Geographic variation in host acceptance by an insect parasitoid: genotype versus experience

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.