http://hdl.handle.net/1903/2767 2013-05-23T18:16:54Z http://hdl.handle.net/1903/13282 Title: Local versus regional processes impacting insect diversity loss from urban headwater streams Authors: Smith, Robert Francis Abstract: Watershed urbanization alters hydrologic, geomorphic, and chemical properties of stream ecosystems and decreases water and habitat quality for stream fauna. The resulting loss of insect diversity from urban streams and the mechanisms resulting from in-stream habitat degradation are well documented. In this study, I focus on how urbanized terrestrial landscapes contribute to diversity loss from stream insect communities. Characteristics of fragmented urban landscapes likely increase mortality, reduce reproduction, and block dispersal by adult stream insects, which can contribute to reduced diversity. I compared the adult caddisfly assemblage among 4 urban and 4 rural headwater streams and found no difference in abundance but lower taxa richness at urban than rural headwaters. Poor in-stream and terrestrial environmental conditions likely caused decreased adult diversity at urban headwaters. I expanded this study by comparing adult and larval assemblages residing in the same 8 headwater streams. Patterns of larval and adult taxa richness, assemblage composition, and urban landscape structure at individual streams suggested that constrained dispersal through urban landscapes (a regional process) and poor environmental conditions in the stream (a local process) both contributed to larval diversity loss from urban headwaters. I also compared flight morphological characteristics of 2 caddisfly species restricted to rural headwaters to 4 species that commonly immigrated to urban headwaters. Wing length and wing aspect ratio did not differ between these 2 groups, but species specific differences for both measures indicated that flight ability may have supported immigration to urban headwaters for certain species. Finally, I used multimodel inference testing to determine what combination of geographic distance, environmental dissimilarity, and land use between streams best explained patterns of taxonomic dissimilarity between in-stream insect communities within 4 Maryland watersheds. All three variables together, based on Euclidean geographic distance, provided the best model fit and supported the hypothesis that regional and local processes structure stream insect communities simultaneously. The overall conclusion from my work was that site specific characteristics of terrestrial urban landscapes caused species specific constraints on adult insect migration that contributed to larval diversity loss from urban streams simultaneously with poor in-stream habitat and water quality. 2012-01-01T00:00:00Z http://hdl.handle.net/1903/13270 Title: EVALUATING THE IMPACTS OF CROTOLARIA JUNCEA, SUNN HEMP, ON ARTHROPOD POPULATIONS AND PLANT GROWTH AND DEVELOPMENT IN A ZUCCHINI INTER-CROPPED AGROECOSYSTEM Authors: Hinds, Jermaine Leabert Abstract: Recently, studies have shown that crop diversification strategies can be effective and sustainable means of suppressing pests, improving crop growth and enhancing beneficial soil organisms. Experiments were conducted in 2009, 2010 and 2011 to investigate the impacts of the tropical cover crop sunn hemp (SH), Crotalaria juncea, utilized as a living mulch and green manure on insect populations, crop growth and the nematode community. When inter-planted as a living mulch, SH reduced populations of cucumber beetles (Acalymma spp. and Diabrotica spp.) compared to monoculture zucchini, Cucurbita pepo, plots. When SH was utilized as an organic mulch, SH treatment plots resulted in significantly larger zucchini plant biomass and yield than monoculture plots. In 2011 when SH was strip-tilled, this resulted in a more nutrient enriched soil as indicated by nematode abundances and calculated soil health indices compared to monoculture plots. Organic fertilizer application resulted in late-season increases in bacteria feeding nematodes 2012-01-01T00:00:00Z http://hdl.handle.net/1903/13251 Title: The role of host-plant hybridization in host-associated population divergence in Phytomyza glabricola (Diptera: Agromyzidae) Authors: Hebert, Julie Byrd Abstract: <italic>Phytomyza glabricola</italic> (Diptera: Agromyzidae) is a leaf-mining fly native to the eastern United States that mines two sympatric native holly species, <italic>Ilex coriacea</italic> and <italic>I. glabra</italic>. Recent work revealed significant genetic divergence between host-associated populations of flies in North and South Carolina, suggesting the populations are host forms and recent work in <italic>Ilex</italic> phylogenetics hint the two holly hosts may hybridize. In this work, I investigated potential ecological speciation in <italic>P. glabricola</italic>, hybridization in its host plants, and how the hybridization among host plants may affect gene flow between host forms of the flies. No-choice mating trials in a greenhouse revealed reproductive isolation between host forms of <italic>P. glabricola</italic> and suggested female flies are capable of making oviposition mistakes resulting in adult offspring on the non-natal host. Based on these results, I used sequences of the nuclear gene EF-1&alpha; and AFLPs to genetically confirm host form status of the flies, and identify <italic>I. glabra</italic> as the ancestral host. In addition, genome scans revealed several loci under divergent selection among the hosts, suggesting the flies may be undergoing ecological speciation. To investigate the role host plants may play in the genetic divergence among flies, I first used AFLPs to confirm hybridization between <italic>I. coriacea</italic> and <italic>I. glabra</italic>. Hybridization rates differed across the geographic range of the species, which was also reflected in the morphology of the leaves. There were no general patterns, however, in the phenotypes of hybrid plants, and no single morphological trait that could reliably identify the hybrids. Finally, I combined genetic data of the flies and the plants to determine whether hybrid plants serve as bridges or barriers for the flies. Population comparisons revealed a significant positive relationship between hybridization in the plants and gene flow in the flies, and individual comparisons indicated flies are using the hybrid plants, albeit at low levels. The results suggest hybrids could serve as bridges between parental species, helping explain how a species from a typically monophagous lineage could expand its host range. 2012-01-01T00:00:00Z http://hdl.handle.net/1903/12192 Title: Genetic and Phenotypic Differentiation as a Consequence of Host Plant Use by Lepidopteran Herbivores Authors: Shlichta, Jennifer Gwen Abstract: In this dissertation, I focused on the role of plant hosts as a driving force leading to phenotypic and genotypic changes in insect herbivores. There are three main questions addressed: (1) Do generalist species' populations have broad diet breadth or do they represent a mosaic of sub-populations, each having narrow diet breadths? (2) How do host plants affect the immune response of polyphagous herbivores? and (3) Do host plants or host plant such as allelochemicals, alter the interaction between herbivore defense and parasitoid counter-defense? Do generalist species' populations have broad diet breadth or do they represent a mosaic of sub-populations each having narrow diet breadths? In Chapter 1, I determined, using amplified fragment length polymorphisms (AFLPs), whether host plant-associated genetic differentiation (HAD) was exhibited by a suite of polyphagous tree feeding macrolepidoptera. I determined this by using polyphagous species that exhibit traits expected to be important in the formation of genetically divergent sub-populations. How does host plant affect the immune response of polyphagous species? In Chapter 2, the objective was to examine the effect of host plant species on the immune defenses of polyphagous lepidopteran herbivores, specifically the intensity of encapsulation measured as percent melanization, of three common forest Lepidoptera species. In Chapter 3, I discuss and assessed the potential role of immune responses in insect outbreaks. I present a brief background on immune responses, discuss the methods used to experimentally measure the components associated with immune response and how immune response varies. Lastly, I draw on the studies available and present several potential hypotheses to stimulate further research. Does host plant, or some aspect of host plant such as allelochemicals, alter the interaction between herbivore and parasitoid? In the final chapter, I explored the ecological consequences of viral-plant allelochemical interactions. The objective of this study was to use a model system, Manduca sexta and Cotesia congregata, to directly test the effect of the allelochemical nicotine and the presence or absence of polydnavirus (PDV) on larval immune responses. PDV allows the parasitoid egg to escape encapsulation (an herbivore defense against parasitism). 2011-01-01T00:00:00Z