The Challenge of Monarch Conservation

Monarch butterfly declines mirror a broader trend of insect losses globally. Insects provide essential ecosystem services—including pollination, pest control, and nutrient cycling—and form the base of many food webs. In economic terms, insect pollination services in the U.S. alone were valued at $34 billion in 2021, and global animal pollination service estimates were between $235-$577 billion. Yet roughly 20% of North American pollinator species are at risk of extinction, and butterfly abundance in the U.S. dropped 22% between 2000 and 2020.

Some monarchs travel up to 3,000 miles and can fly 50-100 miles a day!

USGS

Mounting evidence points to widespread insecticide use—especially neonicotinoids—as a key contributor to these declines, both in North America and Europe. While more field-based, data-driven research is needed, the convergence of findings across studies supports a broad conclusion: monarch conservation is unlikely to be achieved by habitat restoration alone. Pesticide exposure must be directly addressed if monarch populations are to recover and persist.

Following the Science

U.S. Geological Survey scientists and a graduate student at the Missouri Cooperative Fish and Wildlife Research Unit at the University of Missouri partnered with the Missouri Department of Conservation to conduct a literature review of the direct effects of grassland management practices and pesticide exposure on monarchs in North America. Their findings reveal knowledge gaps in understanding direct effects of grassland management activities such as mowing, burning, grazing, and haying on monarch butterfly survival, reproduction, and development.

The review also synthesized a growing body of scientific evidence indicating pesticides may be directly associated with reduction in monarch fitness and survival. Habitat loss and pesticide use are linked to significant declines in butterfly abundance and diversity across North America. Despite extensive conservation efforts, including widespread habitat restoration and milkweed planting, there is little evidence that monarch populations are recovering. Instead, the latest counts show that the eastern monarch population has settled at a ‘new normal’ level of about 8 acres, which is lower than the nearly 15 acre goal identified to prevent further population decline.

Monarch responses to pesticide exposure vary depending on life stage, chemical class, application method, and proximity to treatment areas. However, 70% of studies evaluating pesticide effects on monarchs reported negative effects on at least one aspect of monarch fitness. Laboratory tests show that short-term exposure to commonly used insecticides—including organophosphates, pyrethroids, and diamides—can lead to a significant increase in the death rate among monarch larvae. Neonicotinoids are an insecticide of particular interest because they are widely used in agroecosystems and can kill in two main ways—by stopping molting or by causing central nervous system toxicity at higher doses.

Chronic exposure to harmful chemicals can also cause sublethal effects in monarchs, such as slower larval growth, shorter lifespan, lower flight energy, altered reproductive behaviors, and delayed development. These changes have the potential to significantly reduce monarch fitness and long-term survival.

Measuring real-world pesticide exposure is challenging, as pesticides can drift and contaminate conservation lands and restored habitat far from the initial application location. This broad geographic exposure can complicate habitat conservation and restoration efforts and calls into question assumptions that habitat alone is sufficient to ensure resilient monarch populations.

A recent population model estimated that entire generations of monarchs could be lost near agricultural fields, but indicated the broader metapopulation might still persist. However, this model did not account for chronic or sublethal pesticide effects, which could reduce monarch population viability. Additionally, fungicides, while less studied, may disrupt monarch reproductive behavior and warrant further investigation.

Unpacking the Evidence: The Path Forward

Surprisingly, no published studies have directly measured how grassland management practices such as mowing, burning, grazing, and haying affect monarch life stages. Only two studies since 2019 evaluated how mowing influenced monarchs, however, they focused on effects on milkweed and nectar availability, leading to recommendations on timing of management practices to align with monarch phenology. Without evidence of how monarchs respond directly to grassland management practices, it remains difficult to assess how effective these practices are for protecting monarchs.

“This literature review underscores a key, though often underestimated, component of monarch conservation: ensuring the long-term viability of monarch populations will require addressing factors beyond habitat restoration and milkweed availability” - Dr. Sherri Russell, Missouri Department of Conservation, Science Branch Unit Chief

In contrast, research on pesticide exposure is becoming more comprehensive—and increasingly definitive. Monarchs are likely exposed to multiple pesticides throughout their life cycle through direct spray (aerial or ground-based), larval contact with residues on surfaces of host plants, or ingestion of contaminated plant tissue and nectar.

Notably, milkweed, the only food source for larval monarchs, has been shown to absorb neonicotinoids from treated soils, integrating these chemicals into its leaves and nectar. Neonicotinoid concentrations found in milkweed differ between flowers and leaves and were found to be highest in mature flowers, therefore, the age of milkweed should be considered in exposure risk assessments.

Together these findings indicate that efforts to increase milkweed availability in agricultural landscapes may want to concurrently consider neonicotinoid exposure and/or mitigation. “This literature review underscores a key, though often underestimated, component of monarch conservation: ensuring the long-term viability of monarch populations will require addressing factors beyond habitat restoration and milkweed availability,” said Dr. Sherri Russell, Missouri Department of Conservation, Science Branch Unit Chief.

Wildlife managers play a critical role in shaping pesticide-conscious conservation strategies for monarchs and other grassland-dependent wildlife. Supporting monarch population recovery and long-term viability will likely include future conservation efforts that prioritize reducing chemical exposure while concurrently maintaining or enhancing high-quality monarch habitat.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. We believe our readers will appreciate discovering stories about the exciting fish and wildlife research projects that Unit scientists conduct to solve real-world problems, engage graduate students in experiential learning, and deliver technical assistance to natural resource practitioners. This story was authored by Elyssa McCulloch, ecmvnp@missouri.edu, graduate student, University of Missouri and edited by Dawn Childs, dchilds@usgs.gov, USGS Information Specialist, Cooperative Research Units.