Bluetongue virus (BTV) is an economically important arbovirus of ruminants transmitted by Culicoides biting midges. Vector control using residual spraying or application to livestock is recommended by many authorities to reduce BTV transmission; however, the impact of these measures in terms of both inßicting mortality on Culicoides and subsequently upon BTV transmission is unclear. This study consisted of a standardized World Health Organization laboratory assay to determine the susceptibility ofEuropeanCulicoides species todeltamethrinandaÞeldtrialbasedupon allowing individuals of a laboratory strain ofCulicoides nubeculosusMeigen to feed upon sheep treated with Butox 7.5 pour-on (a deltamethrin-based topical formulation). Susceptibility in the laboratory trialwashigher incolonyC. nubeculosus (24-hLC90 0.00106%), thaninÞeldpopulations ofCulicoides obsoletus Meigen (24-h LC90 0.00203%) or Culicoides imicola Kieffer (24-h LC90 0.00773%). In the Þeld, the pour-on formulation was tested with a total of 816 C. nubeculosus specimens fed upon on the thigh of treated sheep. The study revealed a maximum mortality rate of 49% at 4 d postapplication, and duration of lethal effectwas predicted to be as short as 10 d, despite testing being carried outwith a highly susceptible strain. The reasons for this lowefÞcacy are discussedwith reference both to the potential for lack of spread of the active ingredient on the host and feeding patterns of themajor potential vector species on the sheep host. Practical implications for vector control strategies during BTV incursions are also detailed.