Concern over the vector potential of the Gulf Coast tick, Amblyomma maculatum Koch, with the pathogen Ehrlichia ruminantium Dumler, causative agent of the disease heartwater, has increased the need for fundamental knowledge of tick ecology and behavior, specifically immature tick biology. Texas strain A. maculatum adult male ticks, known to emit attraction-aggregation-attachment pheromone (AAAP), were used to artificially simulate immature tick interaction with adults, in forced environments, on cattle. Artificial areas were grouped by treatment level, which were 1) aggregating, attached adult males, 2) aggregating attached adult females or 3) an empty area with no adults, as a control. Immature ticks were noted to be 6 times more likely to be aggregating in the AAAP treatment area when adult males were present. In the presences of either adult female ticks or no ticks at all, immature ticks were found to be attaching at random within the given area were they where permitted to feed. A second correlation of mortality was noted among immature ticks in the presence of AAAP emitting adult male ticks. In the permitted area where immature ticks could attach and feed, immature ticks were twice as likely to have survived to engorgement if adult male ticks were present in the area as well (53%). There was no difference in the survival rate among immature ticks if adult females were present or no adults at all, 26% and 21%, respectively. The study demonstrated that a significant attraction existed between immature ticks and attached adult males emitting AAAP.

The interaction of tick species and the microclimate that they inhabit is a subject that is frequently studied. The known biology of the Gulf Coast tick, Amblyomma maculatum Koch (Acari: Ixodidae), was used to conduct analyses to determine which meteorological factors regulate Gulf Coast tick survivorship in an area of Texas that is known for reduced occurrence of this tick species. Gulf Coast tick collection records, that indicated the collection of a single tick or multiple ticks from one animal or multiple animals at livestock markets, were obtained from the Texas Animal Health Commission. These records were used as an indicator of adult Gulf Coast tick abundance during each year in the 90 county study area and were used as the dependent variable in linear, quadratic, and cubic regression analyses. Independent variables used in these analyses were precipitation and differing drought thresholds during the peak activity time of the four life stages of the Gulf Coast tick and during combined life stage peak activity times. Linear, quadratic, and cubic regression analyses to measure the effect of precipitation during differing peak activity times of the Gulf Coast tick on adult Gulf Coast tick collection records were not statistically significant. These three regression analyses were also used to measure the effect of increasing drought thresholds, measured using a Keetch-Byram Drought Index, on adult Gulf Coast tick collection records. A determination was made that increasing drought thresholds during the peak activity time of differing Gulf Coast tick life stages reduce the number of Gulf Coast tick collection records the following year.

The aggregation-attachment-pheromone (AAP) of two geographic strains of the Gulf Coast tick, Amblyomma maculatum Koch, was investigated to evaluate practicality of using solid-phase-microextraction (SPME) in an AAP study of Gulf Coast tick. Solid-phase microextraction was used to compare the AAP production in two strains of fed male Gulf Coast tick and demonstrate and confirm the presence of AAP in bioassays. A solid-phase-microextraction (SPME) headspace collection technique was sufficient to capture volatile organic compounds produced by fed and unfed male Gulf Coast ticks. Gas chromatography analysis revealed three major volatile organic compounds were produced in significantly greater amounts (p < 0.05) by fed males than those produced by unfed males. These volatile compounds were produced in significantly higher amount by the third day of feeding by male ticks. However, two of these volatiles remained relatively constant in their production while the primary volatile compound increased in its production until the eighth day of feeding by male Gulf Coast ticks. Also, the relative abundances of these three volatile organic compounds were different between Oklahoma and Texas strains of Gulf Coast ticks. The activity of AAP from fed male Gulf Coast ticks was confirmed using two bioassay techniques. A petri dish bioassay revealed significantly higher numbers of female Gulf Coast ticks attracted to fed-males which also produced significantly greater amounts (p < 0.001) of volatile organic compounds detected by GC analysis. The Ytube olfactometer bioassay revealed that significantly higher numbers of females responded to fed-males or to CO2 when compared to purified air (p < 0.001), but the differences in female response to fed-males and CO2 were not significant (p < 0.391 in Oklahoma strain and p < 0.458 in Texas strain). However, female responses to stimuli containing both fed-males and CO2 were significantly higher when compared to either stimulus alone (p < 0.001).

Amblyomma maculatum (the Gulf Coast tick), an aggressive, human-biting, Nearctic and Neotropical tick, is the principal vector of Rickettsia parkeri in the United States. This pathogenic spotted fever group Rickettsia species has been identified in 8-52% of questing adult Gulf Coast ticks in the southeastern United States. To our knowledge, R. parkeri has not been reported previously from adult specimens of A. maculatum collected in Kansas or Oklahoma. A total of 216 adult A. maculatum ticks were collected from 18 counties in Kansas and Oklahoma during 2011-2014 and evaluated by molecular methods for evidence of infection with R. parkeri. No infections with this agent were identified; however, 47% of 94 ticks collected from Kansas and 73% of 122 ticks from Oklahoma were infected with ". Candidatus Rickettsia andeanae" a spotted fever group Rickettsia species of undetermined pathogenicity. These preliminary data suggest that " Ca. R. andeanae" is well-adapted to survival in populations of A. maculatum in Kansas and Oklahoma, and that its ubiquity in Gulf Coast ticks in these states may effectively exclude R. parkeri from their shared arthropod host, which could diminish markedly or preclude entirely the occurrence of R. parkeri rickettsiosis in this region of the United States.

The recent discovery of Amblyomma maculatum sensu lato (s. l.) ticks in southern Arizona has renewed discussions around species designations for members of the Amblyomma maculatum tick group. Amblyomma maculatum s. l. from Arizona appear to be morphologically intermediate between A. maculatum sensu stricto (s. s.) and A. triste s. s. At present there is no conclusive species designation for the ticks from Arizona. My research focused on analyzing the systematics of both A. maculatum s. l. and Rickettsia parkeri, a common bacterial pathogen transmitted by these ticks. In the laboratory, A. maculatum s. l. from Arizona and A. maculatum s. s. from Georgia readily mated on experimental animals to produce F1 hybrid ticks; there was no difference in fertility with these two populations when compared with homologous populations. However, the F1 hybrids produced during these experiments exhibited diminished fitness and did not produce a viable F2 generation. These results suggest that A. maculatum s. l. and A. maculatum s. s. represent separate biological species. Results of the crossbreeding experiment conflict with recent genetic analyses of A. maculatum s. l. and A. maculatum s. s. suggesting they are a single species. Thus, I developed and optimized 14 microsatellite loci that amplify both A. maculatum s. s. and A. maculatum s. l. These novel microsatellite markers can be used in future analyses of A. maculatum s. l. and A. maculatum s. s. to further test for conspecificity between the two. I also investigated the genetic relationships within geographically distinct R. parkeri strains through development and implementation of a multi-locus sequence typing analysis. I showed that while there is no consistent genetic delineation of strains isolated from A. maculatum s. l. versus A. maculatum s. s., there is a subset of R. parkeri strains from A. maculatum s. l. that appear to represent an intermediate genotype between the North and South American strains. While the biological causes for these results are not immediately clear, coevolution of R. parkeri and A. maculatum s. l. may account for the detection of the intermediate genotype only found in association with A. maculatum s. l.

The public health and veterinary importance of Gulf Coast ticks, Amblyomma maculatum Koch (1844) have become more apparent during the last several decades. In addition, new records of this three-host ixodid tick presently show a geographic distribution throughout much of the southern United States. Rickettsia parkeri, a spotted fever group rickettsia (SFGR) that is commonly found infecting the Gulf Coast tick, was only recently recognized as a human pathogen. Over the last decade, more than 20 human cases of disease caused by R. parkeri have been recognized in the Americas, all of which were similar in presentation to mild Rocky Mountain spotted fever. In addition, a novel, poorly characterized SFGR, “Candidatus Rickettsia andeanae”, was recently identified in A. maculatum from Peru, United States, Chile and Argentina. As the recognition of R. parkeri as a pathogen and “Ca. R. andeanae” as an additional SFGR in A. maculatum only recently occurred, a general gap exists in our understanding of the biology of these SFGRs. The overall objective of this dissertation was to contribute to our knowledge of SFGR infecting A. maculatum. In Chapter 3, we present a prevalence study of R. parkeri, and “Ca. R. andeanae” in A. maculatum from Mississippi where we detected 15.2% R. parkeri-singly infected ticks and 3.1% total “Ca. R. andeanae” infected ticks of which 1.7% were co-infected with R. parkeri. In Chapter 4, we discuss finding four genetically different populations of A. maculatum from Mississippi infected with a homogenous population of R. parkeri, using Single Strand Conformation Polymorphism analysis. Those initial data relating to “Ca. R. andeanae” provided a foundation for studies described in Chapters 5 and 6. We report the first morphological study of “Ca. R. andeanae” using transmission electron microscopy in Chapter 5 and isolation of this SFGR in ,A. maculatum cell co-culture in Chapter 6. We anticipate that results presented in this dissertation will contribute to our understanding of the ecology of ,A. maculatum as a vector for the human pathogen, R. parkeri, and increase the current understanding of both R. parkeri and “Ca. R. andeanae” in A. maculatum.