Monarchs Are Moving Through Arizona, and a Scottsdale Tagging Morning Lets Anyone Help Track Them

If you have noticed an orange-and-black butterfly drifting over a Valley yard this week, there is a good chance it was a monarch on its way south. Early October is the heart of the fall migration in Arizona, and this Saturday a volunteer research group is inviting the public to help catch, tag, and track the travelers.

Southwest Monarch Study will hold a Peak Migration Celebration on Saturday, October 10, from 8:30 to 10:30 a.m. at Scottsdale Community College. Participants will learn to net and tag monarchs, and the group says they will also help radio-tag and track butterflies on their fall journey. Organizers ask people to bring a phone with the Project Monarch app installed so they can be shown how it works. Registration is required by email at info@swmonarchs.org.

What a tagging morning looks like

Tagging a monarch is gentle, quick work once you have been shown how. Volunteers net a butterfly, apply a small numbered sticker to the wing, record where and when it was caught, and let it go. If someone finds that butterfly later, whether in a California eucalyptus grove or a mountain forest in Mexico, the record connects the two points on a map.

The Arizona chapter of The Wildlife Society, which listed the event, passes along practical advice for anyone attending. All participants must complete a liability and photo release form. Sturdy closed-toe shoes, long pants, a long-sleeved shirt, a hat, and sun protection are recommended, along with plenty of water and snacks. Children must be accompanied by an adult. A camera is optional but encouraged.

The Scottsdale morning closes out a short fall schedule. Southwest Monarch Study held tagging events in late September at Rockin’ River Ranch State Park in Camp Verde and Bubbling Ponds Fish Hatchery in Cornville as part of the Flagstaff Festival of Science, and the group’s Gail Morris led a tagging training at Tucson’s Reid Park Zoo on October 3.

Why Arizona monarchs puzzle scientists

For a long time, Arizona was something of a blank spot on monarch maps. North American monarchs were generally described as two groups: an eastern population that winters in central Mexico and a western population that winters along the California coast. Arizona sits right between them, and Southwest Monarch Study has spent more than two decades sorting out where its butterflies go.

The answer turned out to be both. In a summary of its findings, the group reports that 12,088 monarchs were tagged between 2003 and 2014 by 384 people at 276 locations around the state. Recoveries showed Arizona monarchs flying to overwintering sites in Mexico and in California, and the group found a statistically significant link between wind direction at about 1,000 feet and where a butterfly ended up. Monarchs prefer to ride thermals to save energy, so the wind on tagging day appears to matter.

A 2019 study in The Journal of the Lepidopterists’ Society added a striking detail. Of 3,194 monarchs tagged in southeast Arizona from 2014 to 2016, 44 were recovered: 32 in California and 12 in Mexico. For the first time, butterflies tagged at the same place on the same day were found wintering in both regions, a sign that western migration is more complicated than the old two-population picture suggested.

The East Valley has its own place in that record. According to Southwest Monarch Study, the latest migrating monarch it has documented was tagged on November 19 in Chandler and was later seen in Kino Bay, Sonora, on December 14. The group also notes that peak fall migration in Arizona runs from September through mid-October depending on elevation, and that small, loose clusters of monarchs often rest on the southeast side of trees during the peak.

Not every monarch leaves. Southwest Monarch Study reports that small populations stay through the winter in the low desert around Phoenix and Tucson and along the Colorado River. Tagging has shown these winter residents tend to stay within about a three-mile radius, and in most years they begin mating by early February unless a hard freeze knocks them back.

A tag the size of a grain of rice

The radio-tagging piece of Saturday’s event is new technology that has only recently become possible. The Project Monarch Collaboration, led by the Cape May Point Arts and Science Center and Cellular Tracking Technologies, uses a solar-powered transmitter called BlūMorpho that weighs about 60 milligrams, roughly the weight of a grain of rice. It broadcasts on the same frequency as Bluetooth, so ordinary smartphones running the Project Monarch app can pick up its signal passively, along with dedicated receiver stations in the Motus wildlife tracking network.

The results have been remarkable. In a March 2026 release, the collaboration said that more than 400 monarchs were fitted with transmitters during fall 2025 and at least 28 were later confirmed at Mexican overwintering latitudes. One butterfly tagged near Stillwater, Oklahoma, in October 2025 was tracked to central Mexico and then back into Texas the following spring, the first individually tracked monarch confirmed on both legs of the journey. Southwest Monarch Study is one of the project’s partner organizations.

The same release noted encouraging news from the wintering grounds: WWF-Mexico and Mexico’s protected areas agency, CONANP, reported that this past season’s overwintering colonies covered 2.93 hectares, a 64 percent increase from the season before.

How a desert yard can help

Monarchs need two things from a landscape: milkweed, the only plant their caterpillars eat, and flowers that give adults nectar for the long flight. Arizona is unusually rich in the first. A 2015 USDA Natural Resources Conservation Service guide counts at least 41 of the 76 milkweed species known in the lower 48 states in the Desert Southwest.

For low-desert gardeners, the standout is rush milkweed, Asclepias subulata, also called desert milkweed. The USDA guide describes it as a wand-like shrub of slender, bluish-green stems that grows 2 to 5 feet tall, flowers sporadically through the year, and is found along washes and sandy plains below 3,000 feet in Arizona. It is a known monarch host plant, and its upright form fits easily into a xeriscape bed.

The Xerces Society, a national invertebrate conservation group, recommends planting milkweeds that are locally native and grown from regional seed. It does not recommend tropical milkweed, Asclepias curassavica, a common nursery plant that can keep its leaves through mild winters. Xerces says preliminary research suggests that year-round foliage may encourage monarchs to breed out of season and increase infection by a protozoan parasite known as OE. Gardeners in the southern United States who already have tropical milkweed are advised to cut it back to the ground in early fall.

Two other Xerces tips are easy to act on this month. First, ask the nursery whether milkweed and other pollinator plants were treated with systemic insecticides, such as the neonicotinoids imidacloprid or dinotefuran, which can make the plants toxic to caterpillars. Second, skip insecticide sprays on milkweed, which can harm caterpillars and pollinators. When orange oleander aphids show up, Xerces notes that small numbers can be removed by hand or knocked off with a strong stream of water. New plants will need regular water through their first year, especially their first summer.

Gardeners who spot milkweed patches or monarchs can also report them to the Western Monarch Milkweed Mapper, a Xerces community science project that accepts records from Arizona and helps agencies decide where habitat work will do the most good.

More ways to get outside with butterflies

Anyone who cannot make the Scottsdale event has a second option close to the East Valley. Boyce Thompson Arboretum in Superior offers guided butterfly walks with an expert from the Central Arizona Butterfly Association, focused on common local species and their host plants. Upcoming dates are October 10 from 8:30 to 9:30 a.m. and October 24 from 9:30 to 10:30 a.m. The walks are free with admission, limited to 15 people, and filled first come, first served at the Welcome Center.

A monarch that crosses a Valley yard this month may be headed for a California grove or a Mexican mountainside, and for now, nobody can say which without a tag. Every butterfly counted, every patch of desert milkweed planted, and every phone that picks up a tiny transmitter adds one more point to a map that Arizona volunteers have been filling in for more than 20 years.