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Invasive species represent one of the most important threats to biodiversity worldwide, with consequences for ecosystem functioning and the delivery of important ecological services to society. Several hypotheses have been generated to explain the success of exotic plants in their new ranges, with escape from their old natural enemies, such as pathogens and herbivores (the ‘enemy release hypothesis’) and novel defensive chemistry (the ‘novel weapons hypothesis’) receiving considerable attention. Thus far, virtually all studies of exotic plants and insects have been conducted in a strictly bi-trophic framework involving plants and herbivores. On the other hand, it has been argued that a better understanding of the forces regulating community structure and function should include natural enemies of the herbivores. Furthermore, indirect interactions between organisms in the plant roots (below-ground) and shoots (above-ground) are known to strongly effect the behaviour and performance of consumers in the opposite ‘compartment’. Here, we discuss a range of physiological, evolutionary and ecological aspects of plant–herbivore-natural enemy interactions involving exotic plants. Further, interactions between soil and above-ground organisms are explored with respect to studies with exotic plants. We argue that it is important to link population and community ecology to individual-level variation in the physiology and behaviour of insects across several trophic levels in studies with invasive plants. Future research with invaders should also aim to integrate physically separated compartments (e.g. plant roots and shoots). This will facilitate a more complete understanding of the factors underlying the success (or failure) of exotic plants to spread and become dominant in their new ranges. Moreover, these data will also help to unravel the relative importance of top-down and bottom-up processes in regulating communities in which exotic plants have become established. Lastly, we discuss consequences for conservation.

Through artificial selection, domesticated plants often contain modified levels of primary and secondary metabolites compared to their wild progenitors. It is hypothesized that the changed chemistry of cultivated plants will affect the performance of insects associated with these plants. In this paper, the development of several specialist and generalist herbivores and their endoparasitoids were compared when reared on a wild and cultivated population of cabbage, Brassica oleracea, and a recently established feral Brassica species. Irrespective of insect species or the degree of dietary specialization, herbivores and parasitoids developed most poorly on the wild population. For the specialists, plant population influenced only development time and adult body mass, whereas for the generalists, plant populations also affected egg-to-adult survival. Two parasitoid species, a generalist (Diadegma fenestrale) and a specialist (D. semiclausum), were reared from the same host (Plutella xylostella). Performance of D. semiclausum was closely linked to that of its host, whereas the correlation between survival of D. fenestrale and host performance was less clear. Plants in the Brassicaceae characteristically produce defense-related glucosinolates (GS). Levels of GS in leaves of undamaged plants were significantly higher in plants from the wild population than from the domesticated populations. Moreover, total GS concentrations increased significantly in wild plants after herbivory, but not in domesticated or feral plants. The results of this study reveal that a cabbage cultivar and plants from a wild cabbage population exhibit significant differences in quality in terms of their effects on the growth and development of insect herbivores and their natural enemies. Although cultivated plants have proved to be model systems in agroecology, we argue that some caution should be applied to evolutionary explanations derived from studies on domesticated plants, unless some knowledge exists on the history of the system under investigation.

Constitutive and induced changes in plant quality impact higher trophic levels, such as the development 27 of parasitoids, in different ways. An efficient way to study how plant quality affects parasitoids is to 28 examine how the parasitoid larva is integrated within the host during the growth process. In two exper- 29 iments, we investigated the effects of varying nutritional quality of Brassica oleracea on parasitoid larval 30 development inside the host, the diamondback moth (Plutella xylostella). First, we compared larval 31 growth of the specialist Diadegma semiclausum and the generalist Diadegma fenestrale, when the host 32 was feeding on Brussels sprout plants that were either undamaged or were previously induced by cater- 33 pillar damage. Larvae of the generalist D. fenestrale showed lower growth rates than larvae of the special- 34 ist D. semiclausum, and this difference was more pronounced on herbivore-induced plants, suggesting 35 differences in host-use efficiency between parasitoid species. The growth of D. semiclausum larvae was 36 also analyzed in relation to herbivore induction on Brussels sprouts and on a wild B. oleracea strain. Par- 37 asitoid growth wasmore depressed on induced than on undamaged control plants, andmore on wild cab- 38 bage than on Brussels sprouts, which was largely explained by differences in host mass. The effects of 39 induction of wild Brassica on parasitoid development were pronounced early on, but as P. xylostella feed- 40 ing began inducing the previously undamaged control plants, the effect of induction disappeared, reveal- 41 ing a temporal component of plant–parasitoid interactions. This study demonstrates how insights into 42 the physiological aspects of host–parasitoid interactions can improve our understanding of the effects 43 of plant-related traits on parasitoid wasps.

The warty cabbage Bunias orientalis is an invasive pest in much of central Europe, including much of Germany since the 1980s, whereas in other countries, such as The Netherlands, it is a less common exotic species. Here, healthy larvae of Mamestra brassicae, which has been found feeding on B. orientalis plants in Germany, and larvae parasitized by one of its major larval endoparasitoids Microplitis mediator, were reared on both herbivore-induced and noninduced leaves of B. orientalis originating from single large populations growing in The Netherlands and central Germany. Herbivore performance was less negatively affected than parasitoid performance by differences in plant quality. Development times in both M. brassicae and Mi. mediator were shorter on Dutch than German plants and also shorter on noninduced than induced plants. Moreover, survival and body size of the parasitoid was more strongly affected by plant population and induction than survival of healthy M. brassicae. Chemical analyses of defensive secondary metabolites [glucosinolates (GS)] revealed that concentrations of the major GS sinalbin were constitutively expressed in German plants whereas they were induced in Dutch plants. However, in separate feeding bioassays in which preference for induced and noninduced leaves was compared separately, L3 instars of M. brassicae preferred noninduced German plants over Dutch plants but induced Dutch plants over German plants, revealing that changes in primary metabolites or an unidentified non-GS compound mediates population-related differences in plant quality. The results reveal asymmetric effects of plant quality in exotic plants on organisms in the second and third trophic level.

Heliothis virescens (F.) last instar larvae parasitized by the endophagous braconid Cardiochiles nigriceps Viereck fail to attain the pupal stage, due to a parasitoid-induced alteration of ecdysteroid biosynthesis and metabolism. Currently available information on host prothoracic gland inactivation in this host-parasitoid system is reported here. Prothoracic glands of H. virescens mature larvae show a depressed biosynthetic activity, without undergoing gross morphological disruption. The ultrastructure of gland cells is characterized by minor parasitoid-induced changes, with the rough endoplasmic reticulum appearing more developed and electron-dense than in nonparasitized controls. Eventually, the cells of prothoracic glands of parasitized host last instar larvae die but maintain their structural integrity. The inactivation of pupally committed host prothoracic glands is achieved through the disruption of the PTTH signal transduction pathway. The second messenger cAMP appears to be normally produced in response to PTTH stimulation of glands explanted from parasitized host larvae, however the downstream activation of the cAMP-dependent protein kinase does not appear to occur. In fact, a marked underphosphorylation of regulatory target proteins is observed. This underphosphorylation is associated with a significant reduction in general protein synthesis, which appears to be blocked at the translational level, to a redirection of specific protein synthesis and to a drastic suppression of ecdysteroidogenesis. These parameters appeared to be correlated in a kinetic time-course study, confirming their functional link. C. nigriceps polydnavirus (CnPDV) plays a major role in the inactivation of pupally committed host prothoracic glands, while putative factors occurring in the host haemolymph do not seem to be of particular importance at that developmental stage. Southern blot hybridization indicates the occurrence of PKI(protein kinase inhibitor)-like genes in the C. nigriceps genome, which, in contrast, are undetectable in H. virescens. (C) 1998 Elsevier Science Ltd. All rights reserved.

Populations of wild Brassica oleracea L. grow naturally along the Atlantic coastlines of the United Kingdom and France. Over a very small spatial scale (i.e.