In 2009, pyrethroid resistance was confirmed for seven “annual bluegrass weevil” Listronotus maculicollis Kirby (Coleoptera: Curculionidae) adult populations from southern New England. The mechanisms responsible for conferring this resistance were unknown. In this study, topical application bioassays with bifenthrin and bifenthrin combined with synergists affecting three detoxification systems were conducted on four field-collected adult populations of L. maculicollis from Connecticut to determined whether cytochrome P450 monooxgenases (P450s), glutathione S-trans-ferases (GSTs), and/or carboxyl-esterases (COEs) mediated metabolic detoxification. Because a susceptible L. maculicollis laboratory strain does not exist, the most susceptible field-collected population (New Haven) provided a baseline against which all other populations were compared. In the population with the lowest resistance (Norwich), only detoxification by P450s was significant. Detoxification in the population with the second highest level of resistance (Stamford) involved both P450s and GSTs. Detoxification in the population with the highest level of resistance (Hartford) involved P450s, GSTs, and COEs. This study suggests that enzyme-mediated metabolic detoxification plays an important role in annual bluegrass weevil pyrethroid resistance.