While in the past fluctuations in Monarch populations could be explained by factors such as a cold winter, the evidence is now clear that there are human factors putting downward pressure on monarch populations.
(Beyond Pesticides) Monarch butterflies are in the midst of a staggering decades-long population decline that has rapidly accelerated since 2005, research published by an international team of scientists and the University of Florida last month indicates. According to data meticulously collected by researchers, monarchs making their way to central Florida after emerging from their breeding grounds in Mexico have declined by 80% over the last decade and a half. This is roughly the same time frame at which beekeepers began to see precipitous declines in managed honey bee colonies. Researchers point to industrial development and increasing pesticide use as factors that have accelerated the decline of this iconic species.
“A broad pattern is that 95 percent of corn and soybean products grown in the U.S. are Roundup Ready crops that resist glyphosate,” said study coauthor Earnest Williams, PhD, of New York’s Hamilton College in a press release. “That has a national impact. What’s really needed are patches of native vegetation and nectar sources without pesticides. It’s not just for monarchs but all pollinators.”
Beginning in 1985, renowned monarch expert Lincoln Brower, PhD and his team monitored monarch populations at a pesticide-free cattle pasture south of Gainesville, FL. Caterpillars were observed on milkweed, the main source of food for monarchs before metamorphosis, and the numbers of adult monarchs were also recorded for 37 years, spanning what researchers indicate are over 140 monarch generations.
Based on these data, scientists found that monarchs leave Mexico just in time to reach milkweed at its optimal growth stage in central Florida. The timing is critical for monarchs, a deviation of just a couple weeks could result in monarchs missing the opportunity to provide the right conditions for their offspring to survive.
And as researchers indicate, it is critical for them to do so. “Florida is kind of a staging ground for the recolonization of much of the East Coast,” said co-author Jaret Daniels, PhD of the Florida Museum of Natural History. “If these populations are low, then the northern populations are going to be at a similar abundance level.”
Recent reports directly from the monarch overwintering grounds in Mexico paint a very grim picture for the species. Last year monarch populations were 15% lower than the year before. Western monarchs, which overwinter in coastal California forests, are subject to the same declines, with scientists indicating that there is an 86% chance of extinction within 50 years, and nearly 75% chance within 20.
The current research aligns closely with data presented by the World Wildlife Fund, which tracked the Mexican overwintering grounds and recorded a similar 80% decline since the 1990s. While in the past fluctuations in Monarch populations could be explained by factors such as a cold winter, the evidence is now clear that there are human factors putting downward pressure on monarch populations.
Researchers are pointing at a number of factors, with habitat elimination being the primary driving force behind the decline. Industrial development, whether in the form of strip malls and planned communities or intensive monoculture farm fields, contributes to a loss of milkweed that the species requires. Not only are farm fields often displacing monarch habitat, a majority of fields already in use are planting genetically engineered crops designed to tolerate repeated herbicide spraying. While crops are left untouched from the use of chemicals like glyphosate(Roundup), 2,4-D, and dicamba, milkweed is often eliminated from field margins and other near-farm areas due to drift and run-off after herbicide applications.
The class of systemic chemicals linked to the decline of bees has also been shown to harm monarchs. Two studies, one in 2015 and another in 2016, have linked neonicotinoid exposure to the death of monarch larvae. It’s not difficult for these chemicals to make their way into monarch habitat either. Once applied, either through spray, or more commonly, through coated seeds, these chemicals either drift or run off into areas where milkweed is growing. While the insecticides don’t kill milkweed, they can make their way into the milkweed plant, contaminating it. By poisoning the plant, it becomes deadly for monarch caterpillars to feed on its primary food source.
Scientists point last to the unfortunate proliferation of a non-native milkweed species, tropical milkweed (Asclepias curassavica), which can spread parasites to unsuspecting monarchs. The variety is sold in stores and often planted for its color and long growing season, but researchers indicate these properties could lead to monarchs breeding at the wrong time, on the wrong species of milkweed.
While the authors suggest Florida residents plant swamp milkweed (Asclepias incarnata), or butterflyweed (Asclepias tuberosa), they note limitations in that approach. “It’s not as simple as saying, ‘we plant milkweed and the monarch will be saved,’” said Dr. Daniels. “We should think of this as an ecological issue. There are a lot of complexities to any organism and any system.”
Lead author Dr. Brower, a world renown researcher who led the decades long-study Monarchs for decades, died shortly after the release of this research, his final published work. “The best thing we can do is to continue his mission and continue to study and work to conserve the monarch,” Dr. Daniels said of Dr. Brower’s legacy. “I think he would be proud of that mission.”
The monarch is up against complex modern forces – development, chemical pesticides, climate change, logging, and other sources of habitat destruction, disease, and contamination. In order to solve the crisis, advocates need to work on multiple fronts. Help limit the growth of genetic modification in agriculture by purchasing only organic. Also take action in your home and community. See Managing Landscapes with Pollinators in Mind and Hedgerows for Biodiversity. For additional steps you can take to protect monarchs and other pollinators, see Beyond Pesticides’ Bee Protective webpage.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.