ON A SUNNY FRIDAY AFTERNOON in late August last year—not too hot, with just a hint of a breeze, the radiant blue sky streaked with a line of feathery clouds settling over the hills to the west—we had nearly completed our harvest for Saturday’s market. The lettuces and assorted greens were in from the field, wetted down and stored in the cooler; the potatoes were freshly dug; garlic, onions and shallots were set aside; tomatoes, peppers, eggplant and a few remaining cucumbers were packed and ready to go on the truck.

The only task left was to collect the herbs, and for this job the crew split up into pairs: Jake and Gunnar headed off to bring in mint, chives, tarragon and anise hyssop; Laura and Emma to the sage, thyme and marjoram; Josh and Liz to the parsley and basil; and Chris and Derek to rosemary, lavender, oregano and savory. I decided to lend a hand with the parsley and basil, both best-sellers at our stand.

For a while, Josh, Liz and I worked quietly, immersed in our own thoughts. Suddenly, Josh broke the silence. “Keith, quick, come over here. There’s a monarch.” With harvesting knife tucked under my left arm and a half-finished bunch of parsley in hand, I jumped over a few rows and headed in Josh’s direction. Sure enough, flitting along the grassy edge of the field was a large, orange-and-black butterfly with distinctive white spots on its wings—undoubtedly a monarch. It was worth being startled out of my late-day reverie just to catch a glimpse of it.

Not so long ago—5, 10, 15 years—we saw monarchs on the farm regularly (so often, in fact, that one would scarcely bother to comment on an individual sighting). But times have changed. Nowadays, we’re lucky to see even a few monarchs over the course of a season.

According to lepidopterists (those who study moths and butterflies), monarch populations have decreased by more than 90 percent over the past 10 years. Several developments take the blame for this alarming and precipitous decline—climate change and the extreme weather it brings, loss of habitat, the conversion of marginal lands to grow corn for ethanol, and a broad-spectrum herbicide called Roundup (generically known as glyphosate).

Adult monarchs enjoy the nectar of many flowering plants, and in the process perform valuable pollinating services. But when it comes to propagating themselves, the only plant monarchs are drawn to is milkweed. The female lays her eggs on milkweed. The eggs hatch into impressive black-, white- and yellow-striped caterpillars that feed on the milkweed leaves and grow rapidly. The milkweed serves double-duty as both food and protection for the caterpillars. The plant contains toxic glycosides, to which the monarch caterpillars are immune but, by consuming them, the caterpillars themselves become toxic and highly unpalatable to most predators (primarily birds). It’s a good strategy for survival as long as there is milkweed around—without it, monarchs are unable to complete their reproductive cycle.

In North America, there are two semi-distinct populations of monarch butterflies—one west of the Rocky Mountains and one to the east. Both populations undertake extraordinary migrations. The western population summers in the conifer forests of Washington and Oregon, then migrates to spend winters in California. Eastern monarchs spend their summers spread throughout the eastern U.S. and as far north as southern Canada. Beginning in late summer, these paper-winged insects perform what seems to be a miraculous feat of endurance: They fly 3,000 miles south to the Sierra Madre of Mexico, where they cluster together on fir trees at elevations as high as 10,000 feet in locations so remote they weren’t discovered until 1976. Understanding the critical importance of the monarch’s winter habitat, the Mexican government created the Monarch Butterfly Biosphere Reserve in 1986 and expanded it to cover 217 square miles in 2000. In recent years, extensive efforts have been made to promote sustainable forestry practices among the people who live in and around the protected area, about 80 miles from Mexico City.

Milkweed, though not an especially bothersome weed for most farmers and gardeners, is one of Roundup’s many casualties. The greatly expanded cultivation of genetically modified (GM) crops has led to a tremendous increase in the use of Roundup. In 2007, the latest year for which there are EPA estimates, over 180 million pounds of Roundup were sprayed on cropland in this country—more than twice the amount sprayed in 2001. Today, that number is probably much higher. There is increasing evidence that the growing use of GM crops combined with increased spraying with glyphosate is changing entire ecosystems. Indeed, due to its expansive worldwide use, it may be changing the face of our planet. A class of insecticides, neonicotinoids, has been implicated in the disastrous collapse of bee colonies and could harm monarchs, too.

In this country, the Center for Biological Diversity, the Xerces Society for Invertebrate Conservation and the Center for Food Safety have recently petitioned the U.S. Fish and Wildlife Service to list the monarch butterfly as a “threatened species” under the Endangered Species Act, which would make it illegal to intentionally kill monarchs or modify their habitat without a permit. It also could result in the designation of critical habitat areas in which the spraying of herbicides that contain glyphosate would be prohibited. The petition is under review; a ruling isn’t expected until mid-2016.