New studies create uncertainty over what's driving the decline of monarchs at overwintering sites.

New studies create uncertainty over what's driving the decline of monarchs at overwintering sites.

Fred Habegger/Grant Heilman Photography/Alamy

Monarch butterfly studies tell a perplexing tale

Each summer, female monarch butter-flies flutter around their breeding grounds in northern North America in search of nectar, a mate, and a milkweed plant on which to lay eggs. And they have an audience. Thousands of volunteers periodically survey the charismatic black and orange insects, helping scientists track population trends. Others count monarchs as they migrate south each fall to warmer climes. Now, one of the most systematic analyses of these tallies and other data has raised questions about how well researchers understand why monarchs have seen a dramatic decline on their major wintering grounds in Mexico.

The confusing picture emerges from seven monarch studies published this week in the Annals of the Entomological Society of America. One paper suggests that, even as wintering populations of monarchs have plummeted over the past 2 decades, there’s been no similarly steep decline in a key summer breeding area that stretches across the midwestern United States and southern Canada. Others find that some fall migration counts also show no major downward trend. At the same time, the butterflies may be laying fewer eggs overall, concludes one study.

Such findings “present a puzzle,” says ecologist Leslie Ries, now of Georgetown University in Washington, D.C., and an author of one of the studies. They “make me wonder if we really have the strongest evidence to be able to say we know what’s causing the decline that we see in Mexico.” The uncertainty could pose problems for conservationists trying to protect the butterflies.

The ambiguity is fueled, in part, by the monarch’s complex population structure and life cycle, which includes a lengthy migration completed, relay-style, by several generations of insects (see graphic, below). Each spring, many monarchs head north from Mexico to the southeastern United States, where they produce the first of up to three generations that can swell the total population. The offspring then spend the summer feeding and breeding on milkweed throughout the northern United States and southern Canada. In the fall, a final generation makes the trek back to Mexico. (Other regional monarch populations have different migration and breeding patterns.)

The cycle can make it hard to get a handle on summer and fall monarch numbers, but researchers agree on one thing: Over the past 20 years, the number of monarchs returning to the largest known wintering ground in central Mexico’s highland forests has plunged by more than 90% (Science, 
7 February 2014, p. 583). Many have blamed the decline primarily on the expansion of herbicide-resistant crops in the summer breeding grounds, which has led to the wide use of chemicals that kill milkweed. But few studies have systematically examined monarch population trends in these areas.

A. Cuadra/Science

The new papers help fill that gap. One draws on 18 years of monarch sightings collected by citizen scientists. Between 1997 and 2014, at least some monarchs were spotted in all of the butterfly’s historical eastern breeding range (some breed in the far west), suggesting the summer population is hanging on. And a second study, relying on hourly monarch counts made by volunteers at hundreds of summer breeding sites between 1993 and 2014, found no statistically significant population trend, up or down.

There’s “a disconnect” between those results and the wintering data, Ries says. That’s because if milkweed loss is driving the winter decline, then summer populations seemingly should be shrinking, too.

One possible answer, Ries and others say, is that the volunteers may have under-sampled agricultural areas hit hard by herbicides, instead favoring sites popular with butterfly-watchers, such as parks and protected areas. Another view is that the last 7 or 8 years of the summer numbers do contain evidence—albeit statistically weak—of the monarch’s decline. “If they had 9 or 10 years [of data], it probably would be [statistically] significant,” contends entomologist Lincoln Brower of Sweet Briar College in Virginia, who was not involved with the studies.

Other researchers say the egg-laying study suggests summer monarchs are struggling. It found that monarch egg densities on milkweed plants have declined since 2006. “It’s pretty scary,” says biologist and co-author Karen Oberhauser of the University of Minnesota, Twin Cities. “It probably means there aren’t enough monarchs out there to find the remaining patches” of breeding habitat.

The results have also put a spotlight on the need to better understand what is happening to monarchs during their fall migration south, researchers say. Two of the studies examine counts of migrating monarchs made from the mid-1990s to 2014 at stations in northern Michigan and Ontario, Canada. The Ontario tallies show a moderate decline, whereas the Michigan numbers show no clear trend. That suggests monarch deaths occurring farther south during the migration could be responsible for the sharp winter decline, says ecologist Andrew Davis of the University of Georgia, Athens, a co-author of two of the papers. Migrating monarchs run “a gauntlet of dangers,” including predators, parasites, and even speeding cars, he says. “There is a tremendous amount of mortality, and we don’t know how much.”

The biggest knowledge gap is for Texas, Brower says, which is in the middle of both the main fall and spring flyways. Researchers “desperately” need monitoring data from that throughway, he says.

Such studies could help ensure that government agencies and environmental groups focus their time and money on the right solutions. Currently, the U.S. Fish and Wildlife Service and others have focused on conserving or restoring milkweed breeding habitat. “But if the problem is that the monarchs are dying during the migration,” Davis says, “I’m not sure just trying to produce more at the start of the [fall] migration is the answer.” Other steps, such as protecting migratory pathways, may also be needed. The concerns over migration are real, says ecologist Ernest Williams of Hamilton College in Clinton, New York, but “they should be added to—but not replace—the other issues we know to be affecting monarchs.”