College of Science and Mathematics


Biological Sciences Department

Degree Name

BS in Biological Sciences




Francis Villablanca


The decline in milkweed (Asclepias spp.) populations across the country, due to factors including agricultural development and herbicide use, has lead to a correlated reduction of monarch butterfly (Danaus plexippus) feeding and breeding habitat. In addition, the intentional planting of non-native milkweed in areas surrounding overwintering habitat has altered Western monarch butterfly migratory behavior, by allowing monarchs to depart from their ancestral migration pattern (timing and location), therefore decreasing the effective population size, and putting monarchs at risk of higher pathogen prevalence, due, in part, to an obligate protozoan parasite that is able to persist on the non-native, perennial species of milkweed. The presence of native milkweed plants in areas surrounding overwintering sites, such as in the Central Coast of California, provides a unique opportunity to potentially observe transitional resource utilization by monarchs that are present beyond the traditional overwintering season. If actually observed, a shift in phenology (time of resource use) shown by monarchs laying eggs on milkweed could signify a shift in population behavior. In regards to native milkweeds, we find that in the Los Padres National Forest, narrowleaf milkweed (Asclepias fascicularis) is locally restricted to streambeds. We come to this conclusion after mapping the distribution of milkweeds in relation to the streambed and testing hypotheses related to milkweed growth habits. Results show that there was a significant difference in the abundance and distribution of milkweed plants between the years 2016 and 2017. Stem abundance was not conclusively correlated with environment preference for habitats inside vs. outside of the streambed. However, there was a significant increase in the number of monarch adults present in 2017. The information resulting from this study provides insight into current milkweed distribution and monarch behavior, as well as helps to inform future studies on the status of monarchs.

Available for download on Monday, June 12, 2023