Assessing the Spread and Movement of Cotton Leafroll Dwarf Virus and Cotton Infesting Aphids in the Southern U.S.

Abstract

The cotton aphid, Aphis gossypii Glover, is the only known vector to transmit cotton leafroll dwarf virus (CLRDV) to cotton, Gossypium hirsutum L., but the role of the aphid vector in the epidemiology of CLRDV is not defined. The overarching goal of this dissertation is to assess the role of the vector in the epidemiology of CLRDV. The overall objective for Chapter 1 was to conduct a meta-analysis of 289 published effect sizes from 28 studies to better understand the relationships between plant pathogen vectors or vector-borne pathogens and landscape composition, while considering a series of biological, ecological, and environmental factors that may contribute to variation in the relationships. We found that vector and vector-borne pathogens were positively associated with landscape composition metrics, including the percentages of the landscape surrounding the study areas that were crop, non-crop, and natural habitat, but not with habitat diversity. Vector specialists exhibited a significant positive association with various landscape composition metrics while generalists did not, suggesting that specialists, with their narrower host range, may be more dependent upon particular landscape features. Generalists, however, have more host options, and thus their populations may not be as strongly associated with landscape features. Measurements of both abundance and incidence of pests exhibited significant positive associations with landscape composition metrics. For both persistent and non-persistent pathogens, the slope of the relationship between pathogen populations and landscape composition metrics increased as the spatial scale used to calculate landscape composition increased. This meta-analysis demonstrated that vectors and vector-borne pathogens were positively associated with the composition of the landscape, and that these relationships were influenced by different biological, ecological, and environmental factors. Future vector and disease management strategies should consider the impact that landscape composition may have on these pests, and how the landscape, along with cultural practices, can be manipulated to aid in pest management. The overall objective for Chapter 2 was to determine the spatiotemporal distribution, abundance, and seasonal dynamics of the eight species of aphids that have been reported to feed on cotton. In 2020 and 2021, aphids were monitored from North Carolina to Texas using pan traps around cotton fields. All of the species known to infest cotton, excluding A. fabae, were detected in this study. Protaphis middletonii and A. gossypii were the most abundant species identified. The five other species of aphids captured were consistently low throughout the study and, with the exception of R. rufiabdominale, were not detected at all locations. The abundance, distribution, and seasonal dynamics of cotton-infesting aphids across the southern U.S. are discussed. The overall goal for Chapter 3 was to investigate and understand how different biotic and abiotic factors may be driving CLRDV incidence in cotton. Field surveys were conducted in Alabama and Georgia from 2021-2022, to monitor A. gossypii, natural enemy, CLRDV weed hosts, and CLRDV incidence in cotton, alongside collecting data on landscape composition (percentage of cotton and natural habitat surrounding field sites at a 5 km radius), temperature, and precipitation. A structural causal modeling framework was used to assess how these different factors were driving CLRDV incidence in cotton. Aphis gossypii abundance, cotton, and temperature had a significant positive effect on CLRDV incidence. Temperature also had a significant positive effect on A. gossypii abundance, which likely mediated the effect of temperature on CLRDV incidence. Natural enemies, CLRDV weed hosts, natural habitat, and precipitation did not significantly affect CLRDV incidence or A. gossypii abundance. Aphis gossypii is the only known vector to transmit CLRDV to cotton, therefore an increase in vector abundance can increase the likelihood of pathogen presence and transmission. Temperature serving as the only driving factor for A. gossypii abundance in cotton explains the direct positive association between temperature and CLRDV incidence, and further complements the finding that A. gossypii abundance increased CLRDV incidence. Cotton serves as a host for both the vector and the virus, so this increase in cotton may have contributed to inoculum sources and spread that augmented incidence of the pathogen in the landscape. Cotton fields in areas with greater cotton production, higher A. gossypii abundance, and warmer temperatures are at greater risk for CLRDV infection. Warmer regions may experience elevated aphid infestations and population growth, that help increase CLRDV incidence. Our findings identify high-risk regions and the associated causal factors, guiding future research for effective management strategies. They also inform growers and researchers about priority areas for monitoring and management efforts. The primary goal for Chapter 4 was to use DNA high-throughput sequencing to conduct a gut content analysis to reveal the dietary history of A. gossypii prior to colonizing cotton. .Aphis gossypii alatae were collected from cotton fields in Alabama, Georgia, Louisiana, Texas, South Carolina, North Carolina, Virginia, and Mississippi in 2020 and 2021, and used in the gut content analysis. A total of 92 plant genera were detected in this study, 66.30% of which contained at least one species reported to be a host for A. gossypii. Gossypium was detected in more aphids than any other plant genus (66.14%), which is intuitive given that aphids were collected from cotton. Convallaria (17.55%) was the second most frequently detected plant genus, followed by Triticum (8.46%), Prosopis (6.58%), Melilotus (6.58%), and Solanum (5.64%). An additional 86 genera were detected in 85.26 % of the aphids sampled, indicating a high level of variability detected among plant genera. This study highlights gut content analysis as a valuable tool for understanding A. gossypii's dietary history and plant interactions.