Gardener Choice Impacts Monarch Survival: Milkweed Species Identity Influences Arthropod Communities and Monarch Predation
Open Access
- Author:
- Germeroth, Lilly
- Graduate Program:
- Ecology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 19, 2023
- Committee Members:
- Sara Hermann, Thesis Advisor/Co-Advisor
Mary Ellen Barbercheck, Committee Member
Jared Ali, Professor in Charge/Director of Graduate Studies
Jared Gregory Ali, Committee Member - Keywords:
- conservation
monarch butterfly
milkweed
asclepias
community ecology - Abstract:
- The monarch (Danaus plexippus) is a species of butterfly with significant cultural heritage and adoration across North America. It undergoes an impressive yearly migration, radiating North from Central Mexico in the spring across the Central and Eastern United States (U.S.), following milkweed host plants as they sprout across the landscape. Their success is a reflection of the survival and health of the summer breeding monarchs across their thousands of miles journey towards their overwintering site. These migratory monarch butterflies have been in decline since the early 1990’s, which has been met with a desire for conservation engagement from the public. A significant portion of the effort is in supplementing the landscape with their obligate host-plant genus, Asclepias, to account for habitat loss due to agricultural intensification. This is largely facilitated by adding the milkweed, Asclepias syriaca or common milkweed, to the landscape due to its heartiness and wide range of habitat tolerance. However, this is not the only species of milkweed, particularly in cultivated backyard settings, which is a setting with immense conservation potential. In the Northeastern region of the U.S., there are two other native species of milkweed recommended for planting: A. incarnata, or swamp milkweed, and A. tuberosa, or butterfly milkweed. Interestingly, an understanding of monarch use and survival, as well as the interacting community of predatory and herbivorous arthropods that use these milkweeds, in conservation plantings is lacking. Furthermore, since this species is of public conservation concern and effort, it is surprising that little effort has gone into understanding how individual conservation plantings, like those in private garden spaces, affect monarch conservation. In Chapter 1, I report the findings from an observational field study to better understand the arthropod community across these three different species of milkweed. The necessity of this study was further supported by a survey sent out to Pennsylvania gardeners, in which 90% reported having A. incarnata and A. tuberosa in their gardens, showing a demand for understanding the ecology of the monarch on other species of milkweed besides A. syriaca in this region. We sampled each of the three milkweed species in 10 private gardens across the monarch summer breeding season to assess monarch egg and caterpillar presence as well as the community of interacting herbivores and natural enemies found on the milkweeds. We found a differential use of these species by monarchs; the most eggs were found on A. incarnata, the most larvae on A. syriaca, and the lowest numbers of each life stage on A. tuberosa. Each had a unique community of arthropods that varied in abundance across species which translated to different effects of the residing arthropod communities on monarch presence. This result is significant when considering that monarchs face their steepest population bottleneck in their first days of life as eggs and larvae, due to predation by fellow arthropods. In particular, monarch abundance was particularly impacted by aphid presence, potentially due to aphid physical modifications to the plant (honeydew accumulation), modulations of plant defenses, or recruitment of predators, which were also found to be predictive of monarch abundance in certain cases. This research presents a catalog of the arthropods observed across three species of milkweed in this region in private gardens. It also contributes understanding of community influences on monarch abundance, and combined with targeted, manipulative experiments, will elucidate causative effects of community interactions on monarch survival. In Chapter 2, I report the results from a field experiment which assessed monarch predation pressure across two of the milkweed species recommended for planting in the Northeastern region of the U.S. The research conducted for Chapter 1 revealed that each of these milkweed species has a distinct arthropod community, and we sought to assess if this difference in predator community resulted in higher predation pressure on one species over the other. For the research study in Chapter 2, monarch eggs were deployed onto A. syriaca and A. incarnata plants in a home garden and monitored for 24 hours to determine predation pressure between these different species in a conservation planting. We found predation rates for monarch eggs on both species to be intense, with 70% of the eggs exposed to the predatory arthropod community being removed across both species, with predation rates being greatest at night. Predation was more intense on A. syriaca, which, in our observational field study (see Chapter 1), harbors a more abundant predator community compared to A. incarnata. We expect that there is also a different rate of predation for neonate larvae, and future work should examine how the interaction of predator community and challenges of establishing on the plant will impact the survival of these larvae. Together, these studies investigated how the arthropod community varies between species of milkweed commercially available and recommended for gardeners to plant in the Northeastern region of the U.S. Monarchs choose to lay their eggs on A. incarnata, but this does not necessarily translate to A. incarnata contributing the most larvae that go on to reproduce as adults. On A. syriaca, we detected fewer eggs across the summer, but both A. syriaca and A. incarnata harbored similar abundances of larvae. We further examined this difference in survival with a sentinel egg experiment and found that predation was more intense on A. syriaca than on A. incarnata, but both lost more than half the deployed eggs in 24 hours. This finding leads us to believe that in addition to an oviposition preference for A. incarnata, there are more egg predators on A. syriaca than A. incarnata. Asclepias tuberosa had the fewest monarchs overall, but the few eggs that were laid on it appeared to survive to the larval stage, likely due to low interactions with other arthropods on the plant. Asclepias tuberosa also contributes significantly to the larger pollinator community with its abundant nectar resources. Each of these three species has merits for conservation plantings; they each support the monarch in different ways; they all grow in similar conditions conducive to suburban/urban backyards, and they are becoming more widely available through local nurseries. Empowering gardeners with the knowledge to make regionally and ecologically informed decisions is the goal of this research, and will contribute to their actions, collectively making a difference in the conservation of this species cared for by so many.