Immature monarch survival: Effects of site characteristics, density, and time

The eastern North American monarch population is declining, as evidenced by the area occupied by overwintering adults. Recently, decreasing availability of breeding habitat has been most strongly implicated in this decline. An alternative, nonexclusive explanation for the recent population decline is decreasing survival. We used 18 yr of data from the Monarch Larva Monitoring Project, a citizen science program, to determine immature monarch mortality rates over time as well as factors associated with increased mortality. Our data included field measures of mortality from egg to the final larval instar, and mortality due to parasitoids and other causes, assessed by rearing field-collected monarchs. Average egg to fifth-instar survival ranged from ∼7 to 10% across all regions. Survival from fifth instar to adult ranged from ∼60 to 90%, although this overestimates survival because monarchs are not exposed to many mortality factors when reared indoors. Both survival rates showed a great deal of temporal and spatial variation. Survival tended to be higher in sites that were planted and had more milkweed plants. There was a negative effect of per plant egg density on survival, suggesting density dependence. Survival rates appear to be declining from 1997 to 2014, and we discuss possible reasons for this pattern. Finally, we estimate that across all years in the north-central United States, where we have the most data, a minimum number of ∼29 milkweed plants are required to produce an adult monarch that will be part of the fall migratory generation.