yep. Since you already have flies it might seem like it doesnt work at first. You'll have to wait the 10 days or so that it takes for the already existing flies to die off before you start noticing a change (since they feed of the larvae and not the actual flies), but it's definitely worth it.
Since flies reproduce at a much faster rate than the predators do. The flies that are there now will already have a batch of larvae going, etc.
You might want to order a couple months of double orders.