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Ecol Lett. 2011 May;14(5):453-61. doi: 10.1111/j.1461-0248.2011.01604.x. Epub 2011 Mar 7.

Aphids indirectly increase virulence and transmission potential of a monarch butterfly parasite by reducing defensive chemistry of a shared food plant.

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  • 1Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA.


Parasites and hosts live in communities consisting of many interacting species, but few studies have examined how communities affect parasite virulence and transmission. We studied a food web consisting of two species of milkweed, two milkweed herbivores (monarch butterfly and oleander aphid) and a monarch butterfly-specific parasite. We found that the presence of aphids increased the virulence and transmission potential of the monarch butterfly's parasite on one milkweed species. These increases were associated with aphid-induced decreases in the defensive chemicals of milkweed plants. Our experiment suggests that aphids can indirectly increase the virulence and transmission potential of monarch butterfly parasites, probably by altering the chemical composition of a shared food plant. These results indicate that species that are far removed from host-parasite interactions can alter such interactions through cascading indirect effects in the food web. As such, indirect effects within ecological communities may drive the dynamics and evolution of parasites.

© 2011 Blackwell Publishing Ltd/CNRS.

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