Conserving monarch butterflies through habitat management and policy
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Monarch butterflies (Danaus plexippus) are in decline, with a 91 percent population reduction since 1996. Habitat loss is a major contributor to this decline. The milkweed plant (Asclepias spp.) is a critical food resource for monarchs; however, milkweed use by monarchs depends on milkweed species, community structure and landscape context. The main objectives of this dissertation were to: 1) determine best management practices for monarch habitat restoration, 2) understand the effects of oleander aphids on monarch egg abundance, and 3) evaluate the representation of monarchs and monarch conservation actions in State Wildlife Action Plans (SWAPs). To determine best management practices for monarch habitat restoration, we established plots of swamp (Asclepias incarnata) and common (Asclepias syriaca) milkweed in different landscape contexts and assessed the abundance of monarch eggs, oleander aphids (Aphis nerii) and lady beetles (Coccinellidae) weekly from May- September. Structural equation modeling (SEM) revealed that monarch egg abundance was positively related to the presence of aphids on both milkweed species. We also found that wooded edges increased monarch egg abundance on swamp milkweed and wildflowers increased egg abundance on common milkweed. Lady beetles formed a numerical response to oleander aphid prey in the field, but a laboratory study documented that lady beetles were more likely to consume oleander aphids than monarch eggs. To better understand the effects of oleander aphids on monarch egg abundance, we investigated monarch oviposition behavior in response to aphid presence. Behavioral studies conducted in a controlled greenhouse study confirmed the importance of aphids in influencing butterfly oviposition choice. A quadratic equation best described the relationship: at low to intermediate aphid densities the number of eggs laid was positively related to aphid abundance, while egg abundance declined at high aphid densities. To investigate potential mechanisms behind monarch oviposition decisions, we collected volatile organic compounds (VOCs) from plants with different densities of aphids. We found increased expression of three plant volatiles in response to increasing aphid density: one putative pollinator attractant and two associated with plant responses to herbivore feeding. We also examined oleander aphid effects on milkweed flower nectar, a critical energy source for adult butterflies. High densities of oleander aphids significantly lowered the volume of milkweed flower. Experimental work enhances our understanding of the impact of the insect community and landscape context on monarch host plant use and informs habitat management approaches for conservation. Developing policy that incorporates these ideas will enhance the breadth of the impact of this work. We analyzed all US 2015-2025 State Wildlife Action Plans (SWAPs) to assess and compare state-level monarch conservation efforts. We found monarch representation in SWAPs varied widely and lacked geographic alignment with actual conservation needs and interstate coordination. For example, in the contiguous United States, 16 states that occupy critical monarch migration corridors omit listing monarchs as a species of conservation need. Monarch conservation involves both habitat planning and policy change; we hope to inform conservation work through both strategies. We found that effective monarch habitat plantings should include milkweed planted with wildflowers. Land managers should also consider planting milkweed along tree lines or in landscapes that can provide adult monarch refuge from heat and predators. Oleander aphids that colonize milkweed plants do not need to be controlled on milkweed plants, until their population densities reach high levels, since they can be beneficial for larval monarchs and are associated with higher abundances of monarch eggs at lower densities. To implement these findings and conserve monarch butterflies, we can encourage states to include these management strategies in their SWAP documents.
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Ph. D.