The Eastern migratory monarch butterfly population has declined in recent decades, in part because use of herbicide-resistant crops has eliminated their milkweed host plants from crop fields. Monarchs now rely mostly on milkweeds in grassland habitats, representing a potentially important habitat shift. We conducted several experiments to better understand the implications of this change. First, we deployed and monitored monarch eggs in different habitats. We found predation rates are highly variable but can exceed 80% and tend to be higher in perennial grasslands than they are in crop fields. Using laboratory trials and field observations, we found arthropod predators of monarch butterflies are more diverse than previously recognized and identified 36 new taxa from 11 families that readily eat them. We also found that most predation events on monarch eggs occur at night rather than during the day. Finally, we mechanically disturbed milkweed patches during the growing season and measured effects on monarch oviposition, survival, and on the predator community. After disturbance, milkweed sends up new growth which monarchs prefer to lay eggs on. These stems contain fewer arthropod predators, and newly hatched monarch larvae are more likely to survive here than on older stems. Taken together, the results of these studies suggest grasslands differ from crop fields as monarch habitat in multiple regards, and that grassland monarch habitat can be enhanced by low-intensity disturbance during the growing season.
Additional Contributors: Andrew Myers, Sara Hermann, Douglas Landis