The other big reason monarchs are in decline: neonicotinoid pesticides

It's common knowledge that the monarch butterfly—that celebrated orange and black insect whose eastern population is famous for its extraordinary long-distance migration from as far north as Canada to Mexico—is in trouble. However, like the causes themselves, the story of why the charismatic insect isn't faring well has evolved.

The monarch first emerged as a species over a million years ago in the American tropics from ancestors that likely also migrated, albeit within a smaller range. As glaciers began retreating northward approximately 20,000 years ago, milkweed—the only food source for the monarch's caterpillar—did the same. Monarch butterflies began following milkweed's range expansion, while retaining the instinct to return south to their ancestral homeland for overwintering. However, today, the insect faces many obstacles that have made its expansive journey increasingly perilous.

In recent decades, both our understanding of and the causes for why monarchs are declining have broadened. In the 1980s, scientists primarily focused on protecting the species' Mexican roosting sites in the country's oyamel fir forests. In the 1990s, our attention turned to herbicides and Roundup Ready, genetically modified corn (GMO), which has led to a decline in milkweed, the butterfly's larval food source. By the 2000s, climate change and general habitat loss were increasingly cited. However, it wasn't until 2024 that a study revealed that neonicotinoids—a class of neurotoxic, systemic pesticides also introduced in the 1990s—have played an outsized role in the species' decline in recent years. The study, which analyzed 17 years of land use, climate, multiple classes of pesticides, and butterfly survey data across 81 counties in five states across the Midwest found that community-wide declines in total butterfly abundance and species richness, including that of the monarch butterfly, was most strongly associated with the use of neonicotinoid-treated seeds.

Neonicotinoids, or neonics for short, are most often applied in row crop agriculture as a preventative seed treatment. Nearly 100% of field corn and over 60% of soy seeds are treated with these pesticides. Due to neonics' systemic nature, as the plant grows from the coated seed, every part of the plant, from its leaves to its roots, to its nectar and pollen, becomes irreversibly toxic to insects. Adding insult to injury, most of the seed treatment doesn't stay with the seed, but instead leaches into the soil and water, impacting non-targeted vegetation such as wildflowers like milkweed and other important nectar plants, along with bees, deer, songbirds, and even humans. These pesticides also do little to help corn and soy farmers. While they can offer a short window of protection against some pest damage, in many cases, the bad outweighs the good. In fact, they can even decrease yields since they also harm beneficial insects.

However, the pesticide industry, which owns over half of the global patented seed market, has made it difficult for farmers to source non-neonic-treated seeds, particularly for corn. Adding further pressure, farmers who can acquire non-treated seeds from a supplier may be penalized under the supplier's replant insurance policy if their crop fails for some reason, receiving as little as half of the cost to replant their crop. With margins extremely tight for most producers, these pressures make it difficult for them to increase safety for monarchs and other agriculture-adjacent species, even if they would prefer to do so.

WWF is working to reduce the amount of neonicotinoids getting into our environment from agriculture by increasing farmer awareness and choice about what goes on their seeds. We are supporting improved transparency and active farmer decisions about when and why neonic treatments go on their crop seeds, encouraging such treatments only when pest risk in their fields means the use of neonic seed treatments will be effective. This is called agronomic justification—determining pesticide seed treatments based on a field review of whether there is pest risk that can be addressed by neonics or another pesticide. If such a risk exists, farmers should have access to and use the most effective seed treatment for the pest pressures they face. But if a pest risk is not present, farmers should also have easy access to buy non-neonic-treated seeds. By doing so, they can save on input costs while benefiting the soil, water, people, and wildlife, including the monarch butterfly.

Learn more about what WWF is doing to reduce neonicotinoid pesticides and help pollinators.