A hard plastic tray that slides into slots underneath the hive floor. 

With polystyrene hives it is essential the varroa tray is only used for monitoring varroa fall for short periods. For the rest of the year the tray is left out - even all winter. 

This technique has been proven over thousands of hives and is essential for low winter losses. The only exception is the tray can be replaced in the early spring to aid colony build up or when housing a swarm to make the interior of the hive darker.