Density-Dependent Effects of Simultaneous Root and Floral Herbivory on Plant Fitness and Defense

Plants are attacked by multiple herbivores, and depend on a precise regulation of responses to cope with a wide range of antagonists. Simultaneous herbivory can occur in different plant compartments, which may pose a serious threat to plant growth and reproduction. In particular, plants often face co-occurring root and floral herbivory, but few studies have focused on such interactions. Here, we investigated in the field the combined density-dependent effects of root-chewing cebrionid beetle larvae and flower-chewing pierid caterpillars on the fitness and defense of a semiarid Brassicaceae herb. We found that the fitness impact of both herbivore groups was independent and density-dependent. Increasing root herbivore density non-significantly reduced plant fitness, while the relationship between increasing floral herbivore density and the reduction they caused in both seed number and seedling emergence was non-linear. The plant defensive response was non-additive with regard to the different densities of root and floral herbivores; high floral herbivore density provoked compensatory investment in reproduction, and this tolerance response was combined with aboveground chemical defense induction when also root herbivore density was high. Plants may thus prioritize specific trait combinations in response to varying combined below- and aboveground herbivore densities to minimize negative impacts on fitness.

Automated summary

Plants have to regulate their responses to multiple herbivores to deal with a wide range of threats. In particular, they often face simultaneous root and floral herbivory, but few studies have focused on this interaction. This study investigated the combined density-dependent effects of root-chewing beetle larvae and flower-chewing caterpillars on the fitness and defense of a semiarid Brassicaceae herb. The results showed that the impact of both herbivore groups on plant fitness was independent and density-dependent. Increasing root herbivore density had a non-significant effect on plant fitness, while increasing floral herbivore density had a non-linear negative effect on seed number and seedling emergence. Plant defensive response was non-additive and varied with the densities of root and floral herbivores, showing compensatory investment in reproduction at high floral herbivore density and aboveground chemical defense induction at high root herbivore density.