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Although considerable effort has been invested in describing the phenotypic traits required for invasiveness, little is known about their genetic basis. However, as a result of the technological revolution in (functional) genomics, this situation is rapidly changing. Importantly, it has brought molecular genetics within the reach of ecologists, since gene-expression profiling, also called transcript profiling, has become technically feasible even in less well-equipped laboratories. In addition, the frequent use of transcript profiling has lowered its cost and increased the possibility of outsourcing analyses. Nowadays, most laboratories are capable of identifying genes of interest and of monitoring either their individual expression via quantitative PCR or RNA gel-blots or their simultaneous expression using microarrays and high-throughput quantitative transcriptome sequencing. These advances have allowed the ‘ask-the-plant’ experimental approach to be implemented on a genome-wide, and thus unbiased, scale. Moreover, our increased grip on how such candidate genes can be manipulated, via forward and reverse genetics, allowed for assessing the function of alleged ‘plant competition genes’ efficiently. In this chapter we will focus on the molecular mechanisms underlying plant competition, especially those identified through transcript analysis, and discuss how modern molecular tools have illuminated our understanding of the genetic basis of the dynamic and variable ecological phenomenon of invasiveness.

Parasitoids are important natural enemies of many pest species and are used extensively in biological and integrated control programmes. Crop plants transformed to express toxin genes derived from Bacillus thuringiensis (Bt) provide high levels of resistance to certain pest species, which is likely to have consequent effects on parasitoids specialising on such pests. A better understanding of the interaction between transgenic plants, pests and parasitoids is important to limit disruption of biological control and to provide background knowledge essential for implementing measures for the conservation of parasitoid populations. It is also essential for investigations into the potential role of parasitoids in delaying the build-up of Bt-resistant pest populations. The diamondback moth (Plutella xylostella), a major pest of brassica crops, is normally highly susceptible to a range of Bt toxins. However, extensive use of microbial Bt sprays has led to the selection of resistance to Bt toxins in P. xylostella. Cotesia plutellae is an important endoparasitoid of P. xylostella larvae. Although unable to survive in Bt-susceptible P. xylostella larvae on highly resistant Bt oilseed rape plants due to premature host mortality, C. plutellae is able to complete its larval development in Bt-resistant P. xylostella larvae. Experiments of parasitoid flight and foraging behaviour presented in this paper showed that adult C. plutellae females do not distinguish between Bt and wildtype oilseed rape plants, and are more attracted to Bt plants damaged by Bt-resistant hosts than by susceptible hosts. This stronger attraction to Bt plants damaged by resistant hosts was due to more extensive feeding damage. Population scale experiments with mixtures of Bt and wildtype plants demonstrated that the parasitoid is as effective in controlling Bt-resistant P. xylostella larvae on Bt plants as on wildtype plants. In these experiments equal or higher numbers of parasitoid adults emerged per transgenic as per wildtype plant. The implications for integrated pest management and the evolution of resistance to Bt in P. xylostella are discussed.