Why are monarch butterfly populations declining, and how much is it milkweed loss versus overwintering habitat in Mexico

Monarch butterfly populations are declining due to a variety of factors, though scientists continue to study the specific weight of each cause.

• Milkweed Loss: The loss of milkweed is widely recognized as a significant contributor to the decline, particularly due to the widespread use of herbicide-resistant crops in agricultural landscapes across the United States [1, 3].
• Regional Habitat Context: While milkweed loss in the U.S. Midwest is a major concern, research suggests that habitat loss may not be the primary driver in all regions. A 2024 U.S. Geological Survey report indicated that habitat in the migratory corridor (specifically Texas and Mexico) appears relatively intact, with only minimal declines in milkweed and nectar resources observed between 2001 and 2020 [2]. These findings suggest that the migratory journey itself—and the habitat within that corridor—may be more stable than once thought, shifting the focus of the decline toward other factors like pesticide use, which can kill non-target insects and degrade habitats generally [1, 2].

Because scientific findings can vary based on the specific region or timeframe being studied, it is difficult to provide a precise percentage comparing the impact of milkweed loss versus overwintering habitat loss. Current evidence suggests that while agricultural practices in the U.S. are a major pressure point, the dynamics of the decline are complex and involve multiple environmental variables beyond just habitat availability [1, 2].