The diamondback moth, Plutella xylostella (L.), is a global pest of cruciferous crops (Brassicaceae). It has developed resistance to virtually all known insecticides, and biological control has become an important management tool. In North America the parasitoid Diadegma insulare (Cresson) has been used successfully to reduce diamondback moth populations. We document the presence of the α-proteobacterial endosymbiont Wolbachia and its associated bacteriophage WO in P. xylostella and D. insulare and examine the phylogenetic relationships of Wolbachia and WO in both host species. Our results suggest that Wolbachia and WO have been horizontally transferred in this insect—parasitoid system in recent evolutionary history. Knowledge of the dynamics of Wolbachia in P. xylostella and D. insulare may be an important factor in future control of this pest in the field.