Abstract: Throughout the world, vertebrate and invertebrate species are declining with expanding human population and production. More specifically, the insect decline can be seen through the decline of monarch butterflies (Danaus plexippus) in North America and is apparent in both naturally migrating populations. Insects play a large role in balancing ecosystems with behaviors that function to recycle nutrients, decompose matter, support the food web, and many more (van der Sluijs, 2020). Butterflies are significant pollinators and support the reproduction of plants and crops. Monarch butterflies are also used as an indicator species to better understand the health of the ecosystem due to their history of being a densely populated species. Unfortunately, with the many impacting anthropogenic factors including habitat loss, pesticide use, and the changing climate (Halsch et al., 2020), monarch populations have dropped over 90% within 2 decades (Greiner, 2022). This study is done to increase knowledge on the contamination of pesticide chemicals in the plant material of the host plant of monarch butterflies, milkweed, with emphasis on narrowleaf milkweed (Asclepias fascicularis). Done in southern California, primarily on Chumash and Tongva/Gabrielino indigenous lands, 96 samples of narrowleaf milkweed were collected from 20 sites and will be tested for 262 possible compounds using liquid chromatography mass spectrometry. By mapping out the presence of pesticide contamination of narrowleaf milkweed plants, we hope to bring insight on how far chemicals may spread from the site of application. Our results suggest that samples taken from sites closer to restoration areas, where pesticides were used, are contaminated with an abundance of different pesticide compounds. Similarly, sites closer to the boundary line of the Santa Monica Mountains National Recreation Area were highly contaminated. Results varied in the sites between restoration areas and the boundary line, with more contamination occurring at lower elevation. In conclusion, this study suggests that understanding the drift of pesticide chemicals is necessary to foster successful protection and restoration of endangered species through the improvement of management strategies.
