Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Entomology

First Advisor

Dr. Daniel A. Potter

Abstract

The eastern migratory population of monarch butterflies (Danaus plexippus) is in serious decline. Most of the efforts to conserve this iconic insect focus on habitat restoration in the US Midwest. Often overlooked are small butterfly-centric gardens that can act as stepping stones between urban and rural areas. These studies aim to optimize the conservation value of such gardens.

Eight milkweed (Asclepias) species varying in height, form, and leaf shape were compared over two years in a common-garden experiment. I measured milkweed growth, rhizome spread, and bloom periods, conducted bi-weekly counts of monarch eggs and larvae, evaluated suitability for larvae, and quantified bee visitation. More monarchs were found on taller, broad-leaved milkweeds, but there was relatively little difference in larval performance. Asclepias tuberosa attracted the greatest number of bees, whereas bee genus diversity was greatest on A. verticillata.

Gardens containing the identical mix of milkweeds, flowering plants, and grasses but arranged in three different spatial configurations were monitored for monarch colonization over two years. Monarch eggs and larvae were 2.5–4 times more abundant in gardens having milkweeds evenly spaced around the perimeter than in gardens in which milkweeds were surrounded by or intermixed with the other plants. Predator populations were similar in all garden designs. In a corollary experiment, female monarchs laid significantly more eggs on plants that were fully accessible than on milkweeds surrounded by non-host grasses. In addition, I monitored monarch use in 22 citizen-planted gardens containing milkweed and nectar plants in relation to their botanical composition, layout, and surrounding hardscape. Significantly more monarchs were found in gardens having milkweeds spatially isolated and in gardens having 100 m north/south access unimpeded by structures.

The high-profile model system of milkweeds and monarchs was used to test if cultivars have equal conservation value as native wild-types. In replicated gardens I compared two species of milkweed (A. incarnata and A. tuberosa) and three of their cultivars over two years, measuring plant size, defensive characteristics, colonization by monarchs, suitability as host plants, and the bee assemblages, and Lepidopteran communities of each. I found that horticultural selection enhanced defensive characteristics in some cultivars, but did not influence larval growth and development. I also compared defensive characteristics of non-native milkweeds (A. curassavica and Gomphocarpus physocarpus) and their cultivars in the greenhouse and observed similar results.

The European paper wasp or EPW (Polistes dominula) predominantly builds its nests on structures. These invasive wasps forage for soft bodied arthropods, including monarch larvae, which may cause conservation gardens to become ecological traps. I confirmed EPW is the predominant Polistes spp. in urban gardens, documented outcomes between EPW and monarch larvae, and found that predation by EPW was more common in urban gardens than rural grasslands away from structures.

I found that the invasive Japanese beetle (Popillia japonica) aggregates and feeds on flowers of A. syriaca, the monarch’s most important host plant, reducing seed set by >90%. The beetle’s ongoing incursion into the monarch’s key breeding grounds in the US Midwest is likely to limit pollination and outcrossing of wild and planted milkweeds, reducing their capacity to colonize new areas via seeds.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.101

Funding Information

Funding was provided by USDA-NIFA-SCRI grant 2016-51181-235399 (2016) administered through IR4 Grant 2015-34383-23710 (2015), BASF Living Acres Program (2018), US Golf Association (2016), the Horticultural Research Institute (2019), Applewood Seed Co. (2018), University of Kentucky Nursery Research Endowment Fund (2016-2019), and USDA-NIFA Hatch Project no. 2351587000UGGA (2016),and Idlewild Butterfly Farm (2018).

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