Consequences of monarch damage and plant genotype for ant-aphid interactions on the common milkweed Asclepias syriaca

Background/Question/Methods

Ant-aphid interactions provide a well-studied and canonical example of protection mutualisms. There is growing evidence for how these interactions can be influenced by the plants upon which they occur. In turn, plant effects on ant-aphid interactions may feed back to determine levels of herbivory and plant performance. Accordingly, understanding plant influences over ant-aphid interactions is central to predicting under what circumstances ant-tending aphids are beneficial or costly to plants. I will review the results of experiments from several plant systems (Asclepias syriaca, Baccharis salicifolia, Epilobium angustifolium, Valeriana edulis) that investigate the influence of plant genetics on ant-aphid interactions at three hierarchical scales: First, the influence of plant sex in dioecious species, thus investigating the effects of variation in sex determination genes alone.  Second, the influence of intraspecific plant genetic variation, thus investigating the effects of variation in whole plant genomes. And finally third, intraspecific genetic diversity, thus investigating the effects of plant genetic environment.

Results/Conclusions

First, comparing ant-aphid interactions between male and female plants, plant sex influences aphid abundance and rates of ant tending of aphids in multiple plant-aphid-ant systems, thus demonstrating the influence of a relatively small set of sex determination genes upon arthropod community structure and interspecific interactions at higher trophic levels.  Second, aphid abundance and rate of ant tending of aphids varies based upon plant genetic variation, with the effects of ants on aphids ranging from mutualistic to parasitic based upon both plant genotype and whether or not plant defenses were induced by previous feeding damage. Third, genetic variation within patches of coexisting plants is positively associated with aphid abundance and rates of ant tending. Synthesis of these results demonstrates that the strength of plant genetic influences over ant-aphid interactions strengthens monotonically across this hierarchical scale. In addition, aphid-tending ants reduce the density of other, untended herbivores. As a consequence, plant genetic influences over rates of ant tending mediates the relative costs and benefits of ant-aphid mutualisms for the host plants.