We compared host acceptance behaviour between two strains of the parasitoid Cotesia glomerata: one strain from the USA where C. glomerata was introduced from Europe 120 years ago, and one native European strain. In the USA larvae of Pieris rapae are attacked, whereas in Europe both P. rapae and P. brassicae serve as hosts. Pieris brassicae is the preferred host species, but since it is absent in the USA, it has not been available to American C. glomerata for about 350 generations. We observed clear geographic variation in host acceptance between American and European parasitoid strains: American C. glomerata rejected P. brassicae significantly more often than European parasitoids did. Early experience through development in and emergence from the less preferred host P. rapae increased acceptance of this host in European C. glomerata. Host acceptance of the preferred host was `hardwired': it was high regardless of previous experience. Such strong inflexible responses to important stimuli and plastic responses to less important stimuli are observed in many other parasitoid-host systems. However, our results show that 350 generations of selection were sufficient to override this hardwiring in the American parasitoid strain. Host-parasite interactions; Invasions; Host preferences; Geographical variations; Braconidae; Cotesia glomerata; USA none