Genetic variation in plant volatile emission does not result in differential attraction of natural enemies in the field
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Volatile organic chemical (VOC) emission by plants may serve as an adaptive plant defense by attracting the natural enemies of herbivores. For plant VOC emission to evolve as an adaptive defense, plants must show genetic variability for the trait. To date, such variability has been investigated primarily in agricultural systems, yet relatively little is known about genetic variation in VOCs emitted by natural populations of native plants. Here, we investigate intraspecific variation in constitutive and herbivore-induced plant VOC emission using the native common milkweed plant (Asclepias syriaca) and its monarch caterpillar herbivore (Danaus plexippus) in complementary field and common garden greenhouse experiments. In addition, we used a common garden field experiment to gauge natural enemy attraction to milkweed VOCs induced by monarch damage. We found evidence of genetic variation in the total constitutive and induced concentrations of VOCs and the composition of VOC blends emitted by milkweed plants. However, all milkweed genotypes responded similarly to induction by monarchs in terms of their relative change in VOC concentration and blend. Natural enemies attacked decoy caterpillars more frequently on damaged than on undamaged milkweed, and natural enemy visitation was associated with higher total VOC concentrations and with VOC blend. Thus, we present evidence that induced VOCs emitted by milkweed may function as a defense against herbivores. However, plant genotypes were equally attractive to natural enemies. Although milkweed genotypes diverge phenotypically in their VOC concentrations and blends, they converge into similar phenotypes with regard to magnitude of induction and enemy attraction.
KeywordsCommon milkweed Asclepias syriaca Indirect defense Monarch butterfly Danaus plexippus Plant–herbivore interactions Volatile organic chemicals
This work was supported by funding from NSF grants (DEB 0814340 to MDH and IGERT BART to ELW), with additional funding from the Department of Ecology and Evolutionary Biology at the University of Michigan, Ann Arbor. Steve Bertman, Mary Anne Caroll, Huijie Gan, Doug Jackson, Dave Karowe, Andre Kessler, Beth Pringle, Tony Sutterley, Leiling Tao, Shino Toma, Rachel Vannette, Chris Vogel, and all of the staff at the University of Michigan Biological Station provided invaluable advice and support that improved this work. These experiments comply with the current laws of the United States of America, in which the experiments were performed.
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