The wasp sting is a decent knife, dealing more damage than the standard knife and most of the other small melee weapons. It's not as damaging as the combat knife, but it does have a chance to poison foes.
This weapon does not have a 'swing' mode, so the first mode is unchecked. Because the first mode is unused, the second mode (which is used) is assumed not to exist, so the weapon cannot be used. Adding a swing mode to the weapon, or moving the thrust mode to the first mode slot will allow it to be used.