Cabbage root maggots are a multivoltine complex of up to three species including Delia planipalpis, D. radicum, D. floralis (Diptera: Anthomyiidae). The cabbage root maggot overwinters within a puparium and emerges in the spring. Data has shown that continuous cropping of canola on canola contributes to high populations of root maggot so crop rotation is the greatest tool to manage this insect pest whose larval stage feeds upon the canola root. The beneficial organism that attacks both the egg and pupal stages of the cabbage root maggot is Aleochora bilineata, a small staphylinid beetle which is both predator and parasitoid! Aleochora bilineata adults hunt and eat Delia spp. eggs but this beneficial organism also parasitizes Delia puparia so, instead of the fly emerging from the root maggot puparium, a beetle emerges - check it out below!
More information is available below from the new Insect Guide:
Also look up this scientific publication to learn more about Aleochora bilineata in canola production systems in Canada: Broatch J.S., Dosdall L.M., Yang R.C., Harker K.N., Clayton G.W. 2008. Emergence and seasonal activity of the entomophagous rove beetle Aleochara bilineata (Coleoptera: Staphylinidae) in canola in Western Canada. Environ Entomol. 37(6):1451-60.