INTRODUCTION Understanding ecological interactions among insect herbivores and their host plants has long been a goal of ecologists. A substantial body of literature implicates host plant quality as a primary player influencing herbivore- host plant interactions (Thompson, 1988a; Louda & Collinge, 1992). Here, the basic premise is that insects will prefer to oviposit and feed on plants that confer the greatest fitness to themselves and their offspring (Thomp- son, 1988b; Craig et al., 1989; Denno et al., 1990; Fox & Lalonde, 1993; Barker & Maczka, 1996; Craig & Ohgushi, 2002). The preference/performance hypothesis is based on the premise that plants differ in their quality as hosts for her- bivorous insects. Numerous studies show that differences in plant quality reflect the availability of soil nutrients. Total numbers of leaves, main stem diameters and root masses are considered important plant morphological indices (Gartner, 1994; Tekeli & Ates, 2003). Variation in soil nutrients affects the production of plant defensive chemicals (Inbar et al., 2001; Marazzi et al., 2004; Ramona et al., 2005), plant morphology and quality (Chau & Heinz, 2006) and in turn can influence the pref- erence and performance of insect herbivores. Hypotheses of “plant stress” and “plant vigour” predict the responses of herbivores to soil nutrients, as mediated by host plant quality. The plant stress hypothesis of White (1984) predicts that stressed plants serve as better hosts for insect herbi- vores. According to this hypothesis, plants under stress are more suitable for insect herbivores due to increased nutritional quality arising from reduced protein synthesis and increased free amino acids in plant tissues (White, 1969, 1984; Mattson & Haack, 1987) and reduced syn- thesis of defensive chemicals (Rhoades, 1979). The effect of plant stress often varies with the insect feeding guild, the host plant species, and the nature and level of stress (Heinrichs, 1988; Larsson, 1989; Waring & Cobb, 1992). Certain insects (e.g. phloem feeders) prefer stressed plants whereas others (e.g. leaf chewers, leafminers) are negatively affected when feeding on stressed plants (Lars- son, 1989; Koricheva et al., 1998; Inbar et al., 2001). For instance, whiteflies, Bemisia tabaci (Gennadius) and B. argentifolii Bellows & Perring (Homoptera: Aleyro- didae), prefer stressed cotton plants (Flint et al., 1996; Skinner, 1996) while performance of grass miners, Chro- Eur. J. Entomol. 106: 583–594, 2009 http://www.eje.cz/scripts/viewabstract.php?abstract=1491 ISSN 1210-5759 (print), 1802-8829 (online) Bottom-up effects of host plant nutritional quality on Plutella xylostella (Lepidoptera: Plutellidae) and top-down effects of herbivore attack on plant compensatory ability RANA M. SARFRAZ 1 *, LLOYD M. DOSDALL 2 and ANDREW B. KEDDIE 1 1 Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9 2 Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5 Key words. Lepidoptera, Plutellidae, Plutella xylostella, diamondback moth, Brassicaceae, insect-plant interactions, plant vigour, plant stress, soil fertilizer treatments Abstract. The plant stress and plant vigour hypotheses are competing paradigms pertaining to the preference and performance of herbivorous insects on their host plants. Tests of these hypotheses ideally require detailed information on aspects of soil nutrition, foliar nutrient levels and parameters of herbivore fitness, but such studies are uncommon. These hypotheses were tested using the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), reared on its host plant, Brassica napus (L.), grown in an experimental system of five nutrient regimes. Different levels of fertilizer treatments significantly affected the nutrient content of B. napus foliage and this in turn affected the preference and performance of P. xylostella. Ovipositing females discriminated among host plants grown in soils subjected to different fertilizer treatments and selected plants on which pre-imaginal survival was highest, development fastest and longevity of the next generation of adults the longest, even when food was scarce. Plants subjected to herbi- vory by P. xylostella responded by producing elevated levels of some nutrients (e.g., sulphur), but other nutrient levels declined in infested leaves (e.g., nitrogen). Regardless of the rate of fertilizer application, plants compensated for herbivory by increasing root mass compared to un-infested control plants; plants grown in soils receiving the optimum quantity of fertilizer developed the most robust root systems when infested. The plant stress and the plant vigour hypotheses are likely to be at the opposite ends of a con- tinuum of responses between insects and their host plants. Our investigations indicate a complex set of interactions involving both bottom-up and top-down effects, which interact to affect host plant quality, oviposition site selection by female herbivores and the fitness of their offspring. 583 * Corresponding address: Department of Zoology and Biodiversity, University of British Columbia, 2370-6270 University Blvd., Vancouver, British Columbia, Canada V6T 1Z4; e-mail: rsarfraz@zoology.ubc.ca