Monarchs, Migrations, and Milkweed

Written By Cuplet Fern

by Dorothy Caldwell

Monarch butterflies may be the most recognized butterfly in the U.S., but they are found throughout the world. Monarchs can be found in South America, Portugal, southern Spain, parts of the Mediterranean, Australia, and the Pacific Islands including Hawaii, but it is only the northern portion of the Western Hemisphere that the monarchs migrate-no other places in the world. [1] Monarchs have begun their biannual migration and will show up in Florida early to mid-October. The majority of monarchs who come to Florida will head to the St. Marks National Wildlife refuge, one of the top five viewing sites for monarch migration in the United States, pause for a few days to refuel on high-energy nectar plants, and then begin the most perilous leg of their journey, over the Gulf of Mexico heading to the Sierra Madre Mountains of Mexico. There is a smaller portion of monarchs that head down Florida’s east coast to winter in South Florida/Miami, and Key West, and a few even travel further to the islands of the Caribbean. There is a relatively small population of monarchs that stay in South Florida not migrating elsewhere, which has caused some concern and research as to why these monarchs do not migrate. Scientists have come up with a few possible reasons: the presence of nonnative milkweed that does not get cut back and weak and/or diseased individuals that cannot make the flight seem to be the top focus of the research.

A monarch visiting a local garden

There has been a lot of research over many years as to the how and why of this very long and difficult trip. So many questions and the scientists do not have all the answers, but they have made some startling discoveries. One of the most important discoveries is called diapause. Migratory monarchs go into a reproductive diapause- the monarch’s equivalent of hibernation. This diapause allows for a single monarch butterfly to fly from Canada to Mexico, more than 3000 miles, and exist in the cold mountains until its return migration begins in February. These migratory monarchs stay in a juvenile stage of development that does not give them the urge to mate. [2] Monarchs that come across milkweed in bloom on their migration journey south will come out of diapause and become sexually active, again laying eggs. Once they start to lay eggs they will not go back into diapause.  “A warm climate and continuous growing milkweed season keep the species hanging around throughout the year in southern Florida. In North and Central Florida cold winter temperatures can kill them. Monarchs from south Florida and those migrating from Mexico repopulate the area.” [3] “Monarchs can't fly unless they can warm their muscles to 55°F. They can't even crawl below 41°F. Although monarchs can save energy when they are cold, the cold also makes them vulnerable to predators. January and early February are the coldest time of the year. This is a really critical time for the monarch. If the butterfly is also WET, there is only a 10-20% chance of survival for the monarch." [4] The other issue with monarchs in Central and North Florida in January and February is the lack of nectar plants to give them the energy to survive the cold temps. To assist the monarchs in their migration journey, provide them with nectar plants for their return trip to Mexico, and cut back all non-native milkweed so they do not stop their journey and start laying eggs.

Narrowleaf yellowtops (Flaveria linearis) - a great nectar source

Research on monarchs is ongoing; over the past year, new insights have come out. Monarchs prefer to lay eggs on the young, tender leaves of the milkweed plant rather than the older leaves. Entomologists at Michigan State University say, “The results are published in the current issue of Biological Conservation, and they show that strategic grassland management benefits monarchs in two ways: First, monarchs lay more eggs on young milkweed, (new growth after mowing) the sole food source for the butterflies in their larval stage. Second, fewer predators visit immature milkweed; more come during its mature stages, such as when it flowers. “Monarch butterflies scout young milkweed to lay their eggs,” said Nate Haan, MSU postdoctoral research associate in entomology and the study’s lead author. “In terms of a food source, milkweed is more like spinach when it’s young and comparable to cardboard as it ages.” [5]

For the home gardener, keeping all milkweed, native and non-native trimmed down will do a couple of things; encourage monarchs to lay more eggs and assist in controlling the OE parasite, aphids, and other pests. The collective research on tropical milkweed is all telling us one thing - it's bad for migration. 1) It leads to less optimal wing development for long-distance flight; 2) it makes monarchs become reproductive, even if they are migrants; 3) it increases local and population levels of OE, which we already know hinders migration potential; and 4) it reduces their flying ability!” [6]

Cuplet Fern
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