The enormous diversity of plant feeding insects has been exploited by parasitoid wasps. Almost every insect species has at least one parasitoid species. Host associated differentiation has been a major factor in determining both population structure and speciation in herbivorous insects but its influence on parasitoid taxa is unclear. Using the moth-parasitoid community on yucca plants, I examine the role of host associated differentiation in structuring a parasitoid-host community. Data on host range and genetic structure for both groups demonstrate that specialization and host associated differentiation may have a more prominent role in the evolutionary ecology of herbivorous insects than in their associated parasitoids. I explore the potential mechanisms for such an outcome.