Consequences of host plant toxicity across monarch butterfly ontogeny: milkweed challenges from feeding to flying

The decline in North American populations of the Monarch butterfly has been linked to the loss of their milkweed host plant along US and Mexican migration routes. This has resulted in significant milkweed replanting efforts by national initiatives and local enthusiasts. While meritorious in its conception, these replanting efforts tend to use a variety of milkweed species that can vary dramatically in their level of toxicity to Monarch larvae. Moreover, it is unknown how exposure to such varying levels of toxicity as larvae may affect the ability of surviving adult Monarch butterflies to establish viable populations and migrate to Mexican overwintering sites. Using NSF funding and leveraging a large-scale collaborative network of Monarch scientists, the researchers will quantify such effects by examining milkweed-Monarch interactions throughout their life history, under both controlled laboratory- and natural settings. In addition to Monarchs, this project will generally inform habitat restoration efforts aimed at offsetting population declines in other beneficial insects, as consequences of larval dietary history for adult traits important to population health are poorly understood in most insects.

Chemical ecology research on plant-insect herbivore interactions has abundantly focused on the proximate effects of plant defenses and toxicity on insect larval stages. Since insect larvae tend to cause most damage to plants, this focus makes sense, but as a result only limited information is available on indirect, i.e., beyond the larval stages, effects of host plant toxicity on adult insect traits important for population growth and dispersal. The proposed work will address this gap in knowledge by linking variation in plant defense biochemistry experienced by larvae to adult insect fitness outcomes. Using the iconic Monarch butterfly as a focal species, the PIs aim to quantify how variation in toxic traits among ten milkweed species (i.e., Monarch host plants) affect larval growth, adult fecundity and migratory ability, in laboratory and natural contexts. To achieve this aim, they will use an integrative approach that combines state-of-the-art methods in chemical ecology, organismal energetics, biomechanics, and leverage a citizen science driven collaboration with several laboratories located along the annual Monarch's migration routes. This work is pertinent to the focal species as replanting efforts aimed at restoring suitable habitat to combat Monarch population declines use multiple milkweed species without consideration for how larval exposure to varying milkweed defensive chemistry affects Monarch population dynamics and migration. In parallel, this project will generally inform science and habitat restoration efforts aimed at offsetting population declines in other beneficial insects, as consequences of larval dietary history for adult traits are poorly understood in most insects.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.