Abstract
Monarch butterfly numbers in North America have been declining since the early 2000s. This review focuses on the causes of this decline in the eastern migratory population, found roughly east of the Rocky Mountains, and ways to achieve population sustainability. Drivers of eastern North American monarchs' decline include (1) the loss of breeding habitat, caused mainly by the loss of their milkweed host plants in agricultural fields after adoption of genetically modified, herbicide tolerant corn and soybeans, (2) weather, particularly warmer and drier conditions that could push monarchs farther north and lead to lower numbers, and (3) increasing insecticide use. Making habitat broadly available to monarchs across their migratory cycle will make them more resilient, and better able to survive weather-driven declines, and even small tracts of land can provide needed habitat. Sites that are isolated from other habitat patches contain higher egg, larva, and adult monarch densities. Good monarch habitat should include a diversity of native milkweed species and nectar plants that will thrive in local growing conditions. Monarch egg and larva density increases with increasing milkweed density and monarch survival is higher when larvae are less crowded, so a good general rule is that more milkweed is better.