With the ability to generate large amounts of molecular data, landscape genetics studies are an increasingly important tool in the management of pest organisms. Understanding this relationship can inform managers on population boundaries and allows inferences about gene flow and dispersal among putative populations. Within this framework, I examined the effects of common landscape features on population genetic diversity in the tule mosquito, Culex erythrothorax, a pest species endemic to the San Diego region. Female C. erythrothorax mosquitoes were collected from 23 sites spanning six different watersheds running north to south in San Diego County in 2007 and 2008. I sequenced the mitochondrial gene cytochrome oxidase I for 115 individuals, and also quantified diversity patterns for 230 individuals at 10 novel microsatellite loci developed specifically for this study. I tested the relationship between genetic structure and two common landscape features: watershed boundaries and geographic distance. Previous studies have suggested that these landscape features play a role in structuring populations in other mosquito species, although these hypotheses have not been tested in the tule mosquito. Overall, genetic differentiation among populations of C. erythrothorax in San Diego County was weak. Contrary to my hypothesis, watersheds only play a minor role in subdividing populations for this species. A weak isolation by distance pattern was also found to be present in C. erythrothorax, when accounting for the presence of watersheds. Individual multilocus cluster analyses detected a similar pattern, identifying three weakly differentiated gene pools in the region. These findings are interpreted in the context of the power of alternative approaches to data analysis, and also the basic biology of C. erythrothorax. In addition to this landscape genetic study, I also describe a multi-access digital key to the aquatic insect families of San Diego County that I developed for use in bioassessment. Rather than a traditional dichotomous key, this key is designed for non-technical users, such as volunteer groups associated with local watershed monitoring.