Are you speaking of the ramp style DE traps (piece of thin wood with DE glued underneath)?? I tried them last year and found them to be somewhat effective. Not 100% but they did a pretty good job over all.
This type of device doesn't actually trap the beetles. When the beetles attempt to hide beneath it their antennae are cut which disorients them. They then either desiccate from the cuts or become easier targets for the bees to force from the hive. Don't expect to see masses of dead beetles (although you may find little bits of them scattered around). Instead, look for a general lessening of their population.
Hope this helps,
Scott