Entomology Theses and Dissertations

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ODONATA SPECIES COMPOSITION IN AGROECOSYSTEMS: PRELIMINARY SURVEYS WITH AN EMPHASIS ON POTENTIAL FOR BIOLOGICAL CONTROL ON FARMS
(2024) Hartman, Margaret Elizabeth; Lamp, William O; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Adult dragonflies and damselflies are efficient aerial predators that provide ecosystem service as consumers of pest arthropods. However, their role as predators of agricultural pests in agroecosystems has been understudied. The prey of odonates has been historically difficult to quantify but new molecular methods can make diet analysis easier. I conducted visual encounter surveys across four farms in 2020 and 2021. I found odonates were present on all farms surveyed but there were significant differences in abundance and richness. Fecal pellets were collected from 94 odonates in 2021 for prey DNA analysis using next generation sequencing. Nine odonate samples produced exceptional libraries, resulting in a large quantity of identifiable prey sequences. This preliminary study can help future researchers develop best practices for maintaining healthy farm water bodies and optimizing fecal DNA analysis methodology to better understand odonates’ potential for agricultural pest suppression.
UNDERSTANDING NON-TARGET INSECTICIDE IMPACTS AND OPPORTUNITIES FOR BIOLOGICAL CONTROL IN FIELD CORN
(2024) Cramer, Maria; Hamby, Kelly; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Natural enemies can be important regulators of pest populations, replacing chemical pesticides, or where compatible complementing them. To effectively incorporate these biological control services into crop production, we need to understand how pesticides and other management practices impact natural enemies, pests, and their interactions with one another. Using Mid-Atlantic field corn production systems, I evaluated at-planting insecticides and transgenic Bt corn hybrids as tools for insect pest control. In addition, I quantified their impact on predation and on carabid beetle communities. Carabid beetles may be important predators of pest slugs; therefore, I also examined carabid-slug interactions. Finally, I investigated whether a novel pest-management technique (RNAi) could travel up the food chain through pests to predators. At-planting insecticides provided limited pest-management benefits in a three-year field study across Maryland, suggesting that their use could be scaled back to reduce environmental and economic costs. Field and lab experiments revealed carabid beetles are diverse and abundant in corn fields, and a slug predatory carabid species may exert non-consumptive effects on a pest slug. Finally, I determined that a predatory lady beetle species is likely less susceptible to RNAi oral exposure in the adult stage. However, RNAi may still pose risks to predators via trophic exposure. My work supports the adoption of current and future corn pest management practices that protect natural enemies, enhancing biological control ecosystem services and their associated environmental benefits.
Using CRISPR/Cas9 to functionally dissect Blimp1, a newly identified pair-rule gene in the hemipteran Oncopeltus fasciatus
(2024) Reding, Katie; Pick, Leslie; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Genetic screens in the fruit fly Drosophila melanogaster identified a class of mutants displaying half as many segments as seen in wild type fly larvae (Nüsslein-Volhard and Wieschaus 1980). Careful examination of the larval cuticle revealed that one out of every two segments were deleted across the anterior-posterior (AP) axis, an unexpected phenotype suggesting that segmentation in Drosophila follows a ‘pair rule’. Thanks in part to the many genetic tools available for this model species, we now have a clear picture of how the AP axis of the Drosophila embryo is polarized and subsequently divided into distinct segments, and how the pair-rule genes (PRGs) define alternate segments during this process. Since all insects share a similar body plan, it is reasonable to expect that the processes regulating establishment of this body plan would be conserved. However, studies of the Drosophila segmentation gene orthologs in non-model insects suggest that this is not always the case. While the use of model organisms enables an unmatched depth of understanding of the mechanisms underlying development, it comes at the expense of understanding the diversity of these mechanisms across taxa. The milkweed bug Oncopeltus fasciatus (Ofas) (Hemiptera) is a particularly useful insect to study in this regard, as none of the orthologs of the Drosophila PRGs have clear PR-function in this species (Liu and Kaufman 2005b; Auman and Chipman 2018; Reding et al. 2019), while the gene E75A, which has no role in segmentation in Drosophila, is expressed in a PR pattern and its knockdown yields PR segmentation defects (Erezyilmaz et al. 2009). These results suggested that PR-regulation of segmentation in Oncopeltus might require a different set of factors than those discovered in Drosophila. To identify other non-canonical PRGs in this species, I conducted an expression pattern-based screen of transcription factor-encoding genes that are co-expressed with E75A during embryogenesis, and have identified the gene Blimp1 as an Oncopeltus PRG. Like the Drosophila PR mutants, Ofas-Blimp1 mutants display loss of alternate segments across the AP axis. No roles of Blimp1 in insect segmentation had been identified prior to this finding. This result suggests that while insect segmentation may be constrained to follow a pair rule, the genes responsible for regulating PR-segmentation are evolutionarily labile. Further, a major barrier to studying gene function in non-models is the lack of genetic tools such as visible markers and established methods for gene editing. Here I will describe deployment of CRISPR/Cas9 technology in Oncopeltus for targeted mutagenesis. While mutation of the ABC transporter-encoding gene white proved to be recessive lethal, I was able to generate a viable visible marker line by disrupting the X-linked gene Ofas-vermilion (v). Of-v is required for production of dark brown eye pigments, thus Ofas-v mutants have bright red eyes, easily discernible from the black eyes of wild type bugs. I show that a co-CRISPR approach using Of-v as a marker of germline mutation is a helpful strategy to identify mutations of interest at unlinked loci, enabling many future genetic manipulations in this species.
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.
PESTICIDE RESIDUES IN BEESWAX PREDICT THE OCCURRENCE OF VARROA WITH ALLELES THAT CONFER AMITRAZ RESISTANCE.
(2024) Hartel, Eric; vanEngelsdorp, Dennis; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Honey bees are important pollinators and are essential to modern industrial agriculture. One of the largest threats to honey bee health is the parasitic varroa mite Varroa destructor. A common method beekeepers use to control varroa is the application of synthetic varroacides. Following years of treatment, varroa have developed resistance to multiple varroacides leading to treatment failures. This project set out to examine the relationship between pesticide residues within beeswax and resistance alleles in varroa. I expected that the presence of varroacides would be positively related to the occurrence of known mutated alleles that confer varroacide resistance to varroa. We looked for 3 different known varroacide resistance alleles, and only found 2, (N87S and Y215H). Both N87s and Y215H confer resistance to amitraz and her metabolite DMPF. Only one mutation, the Y215H mutation, was widespread, occurring in 68% of the 195 mites we examined. We found that the mutation occurred more frequently in mites that were collected from apiaries that had higher DMPF levels. Surprisingly, the presence of other unrelated pesticides (e.g. not having the same mode of action as amitraz) were also positively correlated with the proportion of mutated alleles found. Both the total number of pesticides found in an apiary, and increasing concentrations of fungicides, insecticides, and varroacides (including and excluding DMPF data), predicted higher odds of finding the Y215H mutation. It is unclear if this relationship is a result of a correlation between pesticide levels, if the mutation help benefits the mites resist pesticide more generally, or if the gene has become fixed in the population. It is important to monitor resistance conferring mutations in the varroa mite population in order to help beekeepers make proper varroa management decisions.
EVALUATING SOLITARY BEE NESTING PREFERENCE – DOES THE GROUND COVER MATTER?
(2023) Barranco, Lindsay; vanEngelsdorp, Dennis; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Solitary bees make up the majority of the world’s 20,000 total species of bees. Over 70% of solitary bees nest underground. Bee fitness is likely determined, in part, by the proximity to appropriate nesting sites. Identifying ground cover management that optimally supports bee nesting could help inform and improve conservation efforts. To date, little is known about the impact ground cover has on ground nesting bees nesting success. To start to fill this gap in knowledge, this study monitored 4 plots, located in Maryland, with different ground cover treatments from early May to mid-July in 2020. Emergence traps were utilized to capture nesting solitary bees within the 4 treatments – solarized bare ground, wildflowers, un-mown grass, and mown grass. When compared to cut grass and uncut grass, we found that emergence traps were 5 to 7 times more likely to catch solitary bees when placed over bare soil plots. This suggests that more bees nest in ground that starts bare or contains significant bare patches. Although the covid pandemic limited the number of sites monitored and reduced the time frame of the study, the preliminary research results provide guidance to homeowners and land managers that wish to manage landscapes in ways that support solitary bee nesting.
Evaluating the Potential Benefits and Sustainability of a Novel Living and Dead Cover Crop Mixture in Mid-Atlantic Crop Production
(2023) Johnson, Veronica; Hooks, Cerruti RR; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Modern vegetable production systems are often characterized by monoculture fields andthe intensive use of tillage and/or synthetic agrochemicals for managing weeds and insect pests. A growing public interest in more sustainable and eco-friendly production practices has resulted in increased demand that crops be produced with lower inputs. Incorporating flowering living mulches and cover crop residues within crop fields can create an environment more hospitable to beneficial organisms and less conducive to pest outbreaks. My dissertation research aims to advance our knowledge in this area by evaluating the impacts of a novel cover cropping tactic which involves combining a perennial flowering living mulch with cover crop residue on insects and/or weeds. Further, it is often suggested that weed management requires a holistic approach; and that cover cropping will not be successful as a sole weed management tactic. As such, another research aim is to investigate whether combining a cover cropping tactic with herbicide sprays would result in better weed suppression and increased yield in sweet corn compared to using cover crops alone. An economic assessment was also performed to further evaluate the practicality of sweet corn producers adopting the management practices being investigated. Cost of seeds, labor and other expenses can be a primary limitation to cover crop usage. To this point, I also evaluated the feasibility of using a single cover crop planting to suppress weeds over multiple cropping systems and field seasons. If a single cover crop planting can be used over multiple seasons, this could reduce the cost of cover crop use. Agricultural intensification and conversion of natural landscapes to crop production fields have contributed to declines in insect biodiversity including natural enemies and pollinators. Advancing our understanding of how increasing vegetational diversity within crop fields influences weed pressure and populations of herbivores and beneficial arthropods, as well as production costs, can facilitate the adoption of practices in annual cropping systems that favor beneficial organisms and conserves insect biodiversity.
A GOOD NEIGHBORHOOD TO RAISE A BROOD: TREE SPECIES DIVERSITY DECREASES PERIODICAL CICADA OVIPOSITION AND TREE RESPONSE
(2023) Jayd, Kristin; Burghardt, Karin; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Natural systems contain diverse assemblages of plants, providing a matrix of potential hosts that herbivores must navigate. Insect-plant host choice patterns are crucial to understanding both herbivore outbreaks and the consequences of outbreaks for plant hosts. Here, I follow the 2021 Brood X periodical cicada mass emergence event in the BiodiversiTREE forest diversity experiment at the Smithsonian Environmental Research Center (SERC) in Edgewater, MD, to uncover whether tree diversity influences cicada oviposition preferences or tree responses to oviposition (flagging), for 15 tree species grown in plots of single species or 12-species mixtures. While cicadas demonstrate clear tree species preferences, the diversity of the surrounding tree neighborhood plays at least as important a role in determining oviposition preference and tree flagging responses. Cicadas were threefold more likely to oviposit in trees grown in single species vs. mixed species plots. While overall, I find a concomitant decrease in tree flagging in diverse plots. I also document that species flag at different rates in response to the same oviposition scar density. Even when accounting for differential oviposition rates, surrounding tree diversity remains an essential additional predictor of tree flagging responses with trees in diverse plots less likely to flag at the same density of scars, suggesting a differential capacity of trees to tolerate damage when growing in single species plots. This study creates a richer understanding of the importance of tree context, specifically surrounding tree diversity, in shaping the ecological ramifications of a mass insect emergence event.
EVALUATING ROADSIDE INTEGRATED VEGETATION MANAGEMENT (IVM) TECHNIQUES TO IMPROVE POLLINATOR HABITAT
(2023) Kuder, Lisa Jennifer; vanEngelsdorp, Dennis; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Pollinator declines have spurred interest in managing road verges as early successional habitats that promote wildflowers. Reduced mowing is a common cost-effective pollinator habitat improvement strategy. Studies on verge management demonstrate the benefits of this method, but also highlight downsides including dispersal of weed seeds by mowing machinery that facilitates establishment and persistence of noxious weeds. Herbicides are commonly used on electric powerline rights-of-way (ROW) to manage invasive plants, as well as establish and maintain low-growing pollinator habitat. This method, however, has not been tested for its suitability for verge management.To explore this knowledge gap, we compare pollinator habitat quality and bee abundance in six Maryland, USA roadsides under three different vegetation management types: frequent mowing, reduced mowing, and spot-spray. For two growing seasons, we took standardized photographs of road verge transects to measure foliar cover using image analysis software (percent cover sampling), counted floral units in fixed quadrats (quadrat sampling), and recorded all insect-pollinated blooms throughout treatment areas and assigned perceived abundance categories to each species (scanning sampling). Management types were compared with respect to the density, native status and diversity of roadside flowers, the proportion of bare ground and leaf litter, and bee abundance. We found that road verges managed under spot spray and fall mow resulted in improved pollinator forage with higher vascular plant cover (4x), floral density (4 and 7x), proportion of native plant species (1.9 and 2.5x), and Hill number’s diversities (3 and 4x) compared to the control. The two treatment groups did not differ in measured metrics, with three exceptions: Spot spray sites had higher proportions of native floral units and potential nesting sites (bare and leaf covered ground); whereas fall mow increased plant species diversity. Surprisingly, we did not detect a significant difference in bee abundance among the three management regimes. However, season, site and year were major predictors of bee abundance. It is not unusual for bee populations to differ substantially throughout and between years Our study demonstrates that during the early transition stage from turf to meadow, spot spraying was similar to fall mowing at promoting roadside pollinator forage. Also, spot spraying resulted in higher proportions of bare ground and leaf litter that could potentially provide nesting and overwintering habitat. Thus there are alternatives to reduced mowing that may have an important role in future conservation efforts. Proper and selective use of herbicides may be especially relevant for areas where noxious weeds are pervasive, or species of concern are at risk from destructive biomass removal (i.e., nesting birds, turtles, and butterfly larvae).
ASSESSING THE SUSCEPTIBILITY TO RNA INTERFERENCE OF THE MILKWEED BUG, ONCOPELTUS FASCIATUS (HEMIPTERA)
(2023) Argaez, Ebony Michelle; Pick, Leslie; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
RNA interference (RNAi) is an effective method to knock down gene expression in insects and other organisms. It has been adopted for basic research, to elucidate gene function, and applied research, to control insect pests. Here, I examined parameters needed for effective RNAi in the milkweed bug, Oncopeltus fasciatus, an emerging insect model species. For two developmental genes, Sex combs reduced, and even-skipped, very small amounts of dsRNA trigger a robust parental RNAi response. The higher the dose of dsRNA applied, the longer the duration of embryos laid with defects. Testing length-dependence, effectiveness decreased with dsRNAs in the 150 bp to 75 bp range. These developmental genes resulted in subtle, gene-specific defects which provided a more sensitive assay than lethality. Finally, effects of RNAi were transmitted across generations through trophic interactions, the first such discovery to our knowledge. This suggests potential unanticipated environmental risk to non-target insects from RNAi-based insecticides.
The Genera Hemiberlesia and Abgrallaspis in North America with Emphasis on Host Relationships in the H. Howard (Cockerell) Complex (Homoptera: Coccoidea: Diaspididae)
(1960) Davidson, John Angus Sr.; Bickley, William E.; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
The work reported consists of revisionary studies of Hemiberlesia Cockerell 1897 and Abgrallaspis Balachowsky 1948 in North America. The interpretation of North America is that of Ferris (1937), "all the mainland from the Arctic regions to and including the Panama Canal Zone." According to Ferris (1942), Hemiberlesia contained 15 species. As a result of this study, only eight of these species are here referred to Hemiberlesia. They are: rapax (Comstock), lataniae (Signoret), popularum (Marlatt), ignobilis Ferris, cupressi (Cockerell), diffinis (Newstead), candidula (Cockerell), and palmae (Cockerell). The species H. coniferarum (Cockerell) is newly assigned having previously been placed in Diaspidiotus Berlese and Leonardi, by Ferris. A tenth species, H. pseudorapax McKenzie, was assigned to this genus by its author. Seven of the 15 species have been assigned to Abgrallaspis. Both these genera appear to be North American in origin. Abgrallaspis was originally created for six species. Three of these occur in North America and were transferred from Hemiberlesia by Balachowsky. They are: palmae (Cockerell), degeneratus (Leonardi), and cyanophylli (Signoret). The last named species was designated as the genotype. Balachowsky (1953) later reassigned palmae to Hemiberlesia and transferred four more North American Hemiberlesia species to Abgrallaspis, namely, howardi (Cockerell), comstocki (Johnson), coloratus (Cockerell), and fraxini (McKenzie). A study of these species in the National Coccoid Collection revealed a complex centering about A. howardi as conceived by Ferris (1938). Usual morphological comparisons of slide mounted adult females failed to yield results, therefore, host transfer experiments were undertaken. A population of "howardi" of Ferris was secured on pachysandra. A total of 2,700 individual crawler transfers were then made to 20 different host plants. These hosts had been chosen because a preliminary study indicated unusual character variation in specimens collected from them. Fourteen of the test hosts (largely ornamentals) were later found to be infested with 14 to 54 per cent of the transferred crawlers. These crawlers were allowed to mature. Adult females were then collected and mounted for study. Six host plant species were completely unacceptable to infestation by the transferred crawlers. Five of these were plum, pear, peach, apple, and pine. A. howardi was described from plum in Colorado, and later recorded from such hosts as pear, peach, and apple. A study of the species Ferris synonymized with A. howardi revealed the test population to be A. townsendi (Cockerell), which was described from an unknown host in Mexico, and later recorded from a long list of ornamentals primarily in the southern and eastern United States. This species is redescribed and the name revalidated. A table is presented showing the variations found in salient taxonomic characters of A. townsendi collected from 14 different experimental host plants. Important variations in the size of the second lobes of A. townsendi were recorded. Second lobe reduction from three-fourths the length of the median lobes to mere hyaline points was observed. Specimens in the last category strongly resemble Diaspidiotus ancylus (Putnam). Aside from these second lobe variations, A. townsendi is a relatively stable species from the standpoint of host determined morphological variables. Avocado was the sixth test host on which transferred crawlers would not develop. Long series of scales from this host are present in the National Collection. They were collected from avocado fruit in quarantine at Texas, from Mexico. This species, A. perseus Davidson, is described as new herein. As here understood for North America, Abgrallaspis contains 13 species. Six were placed in this genus by Balachowsky, and seven by the writer. The last are: flabellata (Ferris) from Hemiberlesia; quercicola (Ferris) from Hemiberlesia; mendax (McKenzie) from Hemiberlesia; oxycoccus (Woglum) from Aspidaspis Ferris; ithacae (Ferris) from Aspidaspis; perseus Davidson as a new species; and townsendi (Cockerell) as a revalidated name. A brief presentation of materials and methods utilized in the host transfer experiments is followed by a discussion of the structural characters used in this work. Descriptions of Hemiberlesia and Abgrallaspis are accompanied by keys and descriptions to all the species in North America. Figures of adult female pygidial characters and scale coverings are provided for all species considered in these two genera.
Elucidating factors to improve biological control of Halyomorpha halys by egg parasitoids
(2023) Potter, Madeline Elizabeth; Shrewsbury, Paula M; Burghardt, Karin T; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Halyomorpha halys Stål (Hemiptera: Pentatomidae), also known as the brown marmorated stink bug, is an invasive species from Northeast Asia, which has now spread to 47 USA states and invaded several other countries. In the USA, H. halys is an economically important pest of fruit, vegetable, field, and nut crops, and it feeds on ornamental plants. A sustainable means of controlling this pest is needed. Here I focus on elucidating features from bottom-up forces (alternate host identity, host plant identity, and habitat type) which may influence top-down forces such as parasitism by H. haly’s key natural enemies, Hymenoptera egg parasitoids. Naturally laid eggs of insects were collected from a tree nursery in 2020 and from diverse habitats throughout Maryland in 2021 to investigate H. halys parasitoids’ alternate insect egg hosts, host plant and habitat associations, and which factor(s) (host plant identity and/or host egg identity) are important to egg mass discovery, or to egg parasitism rate. Effects of host insect feeding guild (herbivore vs predator) and host plant origin (native vs non-native) on parasitism were also examined. One new overwintering insect host and four new in season hosts for Anastatus spp., and five new in season hosts for Telenomus cristatus were found. A diverse array of plant species, particularly native Acer and Quercus species, were found to support alternate host insects. Halyomorpha halys related parasitoids were reared from eggs collected in all habitat types. Host egg order and egg feeding guild affected Anastatus spp. egg unit discovery efficiency and egg parasitism rate. Host plant identity and plant origin affected egg unit discovery efficiency and egg parasitism rate by all H. halys parasitoids. These findings support the importance of having a diverse community of alternate prey and informed plant selections to support parasitoids and their biological control of H. halys and other insect pests.
Urbanization and Landscape Heterogeneity Influence Culex Species Ecology and Genetics in Eastern North America
(2023) Arsenault-Benoit, Arielle L.; Fritz, Megan L.; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Vector-borne disease is an important facet of public health, as they account for nearly 20% of global disease burden. Multiple species, including at least one vector, at least one host, and a pathogen, must interact in vector-borne disease transmission cycles, and thus understanding human risk of vector-borne disease and public health outcomes requires a community ecology framework. Members of the Culex genus, including Cx. pipiens, Cx. quinquefasciatus, and Cx. restuans are sympatric in eastern North America and are vectors of West Nile virus. This dissertation explores the roles of habitat use, community ecology, phenology, and landscape heterogeneity on Culex spatiotemporal dynamics and genetics along urban to rural gradients in eastern North America. Through surveillance of belowground structures in Washington, D.C. over two years, I found that mosquito species of public health importance, including Aedes aegypti, Aedes albopictus, and members of the Culex pipiens assemblage, use these structures for breeding and development. Belowground structures may serve as refugia against extreme climatic conditions and facilitate overwintering survival for non-diapausing taxa and/or taxa with thermal tolerance limitations, potentially expanding their suitable ranges. On an urban to rural gradient in greater Washington, D.C. and Maryland, a complex of cryptic Culex WNv vectors co-occur on the landscape. Using molecular techniques and constrained ordination, I found that these cryptic Culex species were differently distributed at fine spatial scales, likely due to the impacts of urbanization on vector habitat and subsequent niche segregation. Culex pipiens were cosmopolitan and dominant across sites in greater Washington D.C. and Maryland. However, individuals with Cx. quinquefasciatus ancestry were limited to urban and peri-urban sites closest to the city center, and Cx. restuans were most abundant in rural and suburban sites furthest from the city center with dense and heterogeneous canopy cover. Previous work suggested that phenology has a considerable impact on Culex species dynamics; Cx. restuans was thought to be an early season species that cedes to Cx. pipiens over the course of the season. Initially, I did not detect an effect of season on Culex spatiotemporal dynamics when collections were undertaken from June through October, but when I expanded the collection season to include the months of April and May, the influence of season was evident. Therefore, the hallmark “crossing-over” point that is common in the Culex literature happens prior to the local mosquito abatement season in Washington and D.C. and Maryland. During the active surveillance and management period, season has little impact on Culex species abundance as compared to environmental factors measured along our urban to rural gradient. A replicated comparison of the abundance and relative frequency of Cx. pipiens and Cx. restuans along urbanization gradients in Washington D.C., greater Philadelphia, PA and greater Chicago, IL, using gradient forests demonstrated that phenology was consistently the most important predictor of the shift between a Cx. restuans-dominant community and a Cx. pipiens-dominant community. This crossing-over point trended later in the season with increasing latitude. Turnover in species abundance tended to occur at intermediate points along environmental gradients associated with urbanization, like percent impervious surface, percent tree cover, distance to city center, and vegetation index. Results of two analytical approaches (ordination and regression trees) and from three metropolitan areas support Cx. restuans as an early season species that is otherwise associated with sites with cooler temperatures, less impervious surface, more tree cover, a shallower water table, and increased distance from city center. Conversely, Cx. pipiens is more abundant than Cx. restuans in sites that are more characteristic of urbanization. Culex pipiens is globally ubiquitous and was common across site classes in the three localities in this study. This species comprises two bioforms, pipiens and molestus, which are characterized by divergent ecological, physiological, and behavioral traits. These bioforms can interbreed in the field and the lab. However, at all sites analyzed across three northeastern metropolitan areas, analysis of genotypes at a single neutral locus violated assumptions of Hardy Weinberg Equilibrium, suggesting that there is not unrestricted geneflow between bioforms across the landscape. The proportion of molestus alleles increased with increasing percent impervious surface and decreased vegetation, two environmental correlates of urbanization. Molestus alleles may confer an advantage in urban environments because they can leverage human infrastructure to overcome thermal limitations and persist in isolated belowground populations via autogeny and use of mammalian hosts. Overall, Culex WNv vectors are differentially distributed across urban to rural gradients in the northeastern United States. These aspects are influenced by a heterogeneous land use and landscape-level changes associated with urbanization. A clear understanding of vector life history, genetics, interspecies interactions, and distribution across the landscape can improve practitioners’ power and precision in predicting and managing vector borne disease transmission. While some patterns in species distribution and composition were universal across metropolitan areas, there was variation between localities that could significantly contribute to WNv transmission and human disease risk. Therefore, I conclude that modeling, as well as development of surveillance and management strategies for WNv vectors should be implemented locally to have the greatest impact on public health outcomes.
VARROA DESTRUCTOR: ABIOTIC AND BIOTIC CORRELATES TO BODY SIZE AND THE EFFECTS OF SIZE AND HOST TYPE ON MITE TOLERANCE TO ACARICIDE EXPOSURE
(2023) Christmon, Krisztina; Pick, Dr. Leslie LP; Cook, Dr. Steven SC; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Varroa destructor, an ectoparasitic mite of the western honey bee (Apis mellifera), and the viruses it vectors are the most important factors driving high rates of honey bee losses in the United States. Unfortunately, mites developed resistance to some of the pesticides, which creates a burden on beekeepers to keep their colonies healthy. Despite this threat to honey bees, we still know little regarding some of V. destructor’s basic biology. In Chapter 1, I describe the method of a novel system to measure the width and length of the ventral side of V. destructor, which allowed me to assess the size variability of V. destructor in the United States. This chapter is an observational epidemiology study on mite size and its association with year, the time of year, mite density in host apiaries, the virus load of the host apiary, and pesticide exposure in host apiaries. I also conducted a series of experiments to determine whether the mite size findings were biological or due to an experimental artifact. I’ve found a seasonal variation in mite size that is possibly driven by external pressures, arguably acaricide exposure or the diet received by feeding on different developmental hosts. Chapter 2 of my dissertation is the continuum of Chapter 1. I tested the size of mites as a confounding factor in their tolerance to amitraz of mites collected from a field trial and a toxicological bioassay. The field trial and the toxicological assay result suggest that amitraz sufficiently kills smaller mites. Lastly, Chapter 3 investigates how tolerance to acaracide exposure, through feeding on different developmental hosts, affects mites' survival to pesticide exposure or stress. To achieve this, I placed mites to feed on adult or pupal honey bee hosts before exposing the mites to various pesticides. I found that the mites placed on pupa had the highest survival rate 20 hours after a 4-hour exposure to pesticides in toxicological bioassays. Then measured the activities, and two key detoxifying enzymes, Glutathione S-Transferase and Cytochrome P450, were significantly higher in mites that survived the assays. In addition, mites fed on adult bees had a higher activity level of acetylcholine esterase than mites placed on the pupa. From proteomic analysis, I found that mites placed on pupae prior to pesticide exposure had higher levels of stress-induced proteins (heat shock proteins). However, living mites had higher amounts of honey bee proteins, suggesting a more recent feeding event and perhaps a more beneficial nutritional state. Interestingly, surviving mites specifically contained significantly larger amounts of honey bee antioxidant proteins, suggesting the use by V. destructor of host proteins for their survival.These findings contribute to the literature on V. destructor size variability and provide new information on pesticide resistance. My findings highlight the need to factor in size and feeding state when conducting toxicology bioassays. It also provides new insight for future research on Varroa feeding.
UNDERSTANDING HONEY BEE COLONY MORBIDITY AND MORTALITY THROUGH PHYSIOLOGY AND LIFESPAN
(2022) Nearman, Anthony James; vanEngelsdorp, Dennis; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Managed honey bee colonies (Apis mellifera) are a critical component of our agroecosystem. As such, we need to understand and address risk factors that contribute to colony loss. Fundamental to this understanding is a need to detail the connection between individual bee’s physiology, life histories, and colony fitness. In this dissertation I first present an in-depth review of honey bee physiologies important for colony success (Chapter 1); I then examine standard methods for rearing honey bees in a laboratory setting and the importance of individual bee lifespan on colony loss (Chapter 2); followed by identification of honey bee physiologies that relate to chronological age as a means of measuring colony demographics and health (Chapter 3); and then apply potential age- and disease-related physiology measures to determine associations with overwinter colony loss and known and unknown risk factor exposure (Chapter 4). Research indicates honey bee colony loss is largely driven by poor nutrition, pesticide exposure, and parasites and the viruses they vector. Management practices and techniques to mitigate the effects of these risk factors decrease loss rates but do not prevent all of them. New knowledge, therefore, is needed to address the gap in knowledge between risk exposure and colony mortality. As a honey bee colony is a complex interaction between multiple groups of individual bees, collective physiological changes among these groups hold promise for understanding why some colonies die while other do not when exposed to the same risk factors. In one experiment (Chapter 2), I demonstrate the importance of access to water on honey bee lifespan. In a literature review informed by the data obtained from these experiments, I discovered that the median lifespan of laboratory specimen has decreased by half over the past 50 years and that this change is predictive of overwinter loss rates reported by beekeepers since 2006. If the age of individual bees can affect the lifespan of a colony, I posited that physiological measures predictive of individual bee age could be useful to ascertain the demographics of a colony’s population, which would in turn be a measure of colony health. To test this hypothesis, I built upon previous physiology studies and examined age-linked cohorts of bees through the fall transition to overwinter. In doing so I derived a set of easily identifiable physiological measures either predictive of individual bee age or a possible unidentified disease state. I then applied these measures to a retrospective cohort study, where I was able to determine that changes in the prevalence among several physiologies were associated with overwinter mortality and known risk factor exposure. These methodologies and results show promise for the use of physiological measures as a potential pragmatic tool to predict colony survivorship, to diagnose past known and unknown risk factor exposures, and to further advance fundamental knowledge of the role demographics play in societal health.
COMPARATIVE GENOMICS OF CHEMOSENSORY GENE FAMILIES AMONG MEMBERS OF THE HELIOTHINAE
(2023) Guo, Rong; Fritz, Megan L; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Insect chemosensory systems play crucial roles in the perception of chemical signals that regulate sexual behaviors and odors that mediate insect-host plant interactions. These processes, mate-finding and acceptance, as well as host plant identification and use, strongly contribute to diversification and speciation among plant-feeding insects, including the Lepidoptera. Chloridea virescens and Chloridea subflexa are an ideal pair species to study the evolution of insect chemosensory systems because they are closely related but show pheromone-based sexual isolation and divergent host plant preferences. My dissertation focuses on the development of genomic tools that enable investigation into genetic mechanisms of host plant and mate recognition, and applies these tools to examine inter- and intraspecific diversity of chemosensory genes among members of the Heliothinae.In chapter 2, I produced a novel Illumina short read C. subflexa genome assembly and an improved, highly contiguous C. virescens genome assembly. Due to quality limitations common to short read assemblies, I used our Heliothine genomes to examine the feasibility of reference-assisted assembly, an approach that leverages existing high quality genomic resources for genome improvement in closely related taxa. My work demonstrated that reference-assisted assembly has the potential to enhance contiguity and completeness of existing insect genomic resources with minimal additional laboratory costs. Both the potential and pitfalls of reference-assisted assembly are discussed in light of my results. In chapter 3, I manually curated two chemosensory gene families, the odorant receptors (ORs) and odorant binding proteins (OBPs), in C. virescens. In total, I identified 80 ORs, 1 Orco and 49 OBPs. Three types of OBPs were identified according to the number and positions of conserved cysteine residues: 34 classic OBPs, 8 Minus-C OBPs, and 7 Plus-C OBPs. In addition, I used phylogenetic analyses to study evolutionary divergence of OR and OBP gene families among Heliothine moths, which revealed both gene duplications and losses. In chapter 4, I studied the strength and nature of selection on the ORs of field-collected C. virescens and C. subflexa, with focus on the pheromone receptor genes. I characterized the host plant use of these species in central Maryland by comparing the larval densities and infestation rates in 2020 and 2021. Sequencing followed by analysis of selection on field-collected samples indicated that the pheromone receptor, OR6, was under very strong purifying selection in both C. virescens and C. subflexa. AMOVA tests suggested that in C. virescens, host plant-associated population differentiation existed in genes OR6, OR55, OR66 and OR78. Further analyses of genetic divergence analysis focused on OR6 showed that the most highly divergent sites were all in introns. The new genomic tools and analyses of chemosensory gene families described here will serve as a platform for future investigations into the genetic mechanisms underlying host plant specialization and sexual communication among lepidopteran insects.
Quantifying the relative contribution and furthering qualitative understanding of ftz cis-regulatory elements in Drosophila melanogaster
(2022) Fischer, Matthew Douglas; Pick, Leslie; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Embryonic development is coordinated by interactions within gene regulatory networks. This process is orchestrated at the level of transcription through the regulatory properties of enhancers, which direct spatiotemporal expression patterns when bound by specific trans-acting factors. Though enhancers can act upon promoters located at great distances irrespective of orientation, the contributions from these cis-regulatory elements (CREs) are limited by insulators and/or tethering elements that organize chromatin architecture. Much research has been conducted towards understanding the coordination of the segmentation genes that pattern the basic body plan of the fruit fly, Drosophila melanogaster, during embryogenesis. The pair-rule genes (PRGs) of this pathway, such as fushi tarazu (ftz), are expressed in seven alternating stripes across the embryo. These PRGs are required for the development of body segments, and the mis-regulation of a single transcriptional domain can result in the loss of a segment. Here, I have investigated the ftz CREs to more precisely determine their sufficiency to direct expression within ftz stripe domains and their necessity for doing so in the native context of the gene. To investigate the sufficiency, I have generated 36 standardized reporter transgenes from 18 CREs, tested in both forward and reverse orientations. All CREs examined have been inserted into the same XbaI site of the reporter plasmid, and the transgenes have been inserted into the same genomic region. Through in situ hybridization experiments, I have determined that the qualitative patterns conferred by every CRE is orientation-dependent, and I have identified two putative insulators and/or tethering elements, proposed to explain this observation. To investigate their necessity, I targeted four genomic regulatory regions for precise deletion using the CRISPR/Cas9 system to generate seven deletion mutants. Though deletions were expected to cause lethality, most of the mutants are homozygous viable and fertile; only a mutant simultaneously removing two seven-stripe CREs was homozygous lethal. Quantitative gene expression analysis by fluorescent in situ hybridization chain reaction revealed that there is a critical threshold of ftz abundance required in each stripe for segmentation to proceed. In conclusion, I have determined that the ftz CREs are redundant and function together in a non-additive manner.
DEVELOPING A PERRENIAL LIVING MULCH SYSTEM FOR MANAGING PESTS AND AUGMENTING NATURAL BIOCONTROL IN MARYLAND CANTALOUPE SYSTEMS
(2022) Nunez, Demian Antonio; Hooks, Cerruti RR; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This study investigated how alsike clover (Trifolium hybridum) and Virginia wildrye (Elymus virginicus), when interplanted as a living mulch with cantaloupe, (Cucumis melo var. cantalupensis) would impact herbivorous and beneficial arthropod numbers. An additional objective was to determine how these living mulches would impact fruit yield and quality. It was hypothesized that there would be a reduction of cantaloupe pest herbivores and increase in natural enemy abundances in the interplanting compared to monoculture cantaloupe system. Some arthropods conformed to these expectations. However, most had a neutral or inconsistent response to the living mulches. Striped cucumber beetles (Acalymma vitattum), a major pest, were unaffected by the living mulches on most sampling dates. During several periods in both study years, leaf piercing herbivores including aphids were found in greater numbers on cantaloupe interplanted with clover than wildrye and/or monoculture. Spiders were found in greater abundance in cantaloupe interplanted with clover than wildrye or monoculture plantings during several sampling periods. Other natural enemy guilds such as parasitic wasps and piercing predators were inconsistently influenced by living mulch types. Yield was highest in the monoculture plots and living mulch was correlated with changes in fruit texture and color.
The Bee Louse, Braula coeca Nitzsch, its Distribution and Biology on Honey Bees
(1978) Smith, Irving Barton Jr.; Caron, Dewey M.; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
Bee lice were found in 28% of Maryland apiaries and 18% of the colonies examined. In apiaries with lice, 50% of the colonies contained lice. Laboratory tests demonstrated that bee lice had no preference between 1, 5, 15, and 30 day old honey bee (Apis mellifera L.) workers at 25° while there was a preference for 1 day old workers at 34° C. Lice preferred young drones over old drones and virgin and mated queens over young drones at 25 and 34° C. Lice preferred virgin queens over old drones at 25° C while no preference was observed at 24° C. Mated queens were preferred over old drones at 25 and 34° C. There was a preference of lice for foraging age workers over old drones at 25° C while there was no preference at 34° C. Lice preferred both virgin and mated queens over random age workers at 25 and 34° C. Louse larval tunnels were numerous in nucs (4 frame honey bee colonies) stocked with lice from May through August corresponding with periods of nectar flow when bees were capping honey. In field colonies, louse populations decreased in the late spring to a low in early June. During July and after, populations of lice rose with the emergence of new lice. Few immature and adult lice were observed in control nucs having similar populations of bees. In nucs, 1 or more lice were observed on 24% of the queens between August and December. Only 2% of the virgin queens contained lice during the same period. In field colonies, 62% of the queens examined from June through the rest of the season harbored lice; 58% of these lice were pale in color indicating they were less than 1 day old. One louse was observed on 98.6% of the workers with lice, while 1.2% harbored 2 lice and 0.2% had 3 lice; 4 .2% of the lice were on drones. A single bee louse was observed on 3,092 foraging honey bees sampled. One-hundred-seventeen lice were collected on 14,459 bees collected from the brood nest of the same hives. Control samples indicated a 14 to 15% loss of lice during sampling. Tests demonstrated that during visual observations of lice on bees only 49% of the lice present were observed. Fluctuation in louse population levels were similar to those found elsewhere in this study.
Feeding behavior and distribution of Varroa destructor on adult bees of Apis mellifera
(2022) Lamas, Zachary Siqueira; Hawthorne, David J; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Varroa destructor is a competent vector of honey bee viruses and the leading cause of colony losses worldwide. Much about its feeding behavior and distribution on adult bees remains unknown. This work shows that Varroa are promiscuous feeders of adult bees, actively switching from one host to another. Laboratory trials showed there is a large heterogeneity in the host switching rate with some Varroa switching infrequently while others switched at high rates. The consequences of Varroa feeding on adult bees is largely unknown because adult feeding has largely been overlooked. This work shows that there is a high relative risk of death from Varroa feedings. Adult workers die quickly without developing high levels of infection after being fed upon by an infectious Varroa, and confer lower risk to their non-parasitized nestmates than counterparts which were nestmates to longer lived parasitized bees. Further experiments showed communicable routes of virus transmission may explain these findings. Trophallaxis between adult workers allowed for the movement of the pathogen to naïve nestmates. These nestmates act as an infectious reservoir to naïve Varroa showing communicable transmission between hosts can influence the acquisition and subsequent vectoring of the same pathogen by the vector. Another social behavior, cannibalization, was shown to have the same influence on Varroa vectoring. Varroa were also shown to be susceptible to viral acquisition through shared feedings on adult bee and brood hosts. Naïve Varroa readily acquired and then transmitted deformed wing virus when sharing the same host with an infectious Varroa. Collectively this work exemplifies how host social behavior and Varroa-Varroa transmission routes can increase the risk of vectors becoming infectious. Varroa feedings and virus transmission on adult workers cannot describe one of the most glaring features of Varroa infestations. For a portion of the year Varroa aggregate predominantly on adult drones, largely ignoring the worker cohort. Parasite burden only shifts onto workers when drone production ceases.

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