Cascading effects of variation in plant vigour on the relative performance of insect herbivores and their parasitoids

Abstract

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

Carbon–nitrogen stoichiometry in the tritrophic food chain willow, leaf beetle, and predatory ladybird beetle

“…Changes in the concentration of nitrogen in host plants may affect not only herbivorous insects, but also higher trophic levels, including predatory and parasitic insects, through bottom-up cascading effects (Price et al 1980;Teder and Tammaru 2002;Hunter 2003). Plants of better quality may result in an increase in the population density of predators in response to the increased density of prey herbivores (Hunter and Price 1992).…”

Mycorrhiza-induced trophic cascade enhances fitness and population growth of an acarine predator

“…Traditionally, studies on trophic cascades in terrestrial food webs have focused on the aboveground components of tritrophic systems consisting of plants, herbivores, and third trophic level natural enemies (Schmitz et al 2000;Teder and Tammaru 2002;Kagata and Ohgushi 2006). During the last two decades, more and more evidence has emerged that aboveground events may influence Communicated by Stefan Scheu. belowground events and vice versa (van der Putten et al 2001;Wardle et al 2004).…”

Short-term indirect interactions between two moth (Lepidoptera: Noctuidae) species mediated by shared parasitoids: The benefit of being scarce

“…Larvae of four moth species have been found feeding on leaves and stems of T. latifolia (Teder et al, 1999;Teder & Tammaru, 2002). This study was, however, restricted to the two most abundant species, Nonagria typhae Thunberg and Archanara sparganii Esper (Lepidoptera: Noctuidae).…”