The Mariana eight-spot butterfly (Hypolimnas octocula marianensis) and the Mariana wandering butterfly (Vagrans egistina) are endemic to the Mariana Islands and both have recently been listed as federally protected under the U.S. Endangered Species Act. We conducted both targeted surveys and grid transect searches on Guam and targeted surveys across the islands of Saipan, Tinian and Rota to locate native hostplants, eggs, caterpillars and adults of these rare insects. Despite having a relatively common hostplant, V. egistina was not found during this study, even in places it had last been found. Although various life stages of H. o. marianensis were documented, the hostplants of this species were heavily impacted by grazing of introduced ungulates; only in native limestone forest with jagged tower karst topography, did hostplants persist, and the butterfly with them. The reasons for the disappearance of V. egistina are not clear, but do not appear to be related to hostplant availability. Our surveys suggest that control of invasive ungulates on Guam will be the single most effective conservation action for Mariana eight-spot recovery, and is essential to any chance of long-term persistence for the species there. H. o. marianensis has significant, unoccupied, hostplant patches on the three northern islands (Rota, Tinian, and Saipan) which might be considered as sites for future translocation and reintroduction efforts, in order to reduce the risk of extinction.