Do parasitoids diversify in response to host-plant shifts by herbivorous insects?

1. For herbivorous insects, the incorporation of a novel host into the diet , and subsequent formation of distinct host associations (races), is though t to be a significant early step in the speciation process. While many stud ies have addressed this issue, virtually nothing is known about the evoluti onary response of natural enemies to herbivore host-race formation. 2. The hypothesis that the parasitoid wasp Eurytoma gigantea (Hymenoptera: Eurytomidae) has formed host races in direct response to the host shift and subsequent host-race formation by its host, the gallmaker Eurosta solidagi nis (Diptera: Tephritidae) was tested. Emergence time, mating preference, a nd female oviposition preference were determined for parasitoids derived fr om galls of each Eurosta host race. 3. Male and female E. gigantea overlap broadly in their emergence times fro m each Eurosta host race, suggesting that there is no phenological barrier to gene flow. 4. In choice experiments, female parasitoids did not mate assortatively: fe males that emerged from one Eurosta host race were equally likely to mate w ith males from either Eurosta host race. 5. Oviposition behaviour experiments revealed that female parasitoids do no t prefer to oviposit on their host race of origin and that there is no over all preference for one host race, even though fitness is higher when parasi toids are reared from Eurosta galls of the Solidago gigantea host race than when reared from Eurosta galls of the Solidago altissima host race. 6. These results suggest that E. gigantea has not diverged in parallel with its host in response to the herbivore host-plant shift. Further studies ar e needed before the ubiquity of this diversification mechanism can be evalu ated fully.