Endure and call for help: strategies of black mustard plants to deal with a specialized caterpillar

1. Plants have evolved inducible resistance and tolerance mechanisms against insect herbivores. Resistance mechanisms that affect herbivorous insects directly can be effective against generalist herbivores, but will not deter specialist herbivores from attacking the plant. Tolerance mechanisms and indirect plant resistance are more likely effective strategies used by plants when dealing with specialist herbivores. However, inducible indirect resistance and tolerance mechanisms have rarely been investigated within the same study system. 2. We studied multiple tolerance mechanisms and the role of natural enemies in reducing fitness loss of Brassica nigra plants incurred by feeding by the specialist herbivore Pieris brassicae. For this, we measured the changes in carbon and nitrogen triggered by herbivore attack and quantified plant biomass and seed production under field conditions, in the presence or absence of natural enemies of the herbivores. We also assessed whether B. nigra increased selfing rates when exposed to herbivore infestation, and investigated whether infestation by P. brassicae rendered B. nigra plants more attractive to night-active pollinators than control plants. 3. We found that B. nigra flowers are rarely visited by insects during the night, and exposure to herbivores did not influence selfing rates. Brassica nigra plants compensated for herbivory in terms of vegetative biomass. Seed set was negatively affected by herbivory in the absence of natural enemies, but not in the presence of natural enemies. Plants responded to herbivory with drastic changes in nitrogen contents of leaves and flowers, whereas no changes in carbon concentrations were detected. 4. The investment in reproduction or re-growth of vegetative tissues is not sufficient to sustain plant fitness. Reproductive output of flowering mustard plants is only sustained when interactions with the natural enemies of the herbivores are preserved. We conclude that natural enemies of herbivorous insects play an important role as component of the plant's defence strategy. This study reveals that both tolerance and indirect resistance are strategies of this plant species to sustain fitness.