Tritrophic interactions in wild and cultivated brassicaceous plant species.

Plants have evolved a range of defence traits that prevent or reduce attack by insect herbivores. Direct defence traits hamper or reduce the performance and behaviour of the herbivores, whereas indirect defence promote the efficiency of natural enemies to attack the herbivores. Here, I focused on chemical plant defences, both direct and indirect, in brassicaceous plant species. Glucosinolates (GS), which are secondary plant compounds characteristic for the Brassicaceae, may not only affect growth and survival of herbivores, they may also reduce the performance of the herbivore’s natural enemies such as parasitoids. Brassicaceous plant species, like most plant species, emit volatile organic compounds in response to herbivore feeding damage. These herbivore-induced plant volatiles (HIPV’s) can be used by parasitoids of these herbivores to locate a host-infested plant. The production of HIPV’s is a form of chemical indirect defence and is also studied here. The Brassicaceae family contains important crop plants such as cabbages and oilseeds. However, through artificial selection, the chemistry in crop plants may have been modified compared with their wild conspecifics in which defence traits are the result of natural selection. I compared the performance of herbivore and parasitoid species that differed in their food plant, respectively host specialisation, on several cultivated and wild conspecific plant populations. The specialist herbivore-parasitoid systems were not affected by the relatively high GS concentrations in leaf tissues of three mustard species (both wild and cultivated populations). However, the performance of even a specialist herbivore and its parasitoid differed on wild populations of B. oleracea compared to their performance on a B. oleracea cultivar. Development of a generalist herbivore and its parasitoid were even more strongly affected by differences in quality among the wild B. oleracea populations. Differences in plant quality affected not only pupal/adult mass and development time, as was found for the specialists, but also strongly reduced survival. Whether artificial selection has resulted in chemical changes with concomitant effects on the performance of their associated insects depends on the crop trait that has been selected for. Direct and indirect plant defences are usually investigated in separate studies. However, a conflict between direct and indirect defences may arise when a parasitoid female is attracted to a plant on which the development of her offspring is compromised. I studied the performance of two different parasitoid species on three different mustard species and also recorded their attraction to volatiles from host-infested plants. No conflicts between direct and indirect defences were revealed. Parasitoid females were attracted to plants on which their offspring was not negatively affected by the food plant quality of the host. Many studies comparing parasitoid attraction to HIPV’s are conducted in Y-tube olfactometer or windtunnel experiments. However, under natural conditions parasitoids have to search for hosts in heterogeneous environments with different plant species that can be attacked by several herbivores. I observed parasitoid foraging behaviour in a slightly more complex set-up. Female D. semiclausum took more time to find a single host-infested B. oleracea plant in a set-up consisting of B. oleracea plants interspersed with a second brassicaceous plant species, S. alba, than in a monospecific set-up with only B. oleracea plants. Thus, B. oleracea plants may provide the herbivore with a chemical refuge against parasitism in the presence of highly attractive S. alba plants. Comparison of the foraging behaviour and population dynamics of hosts and their parasitoids in managed agricultural and unmanaged natural ecosystems may provide new insights into the mechanisms underlying tritrophic interactions and may help to reveal the importance of bottom-up and top-down control of arthropod herbivores.