The sequestration of plant secondary metabolites by herbivores for their own defense can vary across both hostplant phenology and herbivore ontogeny. How both change through time has rarely been explored. From the herbivore’s perspective, variation in hostplant chemistry may result in dose-dependent levels of sequestration and potentially an upper threshold above which additional sequestration is not possible. From the plant’s perspective, herbivore damage can induce secondary metabolite production to resist herbivores but can also induce susceptibility to future herbivory. Few studies have studied the relationship between temporal variation in hostplant chemistry and sequestration by herbivores. Here we explored variation in defensive chemistry in white turtlehead, Chelone glabra, and sequestration by its specialist herbivore, the Baltimore checkerspot, Euphydryas phaeton, across the development of both the herbivore and its hostplant. In 2012 we sampled hostplants to describe seasonal variation in the concentrations of two iridoid glycosides, aucubin and catalpol. In 2017, we sampled both hostplants and caterpillars over an entire growing season and explored the relationship between hostplant chemistry and herbivore sequestration. In addition, we compared iridoid glycoside concentrations of plants with and without herbivory to gain insight into whether sequestration was the result of herbivore choice or plant response to herbivory.
Results/Conclusions
Overall, we found that plant defensive chemistry varied greatly across the season and was a strong predictor of sequestration by herbivores. In 2012, C. glabra iridoid glycoside concentrations increased more than three-fold early in the season, reaching 8% of leaf dry mass by mid-June and eventually declining by late summer. In 2017, plants followed a similar seasonal pattern and the total concentration of sequestered iridoid glycosides in E. phaeton caterpillars closely mirrored the total concentration of iridoid glycosides in their hostplants. However, the magnitude of sequestration by caterpillars was extremely variable, ranging from two to 20 times the concentrations of hostplants. Moreover, caterpillars exhibited different compositions of the two compounds from hostplants, sequestering a relatively higher proportion of catalpol than that contained in hostplants in May and a relatively lower proportion than hostplants in August. In addition, plants with herbivory had lower iridoid concentrations than plants without herbivory, although the difference was higher in May than in June and the mechanisms driving this result are still unclear. In general, these results suggest that while variation in hostplant chemistry is a dominant factor driving sequestration by herbivores, physiological or ecological factors may affect the relationship between hostplant chemistry and herbivore sequestration.