Plasticity in pollination processes : effects of insect herbivory on cross- and self-pollination in Brassica rapa

How can plants maintain reproduction under stressful conditions? When pesticide use is reduced, insect herbivore presence in agricultural fields may increase. I studied how feeding by herbivorous insects affects various pollination processes in Brassica rapa (field mustard) and its subspecies pekinensis (Chinese cabbage). This study combined field, greenhouse and microscopy studies to better understand the various ways in which insect herbivory affects cross- and self-pollination. Many plant species prefer cross-pollination for reproduction. Cross-pollination mixes genes and prevents inbreeding. However, self-pollination provides reproductive assurance: using own pollen is fast and independent of pollinators. Cross-pollination in Brassica rapa is facilitated by a diverse pollinator community, including bees, hoverflies, and butterflies. The results of field and behavioural studies showed that insect herbivory can alter pollinator attraction, behaviour and community composition. The direction (increased, decreased or maintained) and magnitude of effects depended on the plant genotype as well as pollinator species. Not all pollinators responded the same: some preferred flowers from damaged plants, whereas others preferred flowers from undamaged plants. The effects of herbivory on flower production, seed yield and seed weight were also dependent on plant genotype. To prevent self-pollination, Brassica rapa can recognise self-pollen and subsequently block its germination. With greenhouse and microscopy experiments, I found that under insect herbivory, acceptance of self-pollen increased in one of the tested genotypes. With a large field study, we showed that insect herbivory affects several reproductive traits. Here, we found increased or unchanged outcrossing rates, decreased or unchanged seed siring, and decreased or unchanged plant mate diversity. Seed production and weight were either increased, decreased or maintained compared to control conditions. In conclusion, this thesis shows that insect herbivory alters pollination processes in B. rapa, but that – dependent on plant genotype – this does not necessarily have a negative impact on pollination, reproduction and yield.