Entomology Theses and Dissertations
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- ItemASSESSING 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.
- ItemThe 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.
- ItemElucidating 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.
- ItemUrbanization 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.
- ItemVARROA 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.