ORIGINAL PAPER Effects of plant competition and herbivore density on the development of the turnip root fly (Delia floralis) in an intercropping system Maria Bjo ¨rkman Richard J. Hopkins Peter A. Hamba ¨ck Birgitta Ra ¨mert Received: 24 May 2008 / Accepted: 29 January 2009 / Published online: 20 February 2009 Ó Springer Science+Business Media B.V. 2009 Abstract In this study, interactive effects of plant com- petition and herbivory on plant quality and herbivore development were examined in a greenhouse experiment where cabbage plants [Brassica oleracea L. var. capitata (Brassicaceae)] were intercropped with red clover [Trifo- lium pratense L. (Fabaceae)]. Cabbages were grown with two red clover densities and attack rates by the root feeding herbivore the turnip root fly, Delia floralis Fall. (Diptera: Anthomyiidae). Above ground and below ground cabbage biomass was reduced through intercropping and larval damage. Intercropping also resulted in lower nitrogen and higher carbon root levels compared with levels in the roots of monocultured cabbage. Furthermore, both root nitrogen and carbon levels increased with herbivory. Root neutral detergent fibre (NDF) and lignin content increased in response to both increased plant competition and higher egg densities. For lignin, an interaction effect was observed in the form of elevated levels in intercropped plants subjected to larval damage, while levels in roots of monocultured cabbage remained unchanged. The quality changes brought about by clover competition affected D. floralis development negatively, which resulted in reduced pupal weight. In addition, increased egg density also decreased larval growth. The effects on the development of D. floralis in relation to host plant quality are discussed. Keywords Host-plant quality  Cabbage  Brassica  Root herbivory  Induced response  NDF  Lignin Introduction In natural environments, where competition from other plants and attack by herbivores and pathogens are major challenges, plants are often regarded as facing a dilemma in the allocation of resources between growth and defence (Herms and Mattson 1992). Much of the theory surrounding such trade-offs is based on the aerial parts of plants. How- ever, root herbivory interferes with the basic functions of plant roots, including resource uptake (van der Putten 2003), and this may seriously reduce the capacity of the plant to grow or defend itself and thus to compete in natural systems. At the same time, the constraints placed upon the plant by competition from neighbouring plants may change the quality of the plant as a resource for herbivores and influence the manner in which the plant responds to insect attack. There are two major hypotheses concerning the impact of bottom-up effects on insect herbivory. White (1984) pro- poses in the ‘plant stress hypothesis’ that stressed plants are likely to become more susceptible to herbivores due to the physiological changes that take place in protein synthesis. In contrast, the ‘plant vigour hypothesis’ (Price 1991) predicts that herbivores will preferentially feed on plants or plant Handling editor: Gimme Walter M. Bjo ¨rkman (&) Department of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), P.O. Box 7043, 750-07 Uppsala, Sweden e-mail: maria.bjorkman@bioforsk.no R. J. Hopkins Department of Ecology, Swedish University of Agricultural Sciences, Uppsala, Sweden P. A. Hamba ¨ck Department of Botany, Stockholm University, Stockholm, Sweden B. Ra ¨mert Department of Plant Protection Biology, Swedish University of Agricultural Sciences, Alnarp, Sweden 123 Arthropod-Plant Interactions (2009) 3:55–62 DOI 10.1007/s11829-009-9055-x