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Interspecific competition for limited resources can drive ecological specialization and trait expression. Organisms released from intense competition may exploit a broader range of resources, but if reunited with stronger competitors, survivorship may depend on foraging behaviors that reduce competition. We compared the host selection behavior of the parasitoid Cotesia glomerata from two North American populations that differ in their association with Cotesia rubecula, a superior competitor. Both parasitoids originate from Europe and attack the imported cabbageworm (a.k.a. small cabbage white) Pieris rapae, but C. glomerata was introduced into North America almost a century before C. rubecula. After re-association in North America, C. rubecula has displaced C. glomerata in several regions, but not in other regions. Host selection was measured in female C. glomerata from Maryland (MD) where it coexists with C. rubecula, and in conspecifics from Colorado (CO) where C. rubecula is absent. Unparasitized and C. rubecula-parasitized P. rapae hosts were used in choice tests to examine whether C. glomerata host selection behavior differed based on the population's association history with C. rubecula. We found that C. glomerata from MD had a higher likelihood of avoiding hosts parasitized by C. rubecula (and thus avoiding competition) than did wasps from CO. The ability of C. glomerata to avoid hosts parasitized by C. rubecula may facilitate coexistence in MD; whereas, the lack of discrimination in CO populations of C. glomerata naïve to C. rubecula could contribute to the displacement of C. glomerata were C. rubecula to enter the same habitat.