Monarch butterfly (Danaus plexippus). Photo: Kenneth Dwain Harrelson, Wikimedia Commons

I’ve written about the monarch butterfly (Danaus plexippus), the world’s best known migrating lepidoptera, whose annual 3,000 mile/4,800 km migration from the mountains of Mexico to edge of the boreal forest in Canada is the stuff of legend, more than once in this blog (Monarch Butterflies Are Back!, Plant More than Milkweed to Save Monarchs and Monarchs Arrive in Europe!, for example), but there is much research going on about them and scientists learn more about monarchs each year. 

Here are some interesting recent discoveries:

Winter Numbers Down

Sadly, Mexican authorities report that the number of monarch butterflies overwintering in Mexican forests was down by half during the 2019–2020 winter season. Just 7 acres (2.83 hectares) was covered, a 53% decrease from the 2018–2019 season, when monarchs covered 15 acres (6.05 hectares) of forest.

Jorge Rickards, the managing director of WWF-Mexico, noted that this is not necessarily a cause for alarm, but added that “we must remain vigilant and not allow it to become a trend in the coming years. Conservation is a long-term job.”

New Colony of Monarch Butterflies Discovered in Mexico

For a long time, it was thought that there was only one colony of monarchs in Mexico, at what is now the Monarch Butterfly Biosphere Reserve in the state of Michoacán, but a second one was recently discovered near the Nevado de Toluca volcano, a good 4-hour drive to the southeast. 

For years, locals had remarked the presence of limited numbers of monarchs in the vast (53,419 ha) Nevado de Toluca National Park during the winter, but no one was able to find whether they were just stragglers or if there was a colony hidden there. Just before Christmas in 2018, however, a routine park patrol finally did find a large colony clinging to oyamel firs (Abies religiosa) at an altitude of more than 3,000 meters. In 2020, researchers returned to study the situation and calculated that there were about 20 million monarchs in the new colony, enough so tree branches bend under their weight.

Curiously, while the colony at the Monarch Butterfly Biosphere Reserve stays on the same trees all winter, which has made it into a tourist attraction, the new colony at Nevado de Toluca moves every night, so is harder to find and study.

Local governments currently have no plans to open this new colony to mass tourism.

Plant the Right Milkweed

We all know that monarch caterpillars will only feed on milkweeds (Asclepias spp.) and a few other closely related species, but it turns out that the milkweed species you plant to feed visiting monarchs is very important. 

Butterfly weed (Asclepias tuberosa), the most common milkweed offered in garden centers and a popular garden perennial, is actually the least favorite milkweed species and monarch caterpillars rarely feed on it. It doesn’t have the milky sap of other milkweeds and there may be something in that sap that female monarchs seek when looking for a place to lay their eggs. 

The species most frequented by monarchs is the common milkweed (A. syriaca), a weedy species rarely grown in gardens. One study showed that 85% to 92% of monarchs overwintering in Mexico had fed on common milkweed as caterpillars. That said, most other species studied so far are good host plants as well, including another fairly popular garden species, swamp milkweed (A. incarnata). Apparently, it’s only butterfly weed (Asclepias tuberosa) that isn’t really worth planting if your goal is to provide a host plant for monarch caterpillars..

Another study challenges the commonly repeated belief that only wild species of milkweed should be used in butterfly gardens. It found that hybrid milkweeds are just as attractive to egg-laying females as straight species. 

Cut Your Milkweeds Back 

Research by Nate Haan of Michigan State University shows that female monarchs prefer to lay their eggs on fresh young stems that haven’t flowered yet rather than tougher, aging ones. That’s not a problem early in the season, but Haan discovered that cutting back a third of a garden’s milkweeds in June and another third in July resulted in more monarch butterfly eggs being laid on the resprouting plants. This heavy pruning doesn’t harm the milkweed plants and they quickly rebound.

And cutting back would be a good solution to another dilemma. The tropical milkweed (A. curassavica) is the most popular garden milkweed in the southern U.S., but its habit of continuous bloom well into fall and even winter can be harmful to migration. It’s believed monarchs tend to stop on patches of this plant rather than continue their trip south in a timely manner, disrupting migration. Also, a debilitating protozoan parasite (Ophryocystis elektroscirrha) tends to build up in tropical milkweed, at least in areas where it isn’t killed back by frost in winter, and can weaken caterpillars and cripple adult butterflies. 

Both these flaws can be easily mitigated by cutting tropical milkweed back hard in the fall. With the plant absent from the landscape, the butterflies will continue their migration normally and won’t be as likely to become infected with the disease.

Let Them Be Free

Apparently, monarchs raised indoors as caterpillars largely fail to migrate. They just don’t seem to pick up on the clues (colder weather, dieback of blooming plants) that tell wild-born monarchs it’s time to head south, according to biologists Ayse Tenger-Trolander and Marcus R. Kronforst of the University of Chicago. 

That means the thousands of monarchs sold by butterfly farms to schools, weddings, funerals and other events for mass butterfly releases may be good publicity for the monarch cause, but don’t directly help monarch survival in any way, as they simply are not flying back to Mexico. 

Planting Flowers to Feed Adults

The common belief that planting milkweeds (Asclepias spp.) is all you need to do to save monarchs is incorrect. True enough, female monarchs will only lay their eggs on milkweeds and their caterpillars will only feed on those same plants, but adult monarchs feed on nectar of a wide range flowers. In fact, don’t even particularly like milkweed flowers (they are instead largely pollinated by bees and wasps). 

If you want to create a garden for monarchs (and why not officially start a monarch waystation?), you need to grow more than milkweeds. Instead, try to provide a wide range of flowers, making sure that there are some in bloom throughout the entire growing season (read Plant More than Milkweed to Save Monarchs), especially at the extreme ends of the gardening season, that is, early spring and late fall.

Also, it’s turning out that mixed plantings actually confuse butterflies. They use sight to find their food, relying on plant shapes. So, a “butterfly meadow,” with flowering plants mingling any which way, might not be the best solution. Instead, you’ll get more positive results by growing single plants isolated from others by mulch. And planting in a north-south pattern is best. 

---

So, now you know what to do to make your garden more monarch butterfly friendly!