Monarch butterflies (Danaus plexippus) rely upon milkweed plants for survival, as it is the only plant upon which female monarchs oviposit and it is the sole food supply for monarch larvae. Additionally, cardenolide, a defense toxin within milkweed tissue, is sequestered by monarchs, making them unpalatable to most predators. Eastern and Western monarch populations in the United States are in sharp decline, in part due to the growing scarcity of milkweed, and reestablishing milkweed habitat is crucial to the monarch’s long-term survival. This study aimed to study the effects of plant species diversity and shade on the growth and cardenolide content of two species of milkweed common in the Pacific Northwest (PNW). Milkweed plants were grown outdoors over three summers at the University of Portland, under varying levels of plant species diversity and shade. Four growth measurements and cardenolide concentration were analyzed. Species diversity affected several growth measurements but not cardenolide concentration. Medium shade conditions resulted in increases in three growth measurements and cardenolide concentration compared to low and high shade conditions. The results suggest PNW milkweed can grow under more diverse conditions than previously known, which may be useful information for reestablishment of milkweed habitat supporting the highly imperiled Western monarch population.
Silva, Estrella V.; Mattson, Rylee; Klassen, Katherine; and Dizney, Laurie
"The Effect of Species Diversity and Shade on Milkweed Growth and Cardenolide Concentration,"
RURALS: Review of Undergraduate Research in Agricultural and Life Sciences: Vol. 15:
1, Article 3.
Available at: https://digitalcommons.unl.edu/rurals/vol15/iss1/3