Technical Abstract: Treating perimeters with residual insecticides to provide protection from mosquito vectors has shown promise. These barrier treatments are typically evaluated in temperate or tropical areas using lush, ambient vegetation as a substrate for the pesticide. However, there is an emerging interest to develop this technology to protect deployed US troops in extreme desert environments with sparse vegetation. We used a large remote desert area in the Coachella Valley, California, to evaluate bifenthrin barrier treatments under field conditions of heat, dust, and low humidity on 15.25 x 15.25 m plots of native xeric vegetation using two spray technologies. Through a combination of bioassays on foliage periodically sampled from these plots over 63 days, synchronized with field surveillance of mosquitoes from traps placed within plots, we measured the temporal pattern of bioactivity of bifenthrin barriers under natural desert conditions. Regardless of spray technology, mosquito catch in treated plots was about 80% lower than the catch in control plots one day after treatment. This reduction in mosquito numbers in treated plots declined each week after treatment, but remained at about 40% lower than control plots after 28 days. These field data were corroborated by results from bioassays in the lab which showed significantly higher mosquito mortality on treated vegetation as compared to untreated vegetation out to 28 days post-spray.