I had the same experience last summer feeding exclusively spent brewery grain. The pod was full of larvae but few migrated out into the collection bucket. I added a pvc drain below the coir and gravel that I had in the bottom of the biopod and more larvae exited out that drain than by way of the ramp. They survive just fine in water for at least a day. I've since found that the larvae are great climbers even on vertical surfaces, a five gallon bucket for example, and am speculating that they may be climbing out at night and falling to the ground. I've seen other designs that allow for this exit from pods and are captured in the area surrounding the pods. I'll try that this year.
