We investigated indirect defense in the yellow starthistle (Centaurea solstitialis)-grey garden slug (Deroceras reticulatum)-ground beetle (Pterostichus melanarius and Scaphinotus interruptus) system. In this host plant/ herbivore/predator system, the ground beetles are the primary predator of D. reticulatum, the dominant herbivore of the highly invasive weed, C. solstitialis. The aim of our study was to examine the behavioral responses of two species of ground beetle to olfactory stimuli emitted from yellow starthistle damaged by D. reticulatum. The beetle P. melanarius showed a significant preference for the odor of damaged yellow starthistle relative to the odor of intact plants, while S. interruptus did not. Volatiles from D. reticulatum-damaged yellow starthistle were collected and identified as trans-b-farnesene, germacrene D, bicyclogermacrene, and 1,5,9-trimethyl-1,5,9-cyclododecatriene. No quantitative relationship was observed between beetle plant choice or decision time and the level of herbivory. Similarly, there was no relationship between volatile compound relative abundance and level of herbivory, suggesting that our range of leaf damage produces either undetectable semiochemicals or no variation in volatile emission.

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