Instar-specific effects of host plants on survival of endangered butterfly larvae

1. Outcomes for butterfly conservation can hinge on interactions with host plants during early larval instars. Ontogenetic changes in larvae may cause predictors of survival to shift quickly over time. 2. Survival from instar to instar was measured for an endangered oligophagous butterfly, Euphydryas editha ssp. taylori (Taylor's checkerspot), which was experimentally released in the field on three host plants: Castilleja levisecta, Castilleja hispida, and Plantago lanceolata. Survival rates were quantified, from hatching to the second instar, from the second to the third instar, and from the third to the fourth instar. 3. This study tested whether host plant characteristics (degree of senescence and anthocyanin pigmentation) affected survival during each instar; the effects of oviposition timing and larval group size on survival were also tested, as these vary by group and could affect outcomes for larvae differently on different hosts. 4. Survival rates depended on the host species being consumed, mostly because of a disparity in survival during the transition from the second to the third instar. Survival was lowest on C. levisecta, intermediate on C. hispida, and highest on P. lanceolata. 5. Plant senescence, oviposition timing, and group size were predictors of survival, but their relevance depended on both the host plant being used and the instar being considered. 6. Host plant suitability can vary strongly both within and among species, changing quickly with caterpillar ontogeny during early instars.