1. For herbivorous insects, the incorporation of a novel host into the diet, and subsequent formation of distinct host associations ("races"), is thought to be a significant early step in the speciation process. While many studies have addressed this issue, virtually nothing is known about the evolutionary response of natural enemies to herbivore host-race formation.
2. The hypothesis that the parasitoid wasp Eurytoma gigantea (Hymenoptera: Eurtyomidae) has formed host races in direct response to the host shift and subsequent host-race formation by its host, the gallmaker Eurosta solidaginis (Diptera: Tephritidae) was tested. Emergence time, mating preference, and female oviposition preference were determined for parasitoids derived from galls of each Eurosta host race.
3. Male and female E. gigantea broadly overlap in their emergence times from each Eurosta host race, suggesting that there is no phenological barrier to gene flow.
4. In choice experiments, female parasitoids did not assortatively mate: females that emerged from one Eurosta host race were equally likely to mate with males from either Eurosta host race.
5. Oviposition behavior experiments revealed that female parasitoids do not prefer to oviposit on their host race of origin, nor is there an overall preference for one host race. This occurs even though fitness is higher when parasitoids are reared from galls of the S. gigantea as compared to the S. 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 are needed before we can assess the ubiquity of this diversification mechanism.
Key words. Eurosta solidaginis, Eurytoma gigantea, emergence phenology, gall insect, goldenrod, host-parasitoid evolution, host-race formation, oviposition preference, mate preference, plant-herbivore-parasitoid interaction.