Mosquitoes play an important role in the transmission of pathogens and parasites to humans and animals all over the world. Effective management of vectors requires a clear understanding of vector biology. Novel research that focuses on the factors that influence vector capacity is paramount for elucidating the complex dynamics of vector-borne disease. The research projects presented in this dissertation involve two components of the vector capacity model: density and host preference. The first study investigated how varying CO2 concentration alters mosquito collection rates at a freshwater wetlands in southern California. Host-seeking mosquitoes were collected in CDC-style traps baited with one of six CO2 release rates ranging from 0-1495 ml/min. The species collected were Aedes vexans, Anopheles franciscanus, Anopheles hermsi, Culex erythrothorax and Culex tarsalis. A biting midge Culicoides sonorensis, was also collected. CO2 concentration influenced the collection rates of all host-seeking Diptera sampled in this study with increasing CO2 concentration resulting in increasing collection of host-seeking insects for all flow rates. In the second study, the diel activity of resting mosquitoes was assessed for 24 hr periods on four separate dates at a freshwater wetlands in southern California using resting boxes. Additionally, height of resting flight activity was evaluated using paired small resting boxes placed at ground level or suspended 6 m off the ground to collect resting mosquitoes at canopy level. Aedes vexans and Culex tarsalis were the most abundant species collected in the resting boxes. Our results indicated no significant differences between canopy and ground resting box collections for these two species. Peak collection time from resting boxes occurred 30 minutes before sunrise. The last study investigated the host feeding patterns of Cx. stigmatosoma. Our results confirmed that this species feeds mainly on passerine birds.