Unless you're in a very dry climate, or in the middle of an intense dry season in a dry/wet season climate, there should be plenty of soldier flies around. A
compost pile or even an open, poorly managed latrine are good places to start looking. Mature soldier fly larvae are quite a bit larger than housefly maggots....up to an inch or more in length, and they like stuff that is already composting, not so much on fresh manure or dead animals (although they will feed on both of these with relish once present). A good bait for them is fermenting grain, which won't attract houseflies as much. The adult flies are unique, and resemble wasps more than other flies. Their only purpose is to mate and lay eggs, and they do not feed as adults. So they will not come to food or fresh manure, like houseflies....but are attracted to composting materials. When designing a bait station remember that they don't lay eggs directly in the bait but slightly off to the side or above, so that the baby maggots can get to the stuff easily, but the eggs themselves are less likely to fall prey to other insects already in the compost.....
