When herbivorous insects are rare or endangered, understanding their interactions with host plants can be crucial to success of conservation efforts. Plants differ in their phenology, nutritional value, and secondary chemistry, both within and among species. Importantly, insects’ sensitivities to these differences can differ from instar to instar, since their feeding habits and mobility change ontogenetically.
Taylor’s checkerspot (Euphydryas editha taylori) is an endangered butterfly endemic to grasslands in Pacific Northwest North America. It feeds on harsh paintbrush (Castilleja hispida), which was also used historically, but has also added a novel host, plantain (Plantago lanceolata) to its diet. It also uses golden paintbrush (Castilleja levisecta) at least occasionally, but the history of interactions with this last species is unknown. Both adults and larvae sequester iridoid glycosides from their hosts as a defense against predators. Eggs are laid in clusters, and caterpillars feed as a group.
We placed egg clusters of different sizes on each host species (126 clusters total) and tracked caterpillars from hatching until diapause, after four instars. We quantified survival between instars, to test if predictors of survival differed by growth stage. We also measured iridoid glycoside concentrations when they reached fourth instar.
Results/Conclusions
Caterpillar survival differed by host species and was highest on plantain, intermediate on harsh paintbrush, and lowest on golden paintbrush. Survival differences were strongest during the transition from second to third instar, and did not differ strongly at other stages.
When caterpillars fed on golden paintbrush, survival to second and third instar was limited by plant senescence, with caterpillars dying as plants became necrotic. Larger groups had higher survival to third instar when they ate plantain, but not the other two species, suggesting use of the novel host could select for larger egg clutches.
Both the amounts and kinds of iridoid glycosides sequestered by the caterpillars varied depending on which host plant they used. Individuals that ate golden paintbrush contained substantially lower levels of iridoid glycosides than those feeding on the other plant species, perhaps low enough to fail to deter some predators.
This study illustrates how differences among host plants affect specialist herbivore survival and chemical interactions. By assessing survival of individual instars, we were able to pinpoint effects of these differences on caterpillars as they developed. Our results indicate that this Taylor’s checkerspot population can perform better on its novel host than on the native paintbrush host plants.