The tobacco aphid, a tobacco-feeding form of Myzus persicae is a key pest of tobacco. Severe infestations cause serious reductions in tobacco yield and quality. Although several insecticides are still effective for tobacco aphid control, some forms of the aphid have developed resistance to various organophosphate and carbamate insecticides through the over-production of carboxylesterases E4 and FE4. In 1996, imidacloprid was approved for use on tobacco and imidacloprid and other neonicotinoids are currently used on over 80% of the tobacco acreage in Virginia. Increased tolerance to imidacloprid has been reported in some populations of M. persicae. The present study will assess insecticide resistance to establish baseline information for monitoring resistance in the aphid. Aphids collected from various locations in Virginia in 2004 and 2005 are being screened for resistance to neonicotinoid (imidacloprid), organophosphate (acephate), and carbamate (methomyl) insecticides. Leaf dip bioassays using commercial formulations are being used to quantify insecticide resistance levels for each aphid population. LC₅₀ and LC₉₀ values are being determined and the colonies will be categorized by resistance levels. Microplate and immunoassays are being used to determine carboxylesterases and the levels of E4 and FE4 enzymes, respectively.