# Eastern North American Monarch Butterfly Conservation Needs and Opportunities: What the Science Tells Us

## TL;DR

Monarch butterflies in eastern North America are declining due to habitat loss, herbicide use, and climate change, but conservation efforts like restoring milkweed and nectar plants can help stabilize their populations.

## Contribution

The paper identifies key drivers of monarch decline and emphasizes practical conservation strategies to enhance habitat availability and resilience.

## Key findings

- Monarch declines are driven by habitat loss from herbicide use, climate change, and insecticides.
- Restoring milkweed and nectar plants in diverse habitats improves monarch survival and resilience.
- Even small habitat patches can significantly support monarch populations if they include native milkweed.

## 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 east of the Rocky Mountains, and ways to achieve population sustainability. One key driver of monarchs’ decline is the loss of breeding habitat, caused mainly by the loss of their milkweed host plants in agricultural fields after widespread adoption of genetically modified, herbicide tolerant corn and soybeans and the associated increase in herbicide use. Weather is another driver of monarch population numbers, and climate modeling suggests that warmer and drier conditions in the future could push monarchs farther north or simply lead to lower numbers. The growing use of insecticides to control insect pests has also been implicated in declining monarch numbers. Making habitat broadly available to monarchs will make them more resilient, and better able to survive weather-driven declines. While large tracts of high-quality land will be needed to achieve a sustainable monarch population, even small tracts of land can provide needed habitat. Widespread availability of a diversity of native milkweed species and nectar plants are important components of quality monarch habitat.

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.