There are two main populations of monarch butterflies in the United States — the eastern and the western.
The eastern population is larger and encompasses all states east of the Rocky Mountains and into Canada. These monarchs overwinter in forests on 12 mountaintops in Mexico and migrate north in the spring. It takes about 5 generations of monarchs to make the long trek, with each generation living just five weeks.
Western monarchs live from California to Washington and east to Nevada. This population overwinters in about 300 small colonies in coastal areas from Baja California to just north of San Francisco. Eastern populations of Monarchs are rising and in 2018 in Mexico, the largest hectare area of occupied forest was measured since 2007.
Western monarchs’ populations have declined about 97% since the 1980s to only about 28,429 individuals as counted by citizen scientists in 2018. These volunteers gather each year at overwintering sites on Thanksgiving to monitor butterfly numbers.
The previous year in 2017, 192,668 butterflies were counted. Though populations fluctuate yearly, this unprecedented drop to such low numbers is alarming.
The 2018 numbers represent a 99% drop since the 1980s when there were an estimated 4.5 million western monarchs. A new study has some ideas about what we can do to help.
There are a number of reasons the populations are declining so precipitously, with prescriptions to help stop the struggling monarchs, including protection of overwintering sites.
Western monarchs overwinter in trees, chiefly eucalyptus groves, and activities like tree trimming can disturb butterflies and cause them to abandon sites. In some sites rats have been a big issue, dwelling under boardwalks put in for butterfly viewing and coming out a night to eat butterflies.
Butterflies are susceptible to pesticides and the Xerces Society for Pollinator Conservation recommends no cosmetic use of pesticides on landscaping plants, especially those pesticides containing neonicotinoids.
Leaving milkweeds intact in farm fields and roadsides instead of herbiciding, mowing or tilling them is also very important.
Weather is a big factor affecting both breeding and migration, and adverse weather (rain) can greatly increase mortality rates.
Preserving and restoring the migrating monarch’s summer habitat of milkweed and nectaring plants is key to supporting them.
What can we do as homeowners and citizens to help monarchs? A new study titled “Linking the Migratory Cycle of the Monarch Butterfly to Understand its Population Decline” by Anurag Agrawal at Cornell University may give some new practical advice.
He released a short video called “Beyond Milkweed: monarchs face habitat, nectar threats” that outlines his findings and recommendations.
To date most recommendations have been to plant milkweeds and spring flowering nectar plants so successive generations of northbound butterflies have places to lay eggs and adults have food during their journey.
Agrawal looked at summer population numbers of eastern monarchs and then measured when populations began to decline at the end of summer.
He explains in his video how at summer’s end, eastern population southbound butterflies are in a nonreproductive phase called a diapause, which allows them to save all their energy for the long flight to Mexico.
At this point, food in the form of nectar is most important to their survival and ability to overwinter, which they must build up fat reserves for.
By measuring when populations began to decline, he determined that this begins in the fall as they migrate south, and it likely stems from a lack of nectar sources or having too much distance between them.
This final generation of butterflies for both eastern and western populations must both fly south, and then overwinter until around March when they fly to nearby milkweeds, lay eggs and then die.
At least some of the western monarch population seems to continue to lay eggs on milkweeds on its journey south, with butterfly watchers regularly reporting seeing egg laying until mid- November.
Agrawal emphasizes that fall blooming nectar plants are vital to migrating butterfly’s survival and are usually neglected in recommendations to help them. Milkweeds bloom in the spring and are usually long finished by fall, so will not assist butterflies at this time for nectar.
There are, however, many fall-blooming plants for our gardens that we enjoy just as much as butterflies.
Annual plants include ageratum, zinnia, Mexican sunflower (tithonia), gomphrena, single marigolds, and Salvia farinacea.
Perennials include goldenrod, agastache, asters, salvia, joe pye weed, sedum, helenium, leonotus, mint, oregano, persicaria, Salvia “Indigo Spires,” “Salvia Mystic Spires,” Verbena bonariensis (not “Lollipop”), and California fuchsia (Zauschneria or epilobium).
Shrubs include dwarf buddleia (much easier than the large shrubs), Caryopteris, lantana, Tecoma stans, and abutilon.
Kate Frey’s column appears every other week in Sonoma Home. Contact Kate at: firstname.lastname@example.org, freygardens.com, Twitter @katebfrey, Instagram @americangardenschool