Nosema maddoxi is a newly described microsporidian species that is an entomopathogen of the invasive brown marmorated stink bug (Halyomorpha halys) and three native stink bug species in the United States. Halyomorpha halys is an agricultural and nuisance pest in the US and has been detected in 44 states. Microsporidian species such as Paranosema locustae have been used successfully to control insect pest populations, because they negatively impact their host’s fitness. Since N. maddoxi was present in the field and was also the main cause for the decline of some H. halys lab colonies, it was important to investigate the distribution of this entomopathogen and its impact on this invasive species. One thesis objective focused on the phenology, distribution, infection intensity, and prevalence of N. maddoxi infections in H. halys populations in the US. This study determined that the prevalence of N. maddoxi infection was seasonal, with the highest infection levels occurring in the spring and the lowest in the summer. Prevalence of N. maddoxi infection was also variable among sites. In addition, N. maddoxi infections were widely distributed in established H. halys populations in the US as this pathogen was found in CA, KY, MD, NC, NY, OH, OR, PA, UT, VA, and WV; this included all of the states where H. halys was sampled for this study. The prevalence of N. maddoxi infection was not found to be host density-dependent and low-intensity infections were more common than high-intensity infections in field-sampled populations. Melanized tissues that looked like brown spots were visible through the abdominal cuticle of many infected H. halys and this was identified as a potential indicator of N. maddoxi infection in H. halys. For the second thesis objective, bioassays were conducted to investigate the effects of N. maddoxi infection on the survival of H. halys adult females and nymphs, and on female fecundity, egg viability, and nymphal development. There were two adult female bioassays: 1) treated adult females, in a low spore concentration bioassay, in which each female imbibed 25 µl containing 350,000 spores for 2 replicates and 400,000 spores for one replicate, and 2) treated females, in a high spore concentration bioassay, in which each female imbibed 25 µl containing 875,000 spores. Treated second instar nymphs in the nymphal bioassay each imbibed 2 µl containing 28,000 spores. Results determined that N. maddoxi infection significantly shortened the lives of infected nymphs and H. halys adult females at the higher spore concentration. In addition, N. maddoxi infection negatively impacted female fecundity and egg viability for both spore concentrations. Nymphal development rate and size was not impacted by N. maddoxi infection. In summary, these studies determined that N. maddoxi is widely distributed in H. halys populations, can have high infection prevalence, and shortens the lifespans of H. halys adult females and nymphs, as well as decreasing reproduction, indicating that this newly described microsporidian species is a natural biological control agent of this invasive species in the US.