Troubling and significant declines in monarch butterfly populations in Florida have occured over the last 30+years, according to a research team from the McGuire Center for Lepidoptera and Biodiversity, part of the Florida Museum of Natural History.
A paper by the FLA team was published this summer in the Journal of Natural History, and reports the results of the final study led by world-renowned monarch butterfly expert Lincoln Bower, who passed away earlier this year.
A detailed blog post on the museum’s website provides a summary of the study’s design, methods, and key findings.
The paper reports the results of a 37-year study of monarch populations in North Central Florida. Their findings were “alarming in a number of different ways,” says co-author Jaret Daniels. Monarch “caterpillars and butterflies have been declining since 1985 and have dropped by 80 percent since 2005” (Marchese and van Hoose, 2018).
Monarch butterflies are globally unique and among the most iconic creatures on the planet. They undergo an incredible, multi-generational migration that takes them from Southwestern Mexico, up to east coast and central plains of North America, across the NE and central U.S., and north into southern Canada.
Monarchs generally have a lifespan of just six to eight weeks. But butterflies in the longer-lived “Methuselah,” fourth generation live up to 8 months, long enough to make the near-3,000 mile journey from and back to overwintering grounds in Mexico.
To gain a better sense of the Monarchs incredible journey, check out this short video.
Unfortunately, the stark declines observed in Florida have “occurred concurrently with the decline in the number of monarchs at their Mexican overwintering sites” (Brower et al., 2018)
Florida is an important first stop for migrating monarchs. In a stunning feat of evolution, these little butterflies (remember, at this stage they only live 6-8 weeks) fly from SW Mexico across the warm, tropical waters of the Caribbean and the Gulf of Mexico to arrive in Florida just as the current crop of milkweed plants is developing across the Southwestern U.S.
The adult butterflies can feed off of many different kinds of vegetation, but “their young depend on milkweed as their sole source of nutrition, storing up the plant’s toxins to ward off predators. Monarchs lay hundreds of eggs on milkweed over their brief lifetimes, but just over 2 percent of eggs survive to become fully grown caterpillars” (Marchese and van Hoose, 2018).
Spring Monarch breeding in Florida, which must be carefully timed to the milkweed plants, is critical for monarch populations throughout the central/eastern U.S. and Canada.
“Monarchs rely on Florida for its abundance of milkweed and warm climate to lay the eggs that will help replenish the eastern population” (Marchese and van Hoose, 2018).
But, milkweed populations are also in decline. This research team thinks rising glyphosate use throughout most of North America might be to blame, because of inability of Monarch populations to find enough milkweed at various points along their migratory route.
Glyphosate is lethal to nearly all weeds, including milkweed. Ernest Williams, a co-author of the new paper, explains that “95 percent of corn and soybean products grown in the U.S. are Roundup Ready crops that resist glyphosate.” Milkweed commonly grows in field buffers and hedgerows, areas that are often impacted by drifting herbicides.
The sharp declines in monarch populations began around 2005, roughly coinciding with the most dramatic rise in glyphosate use.
In 1997, U.S. farmers applied about 36 million pounds of glyphosate, or on average, about 0.12 pound per acre across the ~300 million acres of intensively farmed cropland in the U.S. (Data on glyphosate use from “Trends in Glyphosate Herbicide Use in the United States and Globally,” Environmental Sciences Europe, Supplemental Table 18).
Four years later in 2001, use had nearly doubled to 87.5 million pounds of active ingredient, or 0.3 pounds on the average acre, as a result of the popularity of Roundup Ready corn, soybeans, and cotton.
Another four years later in 2005, the volume applied had grown dramatically to 157.5 million pounds, enough to spray an average of 0.5 pound across most of the U.S. cropland base, and definitely enough to either kill or weaken milkweed in or around sprayed areas. No pesticide in history has come close to matching the meteoric rise in glyphosate use in the U.S. from about 1997 through 2007.
Is the decline in monarch butterfly populations an unintended consequence of rising pesticide use?
Other research has also pointed to a link between fewer monarchs and more herbicides:
- An important study by John Pleasants and Karen Oberhauser published in 2012 in the journal Insect Conservation and Diversity found and estimated 58% decline in milkweeds in agricultural areas in the Midwest and a 81% decline in monarch populations from 1999-2010.
- Published by The Royal Society in 2017, the paper by Thogmartin et al. investigated climate and habitat related factors in monarch declines, “results indicated strong negative relationships between population size and habitat loss variables, principally glyphosate use.”
- A 2018 paper by an international team from Minnesota and Australia looked at various factors that may be contributing to monarch declines, concluding that “the key landscape-level change appears to be associated with the widespread use of genetically modified herbicide resistant crops that have rapidly come to dominate the extensive core summer breeding range.”
–Ernest Williams, Professor Emeritus at Hamilton College, New York and study co-author
Brower, Lincoln P., Williams, Ernest H., Dunford, Kelly Sims, Dunford, James C., Knight, Amy L., Daniels, Jaret, Cohen, James A., Van Hook, Tonya, Saarinen, Emily, Standridge, Matthew J., Epstein, Samantha W., Zalucki, Myron P., & Malcolm, Stephen B., “A long-term survey of spring monarch butterflies in north-central Florida,” Journal of Natural History, 52(31-32), 2018, 2025-2046. doi:10.1080/00222933.2018.1510057.
Halle Marchese and Natalie van Hoose, “Florida Monarch Butterfly Populations Have Dropped 80 Percent Since 2005,” Florida Museum, November 8, 2018.