Interactions between dioecious plants and insect herbivore chewing vibrations and life history and laboratory rearing of the red admiral, Vanessa atalanta
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Chapter 1: Life History and Laboratory Rearing of the Red Admiral, Vanessa atalanta The red admiral butterfly, Vanessa atalanta (Linneaus, 1758) (Lepidoptera: Nymphalidae) is a globally distributed species and model organism for studying migration patterns and effects of climate change. Most previous red admiral research focused on wild populations. Establishing laboratory colonies allow for experimentation with a multitude of lab-based plant-insect interactions. We also describe red admiral butterfly life history and laboratory rearing methods. Chapter 2: Interactions between dioecious plants and insect herbivore chewing vibrations Chemical defense theory predicts trade-offs between plant growth, defense, and reproduction. In dioecious plant species males and females grow as separate plants and each sex often investment differentially among growth, defense, and reproduction (Herms and Mattson 1992). Typically, female plants invest in defense whereas males invest in growth. We investigate differences in resource allocation strategies between male and female Amaranthus palmeri, (palmer amaranth) and Urtica dioica (stinging nettle), in the absence of herbivore stimulus in a series of greenhouse experiments. In addition, we investigate the effects of simulated herbivory on stinging nettle male and female growth, defense, and reproductive effort. In the absence of herbivore stimulus, we found no differences in growth rate or specific leaf area and root, leaf, stem mass between both sexes of palmer amaranth and stinging nettle. However, stinging nettle males produce greater floral mass and females possessed a greater density of leaf trichomes in the absence of herbivore stimulus. In response to simulated herbivory, growth of stinging males was greater than females and we no longer observed differences in trichome production or floral mass among sexes. The compensatory growth in response to simulated herbivory of stinging nettle males and greater initial female physical defenses (trichome density) was as predicted. As differences were observed among the two sexes of stinging nettle to simulated herbivory, we chose to investigate other herbivore stimuli to analyze differential resource allocation strategy responses of dioecious plants. Insect herbivores have the potential to severely reduce plant fitness, plants have evolved several mechanisms to detect herbivore feeding cues such as saliva and respond with induced chemical defenses. Plants also detect herbivore caterpillar footsteps, frass, and bioacoustics vibrations (Cocroft and Appel 2014). We investigate if male and female plants respond to insect herbivore bioacoustic stimuli differentially. Using two dioecious plant species (stinging nettle and palmer amaranth) we recorded Spodoptera frugiperda, the beet armyworm, and Spodoptera exigua, the fall armyworm foliage feeding on each plant respectively with a laser vibrometer. Using piezoelectric actuators, we played back the vibration recordings to naive male and female plants. We quantified trichome production among the stinging nettle sexes and leaf chemical defense hormone jasmonic acid production, between both plant species' sexes after exposure to vibrational stimuli. No differences in levels of foliar jasmonic acid between each species' sexes were found, nor in trichome densities of stinging nettle males and females after exposure. Female plants were predicted to have a greater response to herbivore bioacoustic stimuli than males. Simulated herbivory experiments resulted in stinging nettle responding as predicted by chemical defense theory tradeoffs regarding dioecious species, which suggests either our testing method of, or insect herbivory vibrational stimulus itself is not strong enough to elicit a response. Further testing of the modified bioacoustic stimulus application methods and, due to high variation in naive plant foliar jasmonic acid levels among individual plants sexes and species tested, analysis of individual plant jasmonic acid levels before and after the application of the bioacoustic stimuli is suggested to compare to the results of our study.
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M.S.