Contemporary evolution in the invasive weed, Pastinaca sativa, after reassociation with its specialist herbivore, Depressaria pastinacella

Background/Question/Methods

Classical biocontrol is frequently used in the management of invasive weeds but how rapidly invasive weeds can evolve in response to their re-associated natural enemies is largely unknown. The wild parsnip, Pastinaca sativa, is a Eurasian species widely distributed across North America as an invasive weed. This plant has been associated with its coevolved European florivore, Depressaria pastinacella, the parsnip webworm, across the U.S. for over 100 years. In 2004, webworms invaded New Zealand (NZ), where wild parsnips have been free from herbivory since their introduction 150 years ago. We predict that reassociation with a specialist herbivore will result in rapid adaptive evolution in the weed resulting in reduced efficacy of the herbivore over time. We set up reciprocal common gardens with wild parsnips originating in the US and in NZ. Seeds from NZ were collected annually between 2006 to 2009 from a total of nine populations that had either never been infested, infested since 2004 or since 2006. Half the garden was sprayed with insecticide to measure the impact of herbivores. We measured fitness, damage as well as floral chemistry.

Results/Conclusions

Our results indicate that herbivores significantly influence the fitness of wild parsnips. NZ plants infested for 6 years have lower proportional damage and higher fitness in comparison to plants that have never been infested. Further we find evidence suggesting that the evolution of tolerance is favored when resources are not limiting. The capacity of weeds to evolve in response to biological control agents should thus be included in the management of invasive weeds.