Published: July 28, 2021

Monarch butterflly view from undersideMonarch butterflly view from aboveMonarch butterflies (Danaus plexippus) are perhaps one of the most well-known insects in North America. They are easily recognizable by their bright orange wings with black veins and their jet-black abdomens speckled with white spots. Males can be distinguished from females by the two black spots in the center of their hind wing, which females do not have. Can you tell if this Wonder of the Week is a male or female? The caterpillars of this species, found on milkweed, are striped yellow, black and white. The chrysalis is light green with yellow spots along the edge.

Adult Monarchs are migratory insects and can be found in North America from southern Canada in the summer to central Mexico in the winter. There are two distinct groups separated geographically by the Rocky Mountains. The largest group occurs east of the Rocky Mountains and individuals from this group migrate in successive generations in the summer, traveling from their overwintering sites in central Mexico, through Texas, over the plains, and end their journey in the Great Lakes Region. In the fall, these monarchs return to their overwintering site in Mexico. The exact route of travel varies depending on the place of departure. The other group occurs west of the Rockies—less is known about their migratory routes; although Monarchs in California and from Oregon and Washington do overwinter at several sites along the California coast.

Coloradans can see Monarchs passing through the Front Range between mid-June (heading north) and September (heading back south). It is estimated that monarch numbers in the eastern group have declined by about 84% over the last several decades which has been linked to climate change, loss of foraging and overwintering habitat, disease, and use of pesticides in agriculture.1

Like all butterflies, monarchs undergo complete metamorphosis with four main life stages: egg, caterpillar, chrysalis, and adult. Female Monarchs lay eggs on their host plant, many species of milkweed. The egg hatches and the hungry caterpillar feeds on milkweed, which contains cardenolides, or heart poisons. The caterpillars are not affected by the cardenolides and instead sequester this poison in their blood, or hemolymph as it is called in insects. After about two to three weeks the caterpillar has completed growing and it forms a chrysalis. After another one to two weeks, a monarch butterfly emerges from the chrysalis, and continues on its journey. The striking orange coloration serves as a warning to predators, such as birds, that the butterfly is poisonous and should not be eaten. Planting milkweed and discouraging the use of pesticides can help slow the decline of this species.

Common name: Monarch Butterfly
Scientific name: Danaus plexippus (Family: Nymphalidae)
Catalog number: UCMC 0197159
Label data: Sunshine Canyon, Boulder County, Colorado; August 2, 1952; Collected by Don Eff

1. Thogmartin, W. E., Wiederholt, R., Oberhauser, K., Drum, R. G., Diffendorfer, J. E., Altizer, S., Taylor, O. R., Pleasants, J., Semmens, D., Semmens, B., Erickson, R., Libby, K., & Lopez-Hoffman, L. (2017). Monarch butterfly population decline in North America: Identifying the threatening processes. Royal Society Open Science, 4(9), 170760.

2. Reichstein, T., Euw, J. von, Parsons, J. A., & Rothschild, M. (1968). Heart Poisons in the Monarch Butterfly. Science, 161(3844), 861–866.