Grassland disturbance effects on first-instar monarch butterfly survival, floral resources, and flower-visiting insects

Abstract

Many species of conservation concern are disturbance-dependent, relying on periodic ecosystem disruptions to maintain habitat quality. Mounting evidence suggests monarch butterflies are one such organism: they can benefit from growing-season disturbance to grassland habitats in their breeding range, with regenerating stems of milkweed host plants supporting more oviposition and lower densities of arthropod predators. Here we address three questions that were raised by previous work in this system. First, we tested if survival of neonate monarch larvae is enhanced on milkweed stems that regrow after mowing disturbance. Second, we tested if disturbance affects spore densities of the parasite Ophryocystis elektroscirrha (OE) on milkweed leaves. Finally, we documented effects of disturbance on the abundance of floral resources and flower-visiting insects. We found that first-instar monarch survival over 48 h periods was 2.3–2.5 times higher on regenerating milkweed stems than on undisturbed controls. OE spores were not detected on any of the milkweed stems in our study. Disturbance reduced floral resource abundance and floral visits for 3–5 weeks, although some species that were initially suppressed bloomed later in the season with the net effect of extending the bloom period. Our results show grassland disturbance can enhance survival of immature monarchs and could be used strategically to help stabilize the eastern monarch population. More work is needed to understand how disturbance in this system affects resources for pollinators and to optimize habitat management for monarchs and the broader pollinator community.