August 15th, 2022
As the dog days of summer go by there are exciting things happening out in Little Lake Valley. One of the fascinating midsummer blooming plants is milkweed. We have two species that occur in this valley, the narrowleaf milkweed, Asclepias fascicularis, and the showy milkweed, Asclepias speciosa. Supplemental plantings of these milkweed species have also been part of the restoration process. Their flowers are spectacular, and this year is the first time our staff has found monarch caterpillars crawling on them!
Last year this blog reviewed the unique flowers and pollination structures of the milkweed plant, and now that we have eggs and caterpillars on the milkweed, I would like to talk about the amazing Monarch butterfly.
The monarch, Danaus plexippus, in Greek literally means “sleepy transformation.” It is native and widespread in North, South, and Central America, but the population has suffered a devastating decline that has placed the migratory monarch butterfly on the International Union for the Conservation of Nature (IUCN) Red List of threatened species. Monarchs are now classified as endangered.
North American monarchs are migratory and are split into two groups. One group lives on the eastside of the Rocky Mountains and the second group lives on the west side of the Rockies. The subspecies that lives in South America in the southern part are not migratory and are not considered endangered.
According to the World Wildlife Fund, the eastern populations “exhibit the most highly evolved migration pattern of any known species of butterfly.” They embark on a migration route traveling between 1,200 and 3,000 miles from northeast United States and southeast Canada to the mountainous forests of Central Mexico where they hibernate from the beginning of November to mid-March. Much has been in the news about their wintering grounds being destroyed in Mexico and Central America by weather exacerbated by climate change such as flooding and colder temperatures. The eastern group then migrates thousands of miles back from its wintering grounds in Central America up to the Central Midwest in the United States and up into Canada. The western population migrates from the western Rockies to the coastal areas of California and down into Baja California and back again. This population you may be familiar with since they winter in areas such as Bolinas, Santa Cruz, and Pismo Beach. They can congregate in Eucalyptus forests by the thousands if it is a good year.
The reproductive stage of the monarch is simple yet has a complex trajectory that has evolved with the migration to help the butterflies with overwintering. Adults only live two to five weeks during the summer breeding season. During this time monarchs mature enough to breed, and the females lay individual single eggs under the leaves of milkweed (and only milkweed) plants. One female can lay 100 to 300 eggs as she is migrating north or east from the coast. The larvae, or caterpillars, hatch within 3 to 5 days and immediately begin eating their egg case. The larvae only eat milkweed leaves, which gives them toxic flesh since the plants are highly poisonous. Monarchs begin life as a tiny, green, translucent, or grayish-whitish worm and quickly begin growing. In 9 to 10 days, they become large (up to 2 inches long) striking caterpillars that you can see in the photos with yellow, black, and white stripes encircling a green body with black antennae-like extensions coming out of their heads and tail-ends. Monarch butterfly caterpillars are very lovely to observe indeed. Their poisonous skin derived from milkweed provides them protection from most birds. In our area, one of the bird species that can eat adult monarchs is the Black-headed Grosbeak. This bird has a high tolerance for the chemical cardenolides that accumulate in the monarch and will not be poisoned. Further, the latest research shows that grosbeaks target male monarchs almost exclusively, which have less of the toxic chemical than female butterflies. There are also many insect predators that eat the juicy plump caterpillars such as fire-ants and wasps. It takes about 2 weeks and 5 instars, or molts, for them to become a chrysalis and another one to two weeks to metamorphize into an adult butterfly. Their black and orange pattern is a warning to most birds to stay away. As each generation goes through that life cycle the adults continue to fly ever northward (or eastward if they are the western group) until the last generation of the year, which does not reproduce.
This is a complex migration story with each phase of the journey dependent on the last, as it is the last group who are in “reproductive diapause” that then turn south or west to migrate to their wintering grounds by November. This is a fantastic life cycle for a fragile butterfly. The last generation that arrives in the north and the east holds the population success in their DNA, and they don’t reproduce until spring.
Today we found a female monarch butterfly laying her single egg on the underside of a milkweed plant. More caterpillars were observed eating leaves of other plants. It is a very hopeful development that we are happy to encourage by planting more milkweed plants.
Photos by Maureen Doyle