Thesis (M.Sc.(Zoology and Entomology))--University of Pretoria, 2008. / Summary in English. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1959. / Typescript. Abstracted in Dissertation abstracts, v. 19 (1959) no. 11, p. 2711-2712. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 108-115).

Thesis (M.S.)--University of Missouri-Columbia, 2005. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (month day, year) Includes bibliographical references.

Strains of the well-studied plant pathogen Pseudomonas syringae show large differences in their ability to colonize plants epiphytically and to inflict damage to hosts. Additionally, P. syringae can infect some sap-sucking insects and at least one P. syringae strain is highly virulent to insects, causing death to most individuals within as few as 4 days and growing to high population densities within insect hosts. The likelihood of agricultural pest insects coming into contact with transient populations of P. syringae while feeding on plants is high, yet the ecological implications of these interactions are currently not well understood as virulence has not been tested across a wide range of strains. To investigate virulence differences across strains we exposed the sweet potato whitefly, Bemisia tabaci, and the pea aphid, Acyrthosiphon pisum, both of which are cosmopolitan agricultural pests, to 12 P. syringae strains. We used oral inoculations with bacteria suspended in artificial diet in order to assay virulence while controlling for other variables such as differences in epiphytic growth ability. Generally, patterns of pathogenicity remain consistent across the two species of hemipteran insects, with bacterial strains from phylogroup II, or genomospecies 1, causing the highest rate of mortality with up to 86% of individuals dead after 72 h post infection. The rate of mortality is highly variable across strains, some significantly different from negative control treatments and others showing no discernable difference. Interestingly, one of the most pathogenic strains to both aphids and whiteflies (Cit7) is thought to be nonpathogenic on plants. We also found Cit7 to establish the highest epiphytic population after 48 h on fava beans. Between the nine P. syringae strains tested for epiphytic ability there is also much variation, but epiphytic ability was positively correlated with pathogenicity to insects, suggesting that the two traits may be linked and that strains likely to be found on plants may often be entomopathogenic. Our study highlights that there may be a use for epiphytic bacteria in the biological control of insect crop pests. It also suggests that interactions with epiphytic bacteria could be evolutionary and ecological drivers for hemipteran insects.

Mass mortality events (MMEs) can affect communities through large amounts of carcass biomass and significant reductions in population sizes. I conducted laboratory experiments to compare effects of predator and herbivore MMEs on a community using a study system of convergent lady beetles (Hippodamia convergens), pea aphids (Acyrthosiphon pisum), and fava beans (Vicia faba). I used a factorial design crossing input (addition of carcasses) and mortality (removing 75% of the population) of aphids and lady beetles. Lady beetle mortality reduced the top-down pressure on the aphid population, which increased aphid abundance and decreased plant height. Input of aphid carcasses increased aphid abundance. Input and mortality of aphids caused the aphid population to diminish suggesting that top-down effects of mortality were more significant than bottom-up effects of carcasses. My results show that MMEs can have effects through both bottom-up and top-down processes, which is dependent upon who experiences the MME.

Higher percentages of peanuts than soybeans or cowpeas become infected when these crops are growing equal distances from a source of peanut mottle virus (PMV). The total number of aphids trapped in these crops are about equal and the reason for this differential percentage infection has not been demonstrated. Known vectors of PMV such as Aphis craccivora (Koch) and Myzus persicae (Sulzer) comprised 31% of the aphid population in peanuts compared to 14% in soybeans and 17% in cowpeas and could be responsible for the higher number of peanut infections. In addition, trapping of live aphids in peanut fields showed that viruliferoup Rhopalosiphum maidis (Fitch) were present. Laboratory studies confirmed that R. maidis could transmit PMV from peanut to peanut. This is the first report of R. maidis as a vector of PMV. Virus transmission tests using 5 aphids per peanut seedling were conducted in the greenhouse. Myzus persicae (Sulzer), Aphis craccivora (Koch), and Rhopalosiphum maidis (Fitch) were able to vector peanut mottle virus from peanut to peanut 30, 14, and 4%, respectively. In subsequent tests, where only one aphid was used per test seedling, M. persicae, A. craccivora, and R. maidis transmitted PMV at 9, 4, and 2%, respectively. The retention abilities of the three aphid species tested for PMV was low when compared to other experiments (20). Switching varieties from Florigiant to Florunner did not appear to significantly increase virus transmission rates of any of the aphid species tested in comparison to tests done with Florigiant. This is the first report of transmission testing with these three aphid species and PMV V745-473, a strain of the virus isolated from soybean in Virginia and with the Florigiant variety of peanuts. In feeding preference tests, all three aphid species tested showed a preference for settling onto a host, regardless of the host species used, to wandering at random in the feeding arena and not feeding on any host plant. A. craccivora showed the greatest propensity to wander, with only 33% of those aphids tested choosing to settle on a host. In contrast, 58% of R. maidis tested chose to settle on a host. In comparisons of feeding preferences between combinations of two host plants, M. persicae preferred chinese cabbage in 4 comparisons and soybean in 2, A. craccivora preferred both soybean and chinese cabbage in 2 comparisons each, and R. maidis preferred chinese cabbage in 4 comparisons and sorghum in 3. Chinese cabbage, soybeans, cowpeas and sorghum are preferred by these three aphid species over peanuts. Based on feeding preference it would be impossible to separate the PMV vectoring ability of these three aphid species. The three aphid species tested were able to reproduce well on the plants on which they were reared, but only A. craccivora was able to reproduce on peanuts. / M.S.

The potential of Pandora neoaphidis (Remaudiére & Hennebert) Humber as a fungal pathogen for the control of the tobacco aphid, Myzus nicotianae Blackman, on tobacco was evaluated in a 4-year study between 1992 and 1995. The objectives of this study included determination of the seasonal incidence of P. neoaphidis in populations of tobacco aphid on tobacco and non solanaceous host plants, within plant distribution of the pathogen in aphids on tobacco, influence of tobacco cultivars and cultural practices on the incidence of the pathogen, methods of artificially introducing the pathogen into tobacco aphid populations and their potential in controlling aphids relative to chemical control, virulence of the Virginia isolate of the pathogen to tobacco aphid and green peach aphid, M. persicae (Sulzer), from different geographic locations in the eastern United States, and influence of temperature and type of substrate on the developmental morphology of the pathogen. Infections as high as 91% occurred in aphid populations on tobacco under favorable weather. The pathogen survived at moderate levels in the red morph of tobacco aphid on non solanaceous hosts during fall, parts of winter and spring. P. neoaphidis infections in aphids tended to increase towards the upper leaf positions. Incidence of the pathogen in aphid populations varied widely on various cultivars and types of tobacco. Planting date, topping of tobacco, and stage at which tobacco was topped did not influence the incidence of P. neoaphidis in tobacco aphids. Artificial introduction of P. neoaphidis successfully established infections in tobacco aphids, but failed to prevent the build up of aphid populations. P. neoaphidis was equally virulent to the red and green morphs of the tobacco aphid, and the green peach aphid. The developmental morphology of P. neoaphidis was influenced by temperature and was similar on the surfaces of living substrates, tobacco aphid and tobacco leaf, but different on the inert surface of the coverslip. / Ph. D.

The relationship of geographic location, insecticide exposure, time of season and the genotype of red and green morphs of the tobacco aphid, Myzus nicotianae Blackman, on tobacco was investigated in Virginia in 1988 and 1989. Color morph and karyotype were found to be highly related. The translocated karyotype was associated with the red morphs and the normal karyotype was associated with the green morphs. The karyotype of red and green morphs did not change markedly from the beginning to the end of the tobacco growing season. Studies were also conducted to determine if the red and green morphs of the tobacco aphid were developing resistance to acephate (Orthene Tobacco Insect Spray), the most commonly used aphicide on flue-cured tobacco in Virginia. Slight resistance was present in both red and green aphid populations from several counties. Studies were also conducted to determine if males were present in tobacco aphid populations in Virginia. Cool temperatures and short photoperiods which initiate male production in Myzus persicae (Sulzer) with holocyclic life cycles did not cause the production of males in M. nicotianae native to Virginia. / Master of Science

A two year study was conducted to measure the impact of tobacco aphid, Myzus nicotianae Blackman, colonization and cumulative aphid-days on flue-cured tobacco, Nicotiana tabacum (L.). The objectives of the study included quantifying the response of tobacco production variables, cured leaf quality, and cured leaf chemical analysis to the level of cumulative aphid-days. Aphid populations and tobacco plant responses were recorded on an individual plant basis. A gradient in cumulative aphid-days was obtained through the use of temporally distributed aphid colonizations and selective insecticide use. Tobacco aphid populations resulted in yield reductions as great as 22 and 27% in 1988 and 1989, respectively, while gross economic returns were reduced 27 and 32% in the respective years. The responses of tobacco production variables were characterized by a decreasing negative slope; therefore, incremental losses were greatest at low levels of cumulative aphid-days. Regression models were developed to describe crop production responses as a function of cumulative aphiddays. The quality of the cured leaves (grade index) was also responsive to the level of cumulative aphid-days. Changes in both tobacco grade group and quality within a given group occurred with increasing cumulative aphid-days. The occurrence of nondescript tobacco was associated with large aphid populations. The chemical quality of the cured tobacco was also influenced by cumulative aphid-days. The total alkaloid content followed a linear function, while the level of reducing sugars was a nonlinear relationship with cumulative aphid-days. The study also reported on the disproportionate impact of aphid populations upon the within-plant responses of tobacco. A study was conducted to evaluate the impact of tobacco aphid management using eight different action thresholds. Comparison of the two most commonly recommended treatment thresholds (10 and 20% of plants with 50 or more aphids per leaf) revealed no significant differences in the number of remedial treatments required or the yield and gross economic returns. However, use of the latter threshold resulted in a delay of approximately one week for the first treatment and the retreatment interval. / Ph. D.

Alate morphs can benefit aphid populations by facilitating dispersal from deteriorating food sources and by escaping from natural enemies. Wing development, however, imposes constraints on fecundity. The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an important economic pest in the U.S. and Canada. I conducted a series of laboratory and field experiments to determine the environmental factors inducing wing development in this species, and to determine the effects of asexual alate individual production on an A. glycines population under predation. My results reveal that wing induction in A. glycines occurs in response to interactions between crowding and decreased plant quality cues, and that alate aphid production benefits an aphid population under predation by increasing prevalence at a temporary cost to fecundity. My results contribute to the growing knowledge on the production of asexual alate aphids and provide insight into the biology of A. glycines as an agricultural pest. / May 2017