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

Abstract

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.

Preference, performance, and chemical defense in an endangered butterfly using novel and ancestral host plants

“…Host plant affinities are multidimensional and can include female decisions of where to oviposit, innate neonate preference for where to forage, and larval performance when feeding on one host plant or another. Previous work with this population suggests that females prefer to oviposit on either Castilleja species over Plantago 34 , but that survival rates of early-instar larvae in the field are highest on Plantago, intermediate on C. hispida, and lowest on C. levisecta 32 . Plantago and Castilleja spp.…”

“…We would have interpreted a significant correlation between preference for, and performance on, a given host species as evidence that subsets of the population had evolved different host plant specializations, but this possibility was not supported. However, the ordering of oviposition preference (Plantago > C. hispida > C. levisecta) did match the frequency of interaction Taylor's checkerspot from this region probably had with these species over the last several decades 29 , and their survival rates on them in the field 32 . In the field, both Castilleja species senesce before Plantago and early-instar larvae die in especially high numbers on C. levisecta as it begins to senesce in May and June 32 .…”

“…At this site, butterflies oviposit and larvae feed primarily on Plantago, but occasionally on C. hispida as well. Butterflies were collected from the field in 2016 and allowed to oviposit in the lab; the eggs were then deployed for a field study 32 . In late winter 2017, we collected surviving larvae from the field study after they emerged from diapause.…”

“…Plantago can be very abundant in disturbed herbaceous habitats. In the case of Taylor's checkerspot it is not only very easy to find but is also more phenologically available than native hosts because it senesces later 32 . Finally, the iridoid glycosides that Plantago produces, aucubin and catalpol, are already present in both Castilleja species investigated here, indicating that shifting from Castilleja to Plantago does not require metabolic adjustments as might be needed if a herbivore encountered a novel host containing new IG compounds.…”

“…In addition to being generally more abundant than native hosts, its phenology is also more favorable for larvae as Castilleja spp. can senesce quickly in dry conditions 32 . While Plantago cannot have been an ancestral host plant for Taylor's checkerspot, it was the first documented host and some populations used it as early as the late nineteenth century 33 .…”