Why Is the Eastern Monarch Butterfly Population Declining?
- Barbara Murphy
- 4 days ago
- 3 min read
Updated: 3 days ago
By Barbara Murphy
Monarch butterflies have been in the news quite a bit recently. Data gathered on the 2024-25 overwintering population showed a doubling of monarch numbers. However, it’s important to note that the 2023 overwintering population was one of the lowest ever recorded. These fluctuations play a role in the ongoing debate about whether monarchs should be added to the endangered species list. Getting on this list is a complex process, with many species vying for limited resources. Once listed, expenses arise for scientific assessments, legal proceedings, habitat recovery, and compensation to landowners. Since resources are scarce and many species face population declines, understanding the threats to monarchs is crucial.
Only 5% of monarch caterpillars reach adulthood.
Each female can lay up to 400 eggs.
Approximately 100 million butterflies winter in Mexico.
The Eastern Monarch Butterfly Lifecycle
After overwintering in small refuges in Mexico, adult butterflies begin their northern migration in March. Each green arrow on the map represents a reproductive generation. It takes three to four generations of butterflies to complete the northern migration. The final generation remains in a juvenile state (meaning it will not reproduce) and makes the incredible journey from Maine and Canada to Mexico—covering 2,500 to 3,000 miles. Once there, this same butterfly will overwinter and, if it survives, will begin the cycle again in March.
Why Are Monarch Numbers Declining?
The overwintering population in Mexico fluctuates yearly but has declined significantly since the 1990s. There are two camps with different hypotheses as to what is causing the decline. The more established theory is that the decline is due to loss of milkweed mostly attributable to commercial agriculture and pesticides. However, the second camp theorizes that the population decline occurs during the southern migration when no egg laying occurs. The science is still unfolding, but here are some points of interest:
Recent research suggests that monarch populations may be returning to historic norms after a population boom in the 1800s and early 1900s when the Midwest was cleared, expanding milkweed habitat.
Data from the North American Butterfly Association shows that summer monarch populations have actually increased. Early declines were linked to the widespread use of the herbicide Roundup (glyphosate), which kills milkweed. However, glyphosate use plateaued in the early 2000s, yet monarch populations continue to decline.
A 2024 study finds that insecticides now pose a greater threat than herbicides. The research indicates that general butterfly abundance is tied to insecticide use and that neonicotinoids—a widely used class of insecticides—especially impact butterfly diversity.
The commercial or home-based rearing of monarchs can spread the parasite Ophryocystis elektroscirrha (OE). When eggs are densely laid in small rearing chambers, OE spreads easily, deforming and weakening adults, which can lead to early death or an inability to complete the migration.
Some well-meaning gardeners in Florida plant tropical milkweed, which survives through winter. However, this may be problematic, as it allows egg-laying to continue in concentrated conditions, potentially spreading OE and discouraging migration.
Conclusion
As you can see, what is causing the decline of the winter population of monarchs is far from settled. What is agreed on is that the loss of monarchs and their spectacular and unique migration would be a tragedy. Continue to do your part — plant milkweed, reduce or eliminate the use of pesticides, and refrain from raising large numbers of butterflies in captivity.