Gordon Pratt, High Country Conservancy
For years I have heard that the perennial Mexican milkweed (Asclepias currassavica) is bad for monarch butterflies, but that seemed to be like an old wives’ tale since no one could explain why. Recently I found out from a friend that a protozoan parasite (Ophryocystis electroscirrha) hangs out in milkweeds (perhaps in the sap) and at high concentrations the protozoan kills monarch caterpillars. This interaction is natural and has probably been going on for some time. It seems that the caterpillars pick up the parasite from eating the infected milkweed leaves.
Those plants with high concentrations of the protozoan are more likely to kill monarch caterpillars or chrysalises. Since the non-native Mexican milkweed is perennial and does not die back like most of our native milkweeds, it builds up high concentrations of the protozoan. So, for this reason native milkweeds have lower concentrations of the deadly protozoan and are less likely to kill monarch caterpillars. It seems that you can reduce the protozoan population in the Mexican milkweed by cutting them back in the fall and not allowing them to store high concentrations of the protozoans over the winter. It is best to plant milkweeds that are native to our area, like narrowleaf milkweed (Asclepias fascicularis).
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