Are monarch butterflies on the rebound?

The last few years have been tough on monarch butterflies. A widely used agricultural herbicide, known as Roundup, kills the milkweed plants monarchs require for egg-laying. (Genetically modified crops resist the effects of Roundup®.)

The late-summer migration from temperate zones to the mountains of central Mexico can be disrupted by severe September weather. And even after monarchs reach the relative safety of oyamel fir forests in the mountains of central Mexico, winter storms can wreak havoc.

Population decline

Thanks to these and other factors, the monarch population is down 90 percent over the last 20 years. This year, however, monarch watchers report that midwest monarch numbers are up.

Chip Taylor, director of Project MonarchWatch (www.monarchwatch.org), is hopeful despite last winter’s all-time low monarch numbers.

These gorgeous orange and black butterflies occupied just a single acre of habitat last winter.

In his July monarch status report, Taylor predicted, “a modest increase in the number of monarchs in migration and at overwintering sites this winter.”

He bases his optimism on several factors.

1. Though last year’s winter population was low, it seemed to winter well.
2. Weather in Texas in March and April was favorable for milkweed growth.
3. Late spring temperatures in the southern plains helped first-generation monarchs survive and move northward.

Many challenges

Taylor acknowledges the challenges facing monarchs, but sees MonarchWatch’s tagging program, “as a way to monitor their numbers and track the origins of the monarchs that reach Mexico.”

Volunteers tag monarchs by capturing adult monarchs in late summer or raising adult butterflies in captivity and attaching tiny, self-adhesive paper tags with a unique number to the underside of a wing. Much of what we know about monarch migration and survival comes from recovering these tags on monarch wintering grounds.

“The number of monarchs tagged each year roughly parallels the numbers recorded in Mexico each winter, giving us an independent assessment of the numbers in migration,” Taylor explained. “Regional tagging success also helps by demonstrating how monarchs respond to physical conditions and the habitats they use. Thus, tagging is an important tool to help us understand the overall dynamics of the monarch population.”

Lincoln Brower, who began studying monarchs in 1954 and is a retired professor at Sweet Briar College in Virginia, prefers to wait until researchers visit the Mexican wintering sites in December to evaluate the size of the overwintering population.

“We know that monarch density averages about 20 million per acre on the wintering grounds, so after we determine how much winter habitat is occupied, we can more confidently estimate the size of the overwintering population,” Brower told me recently.

Threatened species

In the meantime, monarch conservationists are working to have monarchs listed as a threatened species. The Center for Biological Diversity, the Xerces Society, and the Center for Food Safety are leading this movement.

Though monarchs still number in the millions, they are at risk because the entire eastern population overwinters on just a few acres of montane fir habitat in Mexico. One deep freeze or a snow storm could wipe them out in a matter of days.

Furthermore, here in the U.S., over the last 20 years that have seen a 90 percent population decline, monarchs have lost more than 165 million acres of milkweed habitat.

“Monarchs are in a deadly free-fall, and the threats they face are now so large in scale Endangered Species Act protection is needed sooner rather than later, while there is still time to reverse the severe decline in the heart of their range,” said Brower.

“The widespread decline of monarchs is driven by the massive spraying of herbicides on genetically engineered crops, which has virtually eliminated monarch habitat in cropland that dominates the Midwest landscape,” said Bill Freese, a science policy analyst for the Center for Food Safety. “Doing what is needed to protect monarchs will also benefit pollinators and other valuable insects, and thus safeguard our food supply.”

Learn more

To learn more about monarch conservation, visit www.biologicaldiversity.org or www.monarchwatch.org. At Monarch Watch you can also learn how to get tags for next year and how to obtain milkweed plants for establishing milkweed gardens.