Think of milkweed — all of it, not just the non-natives — as a potential vector for the monarch disease commonly known as “OE.” Just like hand rails carry the common cold virus, any milkweed can carry the protozoan spores for OE, which come from the butterflies. Because people touch hand rails often, they are a good vector for the cold. Many have observed that the monarchs prefer the A. curassavica over native species, so there may be a higher concentration of potential diseases on these plants than on the native plants. In warm climates, this problem is multiplied by a lack of winter freeze that causes all tropical milkweed to die back. Balloon plant (A. physocarpa) does not tend to attract monarchs as much, and no known problems are associated with it. We feel the issues with OE are limited to a very very small portion of the United States. We encourage people to participate in the Monarch Larva Monitoring Project if they are raising monarchs, to help monitor the spread of the disease. http://www.mlmp.org/
Here is a concise, science-based Q&A about tropical milkweed and OE:
The take-home message is in the answer to question #7
• Plant native milkweeds whenever possible.
• If you have tropical milkweed, cut it back from October-February to within 6” of the ground (unless it dies back naturally on its own). Also remove any new plant growth at the base of the plant. If you live in a warm coastal area in the southern U.S. or California, cutting the milkweed back is especially important and it will be necessary to prune frequently (every 3 weeks) as it quickly re-grows.
• Consider gradually replacing your tropical milkweed with native species.
Monarch Watch does not sell tropical species of milkweed, except in very small quantities at our Spring Open House in Lawrence, KS.