There is good news and bad news about monarch butterflies.
In late summer or early fall, monarch butterflies (Danaus plexippus) from our region migrate south thousands of miles to Mexico, where they cluster together in large groups to overwinter. In March, the monarchs migrate back north to breed and lay eggs. The first generation reaches Texas or Oklahoma, where they stop to lay eggs. The caterpillars that develop from these eggs eat milkweed, which contains cardenolides, steroids that can stop the heart. For this reason, monarch caterpillars and adult butterflies are toxic and nasty-tasting to predators.
The caterpillars spin cocoons, and adult butterflies emerge. These butterflies continue the northern migration as temperatures in the south become too hot for their survival. The second or third generation of butterflies reaches New England in the summer.
The final generation in late summer or fall does not lay eggs. Instead, it enters a nonreproductive phase called diapause, and flies 3,000 miles south to overwinter in the forests of central Mexico, beginning the cycle all over again. The northern limit of milkweed plants dictates how far monarch butterflies can travel.
The good news
Scientists have used DNA to understand the size of the monarch population. By looking at monarchs’ genetic diversity, scientists can tell the size of the population. The more genetically diverse a population, the larger its size. As a population declines, its genetic diversity also declines. Using this method, scientists can tell the number of individuals within a population.
Scientists found that the monarch population over the past 25,000 years has expanded and contracted corresponding to increases and decreases in the number of milkweed plants. Populations grew after the last ice age, when North America became lush and green again and milkweed proliferated. There was a second increase 200 years ago, when people cleared much of the eastern forests for agriculture and common milkweed flourished.
Researchers did not find evidence of declines in either milkweed or monarchs in the past 75 years – the DNA of current monarchs shows no decline in genetic diversity. In fact, the monarch population now is larger than it was before people began large-scale farming 200 years ago.
The bad news
Despite the fact monarch populations overall do not seem to be declining, the size of overwintering colonies in Mexico definitely is. Recent data indicates that overwintering monarchs in the forests of central Mexico occupied only 2.2 acres during the winter of 2023-2024. This is 59 percent less than the 5.5 acres from the previous year! In 1996-1997, monarchs occupied 44 acres in central Mexico. They reached their lowest numbers – 1.7 acres – in 2013-2014.
In the past 30 years, there has been another change: Some monarchs do not migrate. In southern Florida nonmigratory monarchs live year-round. Nonmigratory monarchs are also in Southern California, southern Texas, along the Gulf coast, the Atlantic Coast and even northern Mexico.
Scientists are not sure why the nonmigratory population is increasing, but have some ideas: People, especially in the South, plant tropical milkweed (Asclepias curassavica) in butterfly gardens. But tropical milkweed is an aggressive invasive species in Florida and, unlike other types, does not die back in the winter. Migrating monarchs are attracted to tropical milkweed, which stimulates the growth of reproductive tissue. Instead of continuing their migration, monarchs stop and begin breeding. It is believed that many of these monarchs never again migrate.
Another problem with tropical milkweed is that it harbors a one-celled parasite, Ophryocystis elektroscirrha, called OE for short. Because tropical milkweed does not die out in winter, the parasite does not die back either. Monarchs with large numbers of this parasite – which coevolved with monarchs and does not infect other species – are born with crumpled wings and cannot fly; the less infected are smaller, have shorter lifespans, fly poorly or are unsuccessful at mating. Only the healthiest butterflies reach overwintering areas in Mexico; butterflies with this parasite do not survive long migrations. Few overwintering butterflies harbor the OE parasite.
But in Florida, almost all monarchs harbor the OE parasite. As they feed on or lay eggs on tropical milkweed, they become infected. And they spread the infection to migrating butterflies. It used to be that about 1 percent of migrating monarchs harbored OE; now about 10 percent do, resulting in the loss of millions of monarchs during their long-distance fall migration.
Other things that harm monarchs
Summer temperatures are increasing. Monarch eggs and caterpillars develop more quickly when the weather is not too hot. The perfect temperature for monarchs is about 84 degrees Fahrenheit, while temperatures above 95 degrees can kill caterpillars and dry out monarch eggs, decreasing lifespans.
What can people do to help? First, do not plant tropical milkweed – not really a problem in our area, but if you know people in warm Southern states, let them know of its dangers. Also, do not raise monarchs in captive breeding projects, as these butterflies frequently harbor OE. Releasing these butterflies also releases the parasite to healthy migrating populations.
What does winter breeding in the Southern United States mean for the future of monarchs? No one really knows. The OE parasite threatens these Southern butterflies and can spread to migrating butterflies. But on the other hand, as climate change causes milkweed to become established farther and farther north, overwintering in the Southern states decreases the distance of monarch migration.
Reader photo
Brenda Dumas spotted this red admiral in Brookline on July 18.
Have you taken photos of our urban wild things? Send your images to Cambridge Day and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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