Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item ENHANCING BIOLOGICAL CONTROL BY GROUND BEETLES (COLEOPTERA: CARABIDAE) THROUGH AGRICULTURAL DRAINAGE DITCH MANAGEMENT PRACTICES(2024) Shokoohi, Alireza; Lamp, William; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The establishment of semi-natural habitats on crop field margins is an increasingly popular integrated pest management (IPM) tool for conservation biological control of crop pests, decreasing reliance on harmful chemical pesticides. Agricultural drainage ditches are uncropped areas built to mitigate flooding on farms, but they may also provide suitable habitats for beneficial arthropods such as ground beetles, which are generalist predators of many common plant and invertebrate pests. In this study, I aimed to evaluate the potential of drainage ditches as natural habitats that promote biological control by ground beetles. To do this, my objectives were (1) to assess the impact of altered ditch management practices on ground beetle communities within a ditch and (2) to investigate ground beetle community composition and dynamics between ditch and adjacent crop field habitats across the Delmarva peninsula. Addition of straw to ditch banks in the fall increased ground beetle numbers by 97% in subsequent years, and ground beetle activity-density in drainage ditches was proportional to activity-density in adjacent fields for most genera. Results of this study suggest that altering drainage ditch management practices may provide additional ecological benefits by enhancing populations of ground beetles, thus reducing pest damage in adjacent crop fields.Item INDIGENOUS NATURAL ENEMIES OF THE INVASIVE BROWN MARMORATED STINK BUG, HALYOMORPHA HALYS (HEMIPTERA: PENTATOMIDAE)(2013) Jones, Ashley Lynn; Shrewsbury, Paula M; Hooks, Cerruti RR; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Brown marmorated stink bug (BMSB), Halyomorpha halys (Stål), is an invasive species native to Southeastern Asia. Since its arrival into the U.S., BMSB has become an economically important pest in many cropping systems, including woody ornamental plants. Here I have explored the potential impact of indigenous natural enemies on BMSB in woody ornamental nursery systems in Maryland. When sampling for indigenous natural enemies in 2012 and 2013, I found seven species of egg parasitoids attacking BMSB with especially high parasitism rates from Anastatus reduvii. Overall egg mortality averaged 58% and parasitism rates increased from32% in 2012 to 44% in 2013. When sampling for predators as biological control agents, predation was low overall. I found that Arilus cristatus consumed more BMSB than any other predator species tested though low abundances were observed in the field. I also found that the use of sentinel egg masses may underestimate rates of parasitism.Item Ecological Effects of the Biocontrol Insects, Larinus Planus and Rhinocyllus Conicus, on Native Thistles(2005-05-11) Dodge, Gary Jonathan; Inouye, David W; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biological control of invasive weeds is, by nature, a delicate balance between introducing effective biological control agents and not introducing another invasive species. A disconcertingly similar suite of traits is used to describe invasive insect species and to identify appropriate biological control agents (or candidates): good control agents and invasive exotic species are good dispersers, they are good colonizers, they have high reproduction rates, and they are suited for broad distribution. It shouldn't come as a surprise that two previously released weed biological control agents can now be characterized as invasive species. The purpose of this dissertation is twofold: first, it is to explore the ecological relationship between predispersal seed predation and plant population dynamics, and second, it is to elucidate the risks to native plants involved with introduction and redistribution of exotic species. Rhinocyllus conicus and Larinus planus are Eurasian seed-head weevils, introduced and redistributed broadly across the western U.S. to control exotic thistles. Exclusion experiments on native thistles, including one that is rare and imperiled, at sites in Colorado present strong evidence that a decrease in seed production due to herbivory by both R. conicus and L. planus has lead to a reduction in recruitment of the thistles. The density of seedlings in both cases, even in the excluded units, was far below where density dependent effects may play a role in the dynamics of the thistle. Further, a survey of eight western states demonstrates established populations of L. planus and effects on seed production in multiple native species in four states. It also documents the near ubiquity and broad diet breadth of R. conicus. The process for approval of phytophagous biocontrol agents has become more cautious and more efforts are made to prevent nontarget herbivory. Nevertheless, land managers still routinely redistribute previously approved, non-regulated agent insects that appear to pose a higher risk to the native flora. The results of this research will benefit resource managers who wish to consider use of phytophagous insects as biological control agents as well as help ecologists and environmental managers understand the risk probabilities of biological control applications.Item Predator diversity, habitat complexity and the strength of terrestrial trophic cascades(2005-04-15) Finke, Deborah Lee; Denno, Robert F; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Food web complexity is thought to weaken the strength of terrestrial trophic cascades whereby strong natural enemy impacts on herbivores cascade to indirectly influence primary production. Predator diversity can enhance food web complexity by promoting the occurrence of intraguild predation, wherein predators feed on each other and on shared prey. In such cases, theory suggests that the impact of predation on herbivores relaxes and cascading effects on basal resources are dampened. In a terrestrial marsh community, I compared arthropod predator impacts on herbivores and plant productivity between a simple food web with a single predator species and a complex food web with a diverse predator assemblage. I found that enhancing predator diversity dampened enemy effects on herbivores and weakened trophic cascades. The role of intraguild predators in dampening such trophic cascades was determined by factorially manipulating predator species richness (1, 2, or 3 species) and predator trophic composition (strict predators, intraguild predators, or a mixture of both) and measuring their effects on prey suppression and plant productivity. I found that the impact of predator richness on the strength of trophic cascades was dependent on the trophic composition of the predator complex present. Specifically, strict predators additively enhanced planthopper suppression and increased plant productivity with an increase in species richness. However, intraguild predators interacted antagonistically, resulting in greater herbivore abundance and lower plant productivity at the highest levels of species richness. An investigation of the influence of habitat complexity on cascading predator effects revealed that complex habitats with cordgrass leaf litter provided a refuge for predators from intraguild predation and elevated planthopper suppression by the diverse predator assemblage. However, reducing the antagonistic predator-predator interactions and increasing prey suppression did not enhance the conductance of predator effects through the food web to impact positively primary producers, although there was a trend towards greater plant biomass in the complex-structured habitat. Therefore, the possibility exists that changes in habitat complexity might enhance trophic cascades and impact positively productivity by mediating trophic interactions among predators. Overall, interactions between species diversity at higher trophic levels and habitat structure can significantly alter ecosystem function in natural systems.