This is one of those species where the scientific web sites say almost nothing, other than to argue over semantics and exactly who named the insect last, and the pest control sites have a lot to say. (What can I say, nobody seems to want to write a PhD thesis on small, feces-eating flies.)
As the name implies, the root maggot fly (Anthomyia illocata) has larvae that eat the roots of plants. It likes crops, and is therefore considered a pest, as they both damage the plants directly through their eating and allow fungus and bacteria into the plant through their tunneling.
Anthomyiid flies look like small (about 5 mm) black and white houseflies. Illocata has red eyes, and stripes rather than spots as do some of its cousins. Despite the “Don’t eat me!” aposematic coloring, the flies do not actually appear to be poisonous.