Induced resistance to seed predators via leaf herbivory: Implications for individual plants and genotypically diverse patches

Background/Question/Methods

While induced resistance is commonly observed between foliage-feeding herbivores, little is known regarding plant-mediated interactions between foliage-feeders and herbivores that consume reproductive tissues.  Furthermore, while most studies examining induced resistance exclusively consider individual plants, associational resistance/susceptibility with neighboring plants often modifies herbivory on a focal plant.  Here, we test how individual plant responses and genetic variation within a patch of plants influences the interactions between a foliage-feeding herbivore and three seed predators.  First, we conducted surveys of animal abundance and damage by the leaf chewing Japanese beetle (Popillia japonica) and three Lepidopteran seed predators (Schnia florida, Mompha stellella, and Mompha brevivittella) in natural populations of the common evening primrose (Oenothera biennis).  Second, we conducted a manipulative field experiment to test for induced resistance between P. japonica and the dominant seed predator on O. biennis, S. florida.  Third, we manipulated patch-level genotypic diversity of O. biennis to test how associational resistance/susceptibility modifies consumptive relationships between P. japonica and seed predators.

Results/Conclusions

In our surveys of natural O. biennis populations we found a negative relationship between P. japonica leaf damage and seed predation by the two moths that oviposit on flower buds (S. florida and M. stellella), while the seed predator that oviposits on fruits (M. brevivittella), showed no relationship with P. japonica leaf herbivory.  The concentration of jasmonic acid (JA) in flower buds was higher in plants with previous P. japonica leaf herbivory, while JA concentration did not differ in fruits of beetle damaged vs. undamaged plants.  In the manipulative field experiment, we found that S. florida oviposited 62% fewer eggs on plants that had P. japonica leaf damage vs. controls, and consumed 30% more fruits and 115% more flower buds on control plants.  Finally, in genetically diverse patches of O. biennis, we found that leaf consumption by P. japonica was reduced by 21% while seed predation by S. florida and M. stellella increased by 108% and 50% compared to monocultures.  We show evidence of diversity-mediated associational resistance to P. japonica, and speculate that this associational resistance leads to less induced resistance to seed predators in genotypically diverse patches of O. biennis.