Screening trials in the Pesticide Toxicology Laboratory at Iowa State University have focused on characterizing the repellent activity of plant essential oils including catnip (Nepeta cataria), Osage orange (Maclura pomifera), West Indian sandalwood (Amyris balsamifera), and Siam wood (Fokienia hodginsii) against ticks and mosquitoes, including Culex and Aedes sp. Several major components of these essential oils were also isolated and evaluated for repellent activity. Tick climbing assays were designed to assess repellency by measuring responses to a barrier treated with an essential oil. Mosquito repellency was evaluated using a standardized static-air olfactometer. These studies and others have led to the development of mixtures containing different spatial and contact repellent activities. Mixtures consisting of essential oils and some isolated components from catnip, Osage orange, West Indian sandalwood and Siam wood oils, were found to have high residual repellent activity. All mixtures containing both contact and spatial mosquito repellents showed significantly higher levels of repellency as compared to the individual components or oils. Discussion will include trends observed in repellency screening trials regarding the potency of the parent essential oils to their major components and optimization of repellent blends, including residual control.