We haven't seen any monarch butterflies this year at Mill Creek yet, but our milkweed is poised and ready for their arrival.
These plants, Asclepias syriaca, growing among
the shrub roses are "volunteers," started from seeds from a plant that grew last year nearby.
Besides this common milkweed, we also have some swamp milkweed, Asclepias incarnata, growing nearby. The common milkweed began blooming about a week ago, coincidentally just about the time that monarchs first arrived on the scene in our part of the world. The monarchs are attracted to the milkweed flowers for nectar, but more so to the plant's leaves as a place for females to deposit their eggs. Milkweed is the only plant monarch caterpillars will consume; without the important host, monarchs would not be able to survive.
Deposited on the underside of milkweed leaves, monarch caterpillar eggs hatch four or five days after being laid. An enormous eater, the tiny caterpillar consumes 2,700 times its weight and molts five times during this stage of its life cycle, before beginning the next phase by constructing a protective, shiny emerald-green case ringed with golden dots, called a chrysalis. Inside the chrysalis, the caterpillar, now called a pupa, rebuilds itself into an adult butterfly, all within the span of about 10 days to two weeks.
The emerging butterflies live about two to six weeks as an adult, migrating north to reproduce, the females laying eggs and starting another generation, likely the last of four since early March, when the process began in a fir forest in Central Mexico's Sierra Madre Mountains. (Other overwintering sites are in Florida and Baja, Calif.) Each generation of monarchs is pre-programmed to migrate a few hundred miles north, reproduce and die -- excepting each season's last generation, which lives through each winter. Usually in Southern Canada by late summer, each of these butterflies somehow knows it must fly south several thousand miles, back to the very forest, if not the same tree, from which its great-great grandparents hailed. There, it will cluster with groups of tens of thousands per tree until starting the annual process again next March.
There are many types of milkweed, all of which can serve as host to monarchs. Milkweed "sap" contains a poison, cardenolides or cardiac glycosides, which is similar to the heart medicine digitalis. It is poisonous to most vertebrates but harmless to monarch caterpillars, which ingest vast quantities of the substance and then maintain high levels of the poison throughout their life stages. Vertebrate predators either learn to steer clear of monarchs and milkweed through a onetime bad-tasting experience; others just seem to know instinctively to eschew and not chew the poison source.