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Fairchild’s tropical garden column: a world without monarch butterflies?

A monarch caterpillar on its milkweed host plant.
A monarch caterpillar on its milkweed host plant. Fairchild Tropical Botanic Garden

Monarch butterflies are plentiful and seem to be everywhere, so we take it for granted that they will always be here. But soon they could be hanging out with the passenger pigeon and the dodo. Precipitous declines in monarch populations have been reported.

We need to look deeper into what’s going on and what we can do so we don’t lose this beautiful creature.

Butterflies and plants are intricately and intimately connected. Given our many host and nectar plants here at Fairchild, we have loads of butterfly activity throughout the garden. I was recently fortunate to get to watch monarch chrysalises develop, and was incredibly lucky to find one breaking open, with a monarch emerging.

We’ve all seen the photos and read about the basic butterfly life stages: Egg, larva (caterpillar), pupa, adult. But watching the struggle up close is different, both emotional and enlightening. Cheering on this limp, nearly helpless little orange and black folded mass of color for the almost ten minutes it took to be free of the chrysalis made me want to explore the monarch’s world.

Are monarchs really threatened?

Sarina Jepsen, Endangered Species program director at the Xerces Society for Invertebrate Conservation, said that last year’s count of the monarch population that overwinters in Mexico was only about 10 percent of the average population size over the past 20 years — about 33 to 35 million.

“That may sound like a lot of butterflies,” she said, “but it’s low enough that one major storm could be catastrophic.”

In August, Xerces joined with other conservation groups and Lincoln Brower — one of the world’s most renowned monarch biologists — to request that the federal government list the monarch as a threatened species under the Endangered Species Act.

Monarchs are found throughout the world, even on remote Pacific islands. There are three main populations — still the same species — of monarchs in North America.

▪ Those west of the Rocky Mountains generally spend winter months along the California coast, and some fly to Mexico.

▪ The eastern population famously makes the long, multigenerational migration from as far north as Canada to spend winter in central Mexico.

▪ The third group doesn’t migrate at all, staying more or less year-round in South Florida, Texas and the Caribbean.

Why are their numbers diminishing?

“The greatest concern is the decline in milkweed plants, the monarch’s host plant,” Jepsen said.

Many of the eastern population monarchs spend the spring and summer in the Midwest and central Canada, where they lay eggs on milkweed. The hatchling caterpillars must feed on milkweed before pupating and eventually emerging as adults.

In agriculture, much of that milkweed is considered a weed and so is destroyed by herbicides. Especially harmful are herbicides containing glyphosate, which annihilates native milkweeds and other plants, but not the crops genetically modified to tolerate it.

“We’ve seen a 20-fold increase in [glyphosate] use over the last 20 years, and during that same time period monarch populations have declined by 90 percent,” says Jepsen.

Other monarch threats are the same old culprits hurting so many other animal and plant populations: habitat loss, drought (especially for the California population), climate change and extremes of temperature, logging in the Mexican overwintering sites; and pesticides. Of particular concern are the neonicotinoids, a type of insecticide implicated in the decline of bees.

Of course, this all requires a lot more research, and it may never be completely understood. (I still find it incredible that humans invented insecticides and then are amazed that those same insecticides are actually killing insects).

One way people can aid monarchs is by planting milkweed. Ironically, this may be detrimental. Tropical, or scarlet, milkweed (Asclepias curassavica), a non-native to the U.S., is very common. Its bursts of orange and yellow flowers and ease of cultivation make it appealing to home gardeners.

In warmer regions, it will flower year round. Therein lies the problem: migrating monarchs, leaving areas where native milkweed naturally diminishes with cooler weather, may encounter tropical milkweed and be induced to mate and lay eggs — and not migrate.

Additionally, a parasitic protozoan (Ophryocystis elektroscirrha) is infecting the monarch population. Heavily parasitized butterflies may be weakened, unable to mate or deformed and incapable of long-distance flight; many won’t make it out of the chrysalis.

While this protozoan-monarch relationship has probably existed for millennia, multi-generational feeding from the same plants is allowing the parasites to accumulate to abnormal levels. Non-migrating monarchs do seem to show higher infection rates (more details at www.monarchparasites.org).

The Monarch Joint Venture (http://monarchjointventure.org) recommends not planting tropical milkweed north of Orlando, and if you have it already, to trim it back to about six inches throughout the fall and winter to discourage monarchs from establishing colonies.

South of Orlando, however, there is already an established population of non-migrating monarchs, so while tropical milkweed won’t change their life patterns, monarchs passing through the state could be tempted to stay, or be exposed to higher levels of the parasite. Don’t destroy the tropical milkweed you may already be growing in South Florida — our sedentary monarchs rely on it. However eventually replacing it with native milkweeds is a better option.

What else can we do?

Jepsen recommends:

▪ Plant local, native milkweeds.

▪ Avoid herbicides and insecticides.

▪ Protect wild milkweed populations.

▪ Support agriculture that doesn’t harm monarchs; buy GMO-free products.

▪ Support environmental organizations helping to conserve and restore areas butterflies depend on.

▪ Become a citizen scientist.

▪ Visit www.xerces.org for details.

To find milkweed host plants suitable to your area, the Xerces Society provides a milkweed seed finder at www.xerces.org/milkweed-seed-finder/#search.

For Florida, the seed finder indicates planting butterfly milkweed (Asclepias tuberosa); swamp milkweed (Asclepias incarnata); pinewoods milkweed (Asclepias humistrata); longleaf milkweed (Asclepias longifolia); and aquatic milkweed (Asclepias perennis).

Milkweed is so easy to grow, and native milkweed species should be even easier. Plant some, and one day we can tell our grandkids how we helped save the monarch butterfly!

Kenneth Setzer is writer and editor at Fairchild Tropical Botanic Garden