While tropical milkweed (Asclepias curassavica) is not native to the United States, it is commonly grown in gardens across the country. This species is used by monarchs as reported by MLMP volunteers and others. However, conservation organizations like the Monarch Joint Venture recommend cutting back tropical milkweed in areas where it persists or can persist year round. There is strong evidence that year-round availability of host plants causes some monarchs to skip the traditional long-distance migration and reproduce throughout the winter months in parts of the southern U.S. and California. This behavior can have negative implications for the population. For instance, year-round breeding can facilitate greater transmission of the protozoan parasite Ophryocystis elektroscirrha (OE), which decreases monarch fitness and survival. Therefore, the MJV recommends that tropical milkweed should be cut back in the winter and fall months in the southern U.S. and California, and should be gradually replaced with native milkweeds as they become available. (Monarch Joint Venture, Potential risks of growing exotic (non-native) milkweeds for monarchs).
You may be wondering if these conservation recommendations apply if you are reporting data to the MLMP or other monarch citizen science programs. The overall goal of the MLMP is to further the conservation of monarchs, so our recommendation is to follow conservation guidelines like those put forth by the MJV to cut back tropical milkweed in the fall and winter. Winter cut back timing should mimic the availability of native milkweeds in your area.
Ultimately, the decision is yours whether or not you cut back tropical milkweed. If you do decide not to cut it back, please continue to report your weekly monarch density observations for Activity 1, as these data are interesting in comparison to other sites and increase our understanding of the circumstances under which winter breeding occurs. If you do cut back your milkweed, continue reporting on your milkweed until you cut it back, noting in your site profile of the disturbance/cut back date. If adult monarchs continue to be active in your area even after you have cut tropical milkweed back, you can still share your individual observations through our Milkweed and Monarch Observations page. Whatever you decide, your data will be interesting!
Additionally, if you participate inActivity 3: Estimating Monarch Survival make sure you are also testing your monarchs for OE and sending your results to Project Monarch Health. The data from winter breeding monarchs are particularly informative in helping track parasite prevalence in breeding vs. overwintering monarchs.