While secondary chemistry has long been recognized as a key component of plant defense against herbivores, our understanding of the chemical details has been well developed only in a restricted set of model systems.  Here, we report on chemical variation and herbivore resistance in a widespread annual weed, Xanthium strumarium. We conducted a common garden experiment over the summer of 2008 in Eastern Texas.  We asked the question, does chemical variation in X. strumarium influence patterns of herbivore resistance in the field?  Using a common garden approach, we planted 50 seedlings from each of 17 source populations.  Over the course of the summer we scored herbivory three times by visually estimating leaf damage on a sub-sample of leaves.  Sesquiterpene lactone profiles of different source populations were compared using High Performance Liquid Chromatography (HPLC) coupled to UV and Mass Spectrometer detectors. 

Results/Conclusions

Populations differed dramatically in levels of damage from leaf chewing insects.  Mean damage levels per source population ranged from 0.61% damage to 24.17%, almost a 40 fold difference.  Qualitative chemical analyses revealed populations differed in the stereochemical conformation of the lactone ring junction.  This simple chemical marker is highly correlated with damage levels, with plants of one conformation receiving over seven times more damage than the other (1.74% vs. 12.35%). We are currently conducting laboratory feeding trials and more comprehensive chemical analyses to further elucidate this pattern.  Ongoing work is examining the generation of chemical novelty following natural plant hybridization and the consequences for resistance and adaptation.