Fly predators

[Gallo del Cielo]

OK, my fiancé wants to try these fly predators in our coop. We have 13 chickens.

What I can't seem to figure out (or find out online) is that I see the common house fly is the most common fly in our coop (and garage and house). These predators supposedly attack the fly larvae (which I thought were maggots), but we don't have rotten food, or feces (or dead animals) to harbor maggots. Am I missing something?

The fly predators dig down and lay their eggs in the pupae, which are buried just under the soil surface. We've found that they are highly effective with regular application (~3x per year).