A Monarch butterfly’s life cycle, like those of butterflies in general, involves changes in form (metamorphoses) in four stages: egg, larva (or caterpillar), pupa, and adult.

Why is milkweed important to the Monarch butterfly?

It is the ONLY plant on which their eggs are laid and subsequently, it is the ONLY plant which the hatching larvae (caterpillars) eat. The milky, sticky sap from the leaves is actually toxic to many other animals, but Monarch caterpillars, amongst other animals, can safely absorb those toxins, rendering the adults unpalatable to most predators.

Between migrations, there are generally four generations of Monarch butterflies. In February and March of each year, overwintering Monarchs will emerge from their dormancy, and as they migrate northwards, they lay eggs on milkweed along the way. The reproductive cycle continues and by the end of August, up to four generations will have evolved. This final (fourth) generation has been dubbed the “supergeneration,” for it is the one that will migrate southwards on its long journey, and live for seven to eight months, in contrast to the previous generations’ lifespan of four to six weeks.

Interestingly, Monarchs that live west of the Rocky Mountains migrate to Pacific Grove, near Monterey in California, whereas Monarchs living east of the Rockies migrate down to central Mexico. The exact location of where the Monarchs roosted in Mexico was a mystery until 1975. That year, Catalina (Aguado) Trail and her partner Ken Brugger stumbled upon the breathtaking site of millions of roosting Monarchs in the oyamel fir trees of the Michoacan province, near Angangueo. Trail and Brugger were participants in The Great Butterfly Hunt, the very first citizen science project, launched in 1935 by U of T professor Dr. Fred Urquhart. 

It is clear that milkweed is a critical element upon which the survival of the Monarch depends. Unfortunately, large declines in Monarch butterfly populations have been observed over the past two decades, largely due to a drastic reduction in milkweed across North America. As is the case with most aspects of pollinator declines, herbicide use and farming practices have led to a loss of 140 million acres of the vital milkweed plant.

Although Monarch butterfly larvae are host-specific for food, their activities as adult pollinators are more general, pollinating other plant species in addition to milkweed.