According to the USDA they use all nine species, caveat, this study was done in milkweed that occurs in Iowa:
Female monarchs will lay eggs on all nine milkweed species, but they prefer some over others.
- Swamp milkweed (Asclepias incarnata) and common milkweed (A. syriaca) averaged the highest number of eggs.
- Monarch caterpillars hatching from eggs laid on tall green milkweed (A. hirtella) and prairie milkweed (A. sullivantii) had the lowest survival rates.
- The height and number of blooms on the milkweed plants across all nine species weren’t factors influencing the female butterflies’ egg-laying preferences.
The findings indicate that while female monarchs do make choices, they don’t specialize in reproducing on a single milkweed species. What’s more, their egg-laying preference can change according to the time of season, the prevalence and habitat of the milkweed species they encounter, and the plants’ robustness and maturity.