"Imp Catching The only combat monster that can also be hunted is the ordinary imp. Imp hunting requires magic box traps, which can be purchased from the hunting shops. Activate a magic box trap near a location known for imps, such as south of Edgeville, south of Falador, Lumbridge, Varrock, Brimhaven/Karamja, and south of East Ardougne. Smoking the trap has no effect. You can optionally bait the trap with a coloured bead (obtained, ironically, from killing imps), but they will steal the bead fairly regularly, and can be caught without bait. You can have more than one imp-in-a-box at a time, but they cannot be traded." Straight from the hunter guide, you don't need to bait the traps. And i believe GWD imps can't be caught don't know why tho.