Myers, A. T. 2019. The interacting influences of habitat context and predators on monarch butterfly (Danaus plexippus L.) oviposition and survival in agricultural landscapes. Dissertation, Michigan State University, East Lansing, MI.
Monarch butterflies (Danaus plexippus L.) are among the most recognizable and beloved insect species in North America. However, concern about the long-term persistence of monarch populations has grown during recent years due to the declining overwintering aggregations of both the eastern and western population segments. One hypothesized driver of the decline of the eastern population segment is the reduction of milkweed host plants from agricultural landscapes through the use of herbicide-tolerant row cropping systems. Proponents of this hypothesis reason that with access to fewer host plants, monarchs are not able to lay their full complements of eggs during their lifetimes, resulting in lower population growth rates during the summer breeding season in the core of the breeding range. This hypothesis has motivated research efforts to determine which species of milkweed attracts the greatest oviposition rates and where these plants can be positioned to maximize egg laying by monarchs. Here I propose that monarch conservation efforts would also benefit from increased knowledge regarding the impact of predators on monarch eggs and larvae, as boosting survival during early life stages could substantially increase monarch breeding productivity. I performed several field experiments to investigate how habitat context and interactions with predators influence monarch oviposition and predation pressure. Using sentinel milkweed host plants and monarch eggs placed in various habitat treatments, I found that monarchs exhibit oviposition habitat preferences that vary between corn and grasslands depending on the year. I also showed that monarch egg survival over 72 h varies by year, but can be as low as 10% in grasslands. These experiments demonstrate that numbers of eggs observed by previous weekly surveys of various habitats reflect both the effects of oviposition preferences and predation rates and that grasslands represent relatively risky monarch egg habitats. Next, I sought to determine which predators were responsible for monarch egg mortality in grasslands by video monitoring over 150 monarch eggs. I found that a diversity of predators consumed monarch eggs, with a plurality of eggs consumed by spiders and a majority of eggs consumed during nocturnal hours. Finally, I sought to determine how the presence of ants influences monarch oviposition patterns and how aphid presence and species on host plants shapes monarch interactions between ants and monarch neonates. I measured first instar survival at 96 h on plants infested with Myzocallis asclepiadis, Aphis asclepiadis, or no aphids and with and without ants excluded and found the lowest monarch survival on Myzocallis-infested plants , but this effect disappeared when ants were excluded. I also found that wild monarchs laid significantly more eggs on plants with ants excluded, indicating that monarchs avoid ovipositing on plants with ants. Taken together, these studies demonstrate that reducing predation pressure on monarchs during early life stages has the potential to serve as an effective way to increase monarch productivity on limited land space in agricultural areas and provides important information regarding how habitat context and interactions with predators and other arthropods interact to influence monarch oviposition patterns and egg and early larval survival. I interpret the results of these studies in the context of monarch conservation and make recommendations for further study.
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