Apithor works really well for me.
Putting them on top of the frames is contrary to the manufacturers instructions.
If moisture entered the hive and ran through the trap You may kill your bees and/or end up with Fiprinol residue in your honey.
"With a hive tool, paint scraper or similar implement remove wax and debris from a sufficient area of the bottom board to accommodate the harbourage. Place harbourage, flat surface down, on the bottom board with the slot ends aligned away from the hive entrance.
The harbourage must sit flat on the bottom board such that beetles cannot shelter underneath. In hives with corrugated or distorted bottom boards apply a thin bead of silicone sealant to the outer edge of the underside of the harbourage and press down firmly onto the bottom board. Unless stuck to the bottom board a thin wire may be attached to the harbourage to facilitate later removal from the hive via the hive entrance thus removing the need to open the hive. Monitor harbourages for damage or for 'waxing up' of the slot entrances. Replace if damaged or when effectiveness declines. Remove harbourages when control has been established or after 3 months."
"NOT TO BE USED FOR ANY PURPOSE, OR IN ANY MANNER, CONTRARY TO THIS LABEL UNLESS AUTHORISED UNDER APPROPRIATE LEGISLATION."