Monarch caterpillar feeding on Asclepias fascicularis
All of our local native milkweeds are perennials, but like a lot of our drought-adapted plants, die back and go dormant during the long late summer and fall drought. Many gardeners, knowing there is a monarch butterfly/ milkweed connection, try to keep the milkweeds green all year, or use non-native milkweeds that stay green. Cal Poly’s Dr. Francis Villablanca has shown that winter breeding by monarchs will take place if green milkweed is available, which would not normally happen in the overwintering populations in SLO County. Nonstop breeding on the same plants can perpetuate the transmission of a devastating parasite called OE, for Ophryocystis elektroscirrha.
Normally, the transmission cycle is broken when milkweeds go dormant. The infection can kill adults as they emerge from their chrysalis, while mildly infected monarchs fly poorly, don’t reproduce normally, and die early. These very sick butterflies can then carry spores of the pathogen into the milkweeds in other gardens or along the entire migration route.
You don’t have to tear out a non-native milkweed if you cut it way back. While the infection issue is much greater for the central USA migration paths, it is critical that we take preventive actions on the coast, especially since we are still determining how bad it actually is in California.
Many thanks to Dr. Villablanca of Cal Poly on putting all of this together.