Plant Genotype Shapes Ant-Aphid Interactions: Implications for Community Structure and Indirect Plant Defense
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Mooney, Kailen A.; Agrawal, Anurag A.
Little is known about the mechanisms by which plant genotype shapes arthropod community structure. In a field experiment, we measured the effects of milkweed (Asclepias syriaca) genotype and ants on milkweed arthropods. Populations of the ant?tended aphid Aphis asclepiadis and the untended aphid Myzocallis asclepiadis varied eight? to 18?fold among milkweed genotypes, depending on aphid species and whether ants were present. There was no milkweed effect on predatory arthropods. Ants increased Aphis abundance 59%, decreased Myzocallis abundance 52%, and decreased predator abundance 56%. Milkweed genotype indirectly influenced ants via direct effects on Aphis and Myzocallis abundance. Milkweed genotype also modified ant?aphid interactions, influencing the number of ants attracted per Aphis and Myzocallis. While ant effects on Myzocallis were consistently negative, effects on Aphis ranged from antagonistic to mutualistic among milkweed genotypes. As a consequence of milkweed effects on ant?aphid interactions, ant abundance varied 13?fold among milkweed genotypes, and monarch caterpillar survival was negatively correlated with genetic variation in ant abundance. We speculate that heritable variation in milkweed phloem sap drives these effects on aphids, ants, and caterpillars. In summary, milkweed exerts genetic control over the interactions between aphids and an ant that provides defense against foliage?feeding caterpillars.
The American Naturalist
This work was funded by National Science Foundation grant DEB?0447550, the Biogeochemistry and Biocomplexity Initiative at Cornell University, and the University of California, Irvine, School of Biological Sciences.
University of Chicago Press
ant-aphid mutualism; indirect defense; plant genetic effects; community ecology
Mooney, K. A., & Agrawal, A. A. (2008). Plant Genotype Shapes Ant?Aphid Interactions: Implications for Community Structure and Indirect Plant Defense. The American Naturalist, 171(6), E195–E205.