Not Easy Being Mead's: Comparative Herbivory on Three Milkweeds, Including Threatened Mead's Milkweed (Asclepias meadii), and Seedling Ecology of Mead's Milkweed

Abstract

Chapter 1: Comparative Herbivory and Herbivore Effects on Reproduction for Three Milkweeds (Asclepias) in Two Landscape Contexts. The ability of herbivores to regulate plant populations depends on many factors including herbivore species, abundance, and phenology. These factors can be dynamic across the landscape and between plant species. Where plant species share common herbivores, they may also interact with each other through apparent competition, a form of associational susceptibility. Milkweeds (Asclepias) are an excellent system for studying the relationships between herbivores, plants, and plant reproduction in a complex community. Milkweeds are well-known for their plant defenses and the guild of specialist insects that feed on them, but generalist herbivores also feed on some milkweeds. I compared herbivore activity and its effect on plant reproduction for three milkweed species (Asclepias syriaca, Asclepias viridis, Asclepias meadii) in eastern Kansas. I selected plants of each species at two field sites in distinct prairie landscapes and monitored them for herbivory throughout the growing season, from before bud formation to seed pod maturation. Milkweed herbivores showed major differences in abundance and phenology between plant species and study site. Damage from herbivores was implicated in the reproductive failures approximately half of all A. meadii and A. viridis ramets but only a small portion of A. syriaca ramets. One of the milkweed species, A. meadii, is federally threatened, so a better understanding of this milkweed-herbivore community has conservation management implications. High rates of herbivory on A. meadii suggest that herbivore control measures, especially for deer, would be beneficial to some populations of this rare plant. Chapter 2: Influence of seed characteristics and site conditions on establishment of a threatened prairie milkweed, Asclepias meadii, in Kansas. Population restoration and reintroduction are critical aspects of many plant conservation efforts but factors affecting the earliest life history stages, critical to the establishment of new individuals, are often poorly understood. I investigated the influence of seed characteristics and manipulations of the field environment on seedling emergence and seedling growth in Asclepias meadii (Mead's milkweed). Seeds of known mass and maternal plant were reared in a greenhouse and also in experimental restoration plots with combinations of burning and soil disturbance treatments. Seed mass was positively correlated with seedling emergence but not seedling growth. There was no detectable effect of burning on seedling emergence, but it had a negative effect on seedling growth. The effects of soil disturbance on both seedling emergence and growth were non-significant. Mass may be a useful metric for evaluating seed stocks for reintroduction and the quality of seeds produced in restored populations. Pre-emergence manipulations of a restoration site do not facilitate emergence or growth and may even be detrimental to restoration efforts. High survivorship of seedlings during their first year and overwinter suggests that direct sowing of seeds into the field is an effective restoration technique for A. meadii.