Abstract. Oviposition site selection is crucial for the reproductive success of herbivorous insects. According to the preference-performance hypothesis, females should oviposit on host plants that enhance the performance of their offspring. More specifically, the plant vigor hypothesis predicts that females should prefer large and vigorously growing host plants for oviposition and that larvae should perform best on these plants.The present study examined whether females of the monophagous leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) prefer to oviposit on large host plant individuals of the meadow clary and whether large host plants are of higher nutritional quality than small host plants. Subsequently, it was tested whether the female preference correlates with offspring performance and survival.In the field, females preferred large host plant individuals for oviposition and host plant quality, i.e. leaf nitrogen content, was significantly higher in leaves of large than of small host plants. In the laboratory, larval development time was shorter on leaves of large host plant individuals than on small host plant individuals, but this could not be shown in the field. However, a predator-exclusion experiment in the field resulted in a higher survival of larvae on large host plants than on small host plants when all predators had free access to the plants. On caged host plants there was no difference in survival of larvae between plant size categories.It is concluded that females of C. canaliculata select oviposition sites that enhance both performance and survival of their offspring, which meets the predictions of the plant vigor hypothesis.

Abstract.Interactions between herbivorous insects and their parasitoids occur in highly structured and complex environments. Habitat structure can be an important factor affecting ecological interactions between different trophic levels. In this study the influence of plant architecture and surrounding vegetation structure on the interaction between the tansy leaf beetle, Galeruca tanaceti L. (Coleoptera: Chrysomelidae) and its egg parasitoid, Oomyzus galerucivorus Hedqvist (Hymenoptera: Eulophidae), was investigated at two small spatial scales in the field. It was expected that high and structurally complex plants or vegetation represent an enemy free space for the herbivore by making host search more difficult for the parasitoid. At the scale of individual plants, plant height had a positive influence on herbivore oviposition and egg clutch height a negative impact on parasitism. In addition, the beetle was more likely to oviposit on simple plants than on plants with branches, while the parasitoid remained unaffected by the degree of branching. At the microhabitat scale (r = 0.1 m around an oviposition site), both height and density of the vegetation affected beetle oviposition positively and egg parasitism negatively. The herbivore and its parasitoid, therefore, were influenced in opposite ways by habitat structure at both spatial scales investigated, suggesting the existence of an enemy free space for the herbivores' eggs on tall plants and in tall and complex vegetation. This study indicates that structural components of the environment are important for interactions among organisms of different trophic levels.

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Abstract. The importance of olfactory versus contact cues for host plant recognition was investigated in the tortoise beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae), which is strictly monophagous on meadow sage. The reaction of adult beetles to olfactory and contact host cues was tested using three bioassays (locomotion compensator, six-chamber-olfactometer, 'stem arena') to account for different behavioral contexts. Bioassay-guided fractionation of plant extracts was elaborated to characterize the nature of contact stimuli. The beetles were only slightly attracted to odors from small amounts of leaf material. However, when contact cues were provided additionally, the beetles showed strong preferences for samples of their host plant over controls. Bioassayguided fractionation led to isolation of at least two non-polar contact stimuli acting in concert that are sufficient for host plant identification in C. canaliculata.

In the literature, various environmental factors are described as being capable of influencing the reproductive output of insect females irrespective of their body size. Still, female body size or weight is widely used as a proxy for fecundity. In the present study, a seven-year data set on the autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae), was used to analyze whether the body weight-fecundity relationship in this capital breeding, cyclic forest defoliating lepidopteran is constant across years. Ambient temperature conditions and density of conspecifics during larval development, the length of the pupal period, as well as moth densities in the parent generation were examined as factors capable of modifying the body weight-fecundity relationship. While the regression slope of potential fecundity (total egg numbers per female) on pupal mass was constant across years, the mean total egg number per given body weight (the regression intercept) was significantly different between years. This residual variance in egg numbers after controlling for the effect of pupal mass was best explained by the pooled geometrid density (autumnal and winter moths) in the parent generation. The total egg number per given body weight decreased with increasing density of geometrid moths in the parent generation. Thus, maternal density effects on offspring fecundity were found in this system. Their rather weak nature suggests, however, that this maternal effect alone does not have the potential of causing cyclic population dynamics in the autumnal moth.