentially improved the conditions for natural enemies of D. radicum. Activity density of the predator and pupal parasitoid Aleochara bipustulata was higher in plots with CS, at the time of D. radicum egg-laying, in two out of three years. In 2009 this was associated with a significant reduction in the number of infested cabbage plants. Other natural enemies, such as Cynipoidae wasps, were also more abundant in plots with CS. A parallel semi-field experiment showed that T. rapae could parasitise significantly more D. radicum larvae when given access to flowering buckwheat plants. However, parasitism by T. rapae and A. bipustulata was not higher in field plots with CS in any study years. On the other hand, egg-laying by D. radicum was not enhanced despite the provision of food plants which were not solely selective for natural enemies.