Abstract. 1. Consequences of variation in food plant quality were estimated for a system consisting of two monophagous noctuid herbivores and three ichneumonid parasitoids.2. In a natural population, pupal weights of the herbivores in this system, Nonagria typhae and Archanara sparganii, were found to be highly variable. Pupal weights increased strongly and consistently with the increase in the vigour of the host plant, Typha latifolia, providing support for the plant vigour hypothesis. Correspondingly, as the moths do not feed as adults, a strong, positive correlation between host vigour and fecundity of the herbivores would be expected.3. There were strong and positive relationships between adult body sizes of the parasitoids and the sizes of their lepidopteran hosts. Moreover, a direct, positive link between plant quality and parasitoid size was documented.4. For all three parasitoids, cascading effects of plant quality on body size were weaker than for the herbivores. Differences in the importance of adult feeding and oviposition behaviour suggest that dependence of fitness on body size is also weaker in the parasitoids than in the moths. It is therefore concluded that the numerical response of the herbivore population to a change in plant quality should exceed the corresponding response in the parasitoids.5. The results of this work imply that variation in plant variables may affect performance of different trophic levels to a different extent. It is suggested that the importance of adult feeding for the reproductive success (capital vs. income breeding strategies) in both herbivores and parasitoids is an essential aspect to consider when predicting responses of such a system to changes in plant quality.Key words. Adult feeding, Archanara, body size, cascading effects, Ichneumonidae, Nonagria, population dynamics, Typha latifolia.
IntroductionTrophic cascades can be defined as strong interactions within food webs that influence system properties and embrace at least two trophic links (Pace et al., 1999). Both bottom-up and top-down trophic cascades may influence community-level processes and population dynamics of single species (Polis & Strong, 1996;Polis et al., 2000;Sinclair et al., 2000). In terrestrial ecosystems, one of the most universal bottom-up cascades is based on plants (Price, 1992). In particular, heterogeneity in food plant quality has a strong potential to affect body size in herbivorous insects (Hunter & Price, 1992;Price, 1992). In turn, body size of an adult insect is frequently a good predictor of life-history parameters (e.g. fecundity, longevity, and survival) that have direct relevance to population dynamics (HoneÏ k, 1993;Nylin & Gotthard, 1998; but see Leather, 1988). Most data on the effects of plant quality on insect performance come from laboratory studies, however less is known about the range of food plant-induced size variation