Plant genotype, not nutrients, shape aphid population dynamics

Background/Question/Methods

Within species variation can shape plant-insect interactions, but little is known about whether nutrient availability may mediate the effects of within species variation on the population dynamics and community structure of associated insects. Here, we report on an experiment designed to assess the combined and relative effects of nutrients and intraspecific variation of Solidago altissima, a dominant old-field plant species, on aphid population dynamics. Specifically, we asked: 1) What are the effects of within-species variation and nutrients on aphid population dynamics in the presence and absence of predators and parasitoids? 2) Are there particular plant traits that covary with genotype and influence aphid population dynamics? We propagated 20 Solidago genotypes in a common garden environment consisting of ninety-two m² plots, each containing six Solidago individuals in either a monoculture (all six individuals of the same genotype) or a mixture (all six individuals of six different genotypes). We established four nutrient treatments: control, soil nitrogen [N] addition, soil phosphorus [P] addition, and soil N and soil P addition. We randomly selected two individual Solidago ramets within each plot colonized by aphids, controlling for predators by caging one ramet and leaving the other uncaged, and tracked aphid abundance weekly for seven weeks.

Results/Conclusions

We found that aphid abundance differed significantly among Solidago genotypes, but only in the absence of their predators. We found no effect of nutrients on aphid abundance (P>0.05). Additionally, plant genotype, not nutrient manipulations, influenced plant traits such as flowering phenology, leaf damage by other herbivores, and leaf toughness (P<0.03 in all cases). These traits accounted for significant variation in aphid population size over the course of the experiment. Our results suggest that intraspecific variation can indirectly affect herbivore population dynamics through the mediation of plant traits associated with susceptibility to herbivory, and, at least in this experiment, outweigh the effects of nutrient manipulations.