Plant species variation in bottom-up effects across three trophic levels: a test of traits and mechanisms

1. An increasing number of studies have addressed the mechanisms by which plant inter-specific variation influence interactions at higher trophic levels, but little is known about the underlying plant traits driving these dynamics. 2. Here we investigated the effects of host plant species on herbivore-parasitoid interactions and the underlying traits driving such effects. For this, we measured the abundance of seed-eating bruchids and their parasitoids across seven sympatric populations of the bean species Phaseolus coccineus and Phaseolus vulgaris in Central Mexico. To investigate the mechanisms underlying differences between bean species in bruchid-parasitoid interactions, we carried out two laboratory experiments to test whether bruchid and parasitoid performance differed between plant species. We also measured seed size and phenolic compounds to investigate if seed traits mediate bruchid-parasitoid interactions by influencing herbivore susceptibility or resistance to parasitoids. 3. Field surveys revealed that the rate of parasitoid recruitment to bruchids was significantly higher onP. vulgaris than onP. coccineus. Subsequent laboratory bioassays indicated that bruchids developed more slowly and exhibited lower fitness on P. vulgaris seeds than on P. coccineus seeds. Accordingly, we found that bean species differed in seed size, with P. vulgaris having smaller (less nutritious) seeds, which explains why bruchid development was slower on this plant species. 4. These results provide a mechanism for why bruchids exhibited higher parasitism rates on seeds of P. vulgaris in the field which could be due to Slow-Growth/ High-Mortality effects, a smaller physical refuge provided by the seed, or both factors. The roles of these mechanisms remain inconclusive without further study.