Taylor’s checkerspot, Euphydryas editha taylori, is a rare and federally endangered butterfly native to Pacific Northwest coastal prairies. Active programs work to restore native prairie habitat for E. e. taylori, while a parallel program rears butterflies in captivity and reintroduces them to the restored habitat. However, despite successful establishment of dense E. e. taylori larval host and nectar resources at reintroduction sites, growth of reintroduced populations has been unsatisfactory. We sought to determine how E. e. taylori adult females utilize restored habitat during oviposition, a crucial life history event. We surveyed for egg masses in 100 plots at two reintroduction sites in 2016. We quantified density of two alternative larval host plants (native and non-native), availability of bare ground used for basking, and density of plants likely to be used as sheltering resources in inclement weather. We also monitored plot air temperatures and counted flowering stalks of nectar-providing forbs in wider areas centered on plots.
We documented 17 E. e. taylori egg masses, all of which occurred at one site. No egg masses occurred in plots at the other site, despite observations of adults in flight. Eleven of the egg masses were found on the non-native host plant. Probability of an oviposition increased significantly as density of the non-native larval host plant increased. There was no clear relationship between oviposition probability and density of the native host; we also did not detect any effect of ground-level basking area, density of sheltering plants, or nearby nectar abundance and composition on oviposition probability. Temperatures likely to be experienced by larvae varied across plots, but we did not detect strong effects of microclimate on oviposition choices. With respect to oviposition, the most important feature of restored habitat for E. e. taylori is density of host plants, particularly the non-native host. Since density of the non-native host plant is generally high in restored habitat, lack of oviposition sites is probably not limiting, and other explanations must be found for the slow recovery of reintroduced populations.