Management practices at western monarch butterfly overwintering sites are often based on the "common knowledge" that monarchs "prefer" eucalyptus. Yet whether or not monarchs prefer eucalyptus has never been formally tested, in part because such a test would require that monarchs have a choice (options) and express a preference given those options. A test of the hypothesis is provided here. The study compares monarch tree use versus the tree species availability at five Central California overwintering sites across three overwintering seasons. The results show no support for the hypothesis that monarchs prefer eucalyptus. In fact, when monarchs used a tree species disproportionately given the tree's availability, they were usually clustered on native conifers rather than on eucalyptus. These findings lead to a recommendation against simply planting more eucalyptus as a habitat management strategy. Instead, the findings lead to a recommendation that overwintering groves be managed to include and maintain a mixture of native tree species such as Monterey cypress (Hesperocyparis macrocarpa) and pitch canker-resistant Monterey pine (Pinus radiata).
O.e prevalence in Southern California monarchs breeding on non-native milkweed. Satterfield et al 2016.
In 2015 Dara Satterfield and Sonia Altizer showed that eastern monarchs that do not migrate are much more likely to have a high prevalence of O.e. than monarchs that do migrate. Their result is based on winter breeding monarchs from the Gulf Coast of the U.S., as compared to monarchs overwintering in Mexico. The research team predicted that winter breeding monarch butterflies in Southern California would show a similar pattern. Indeed, citizen science data from Southern California showed that monarchs reared on non-native milkweed in the winter months had a nine times greater proportion of the population that was heavily infected with O.e. versus migratory monarchs from Southern California overwintering sites. The current recommendation is to NOT plant milkweed near the coast, but instead to focus on nectar plants. If milkweed is planted near the coast, then it should be a local native milkweed (as these all die back in the winter).