When an Emergence Hole opens up, toss a grenade into it to automatically close the hole, and therefore keep the Locusts from coming out.
The Hammer of Dawn makes easy work of Emergence holes. You must activate it by targeting an enemy first, and then aim it toward the Emergence Hole.
Fire a Torque Bow arrow into the hole, which can be difficult due to the straight line of fire given by the bow. However, if you are close enough to fire while the line of fire is arced, it is possible to make it into the hole. For the dead-eye player, a Torque Arrow stuck in the top of the hole will close it equally well.
It isn't necessary to put an explosive in the hole-- it only needs to hit very close to it.
When there is an Emergence Hole, take out your Lancer while over the hole and hold B. When you do this, the first thing and last thing each Locust sees when they emerge is your chainsaw going through them.