Asclepias is a genus of plants commonly known as milkweed. When broken, the stems or leaves of milkweed issue a milky sap that defends the plants from being eaten. Another common factor amongst the milkweeds is the unusually generous nectar that their flowers provide to pollinators. This makes them extremely attractive to domestic and wild bees, and an important food crop for adult butterflies, including the endangered Monarch butterfly. However, recent studies seem to indicate that the plant that the Monarch prefers to lay her eggs on is Asclepias incarnata (Swamp Milkweed). This is the milkweed favoured as a food plant for the Monarch caterpillar. The adult will feed from any of the milkweeds, but the caterpillar is primarily dependent on this species. Studies show that the adult female will also lay her eggs on A. syriaca and A. speciosa in laboratory settings.
To add confusion, a couple of milkweed species were, until recently, listed as noxious weeds, so the availability of their seeds was very limited. In a (sensible, we think) effort to conserve the Monarch's feeding habitat, these plants have been de-listed. Campaigns are underway to plant Asclepias enthusiastically for pollinator conservation — and to plant A. speciosa for Monarch caterpillar conservation.
The Monarch butterflies that have received the most attention in the last couple of years migrate between central Mexico and southern Canada east of the Rocky Mountains (largely into Ontario and Quebec). A western population migrates between south coastal California and the BC interior. There is limited, but occasional crossover between populations. While Monarch butterflies are rare in south coastal BC, all Asclepias species are coveted food plants for most butterflies.