Something is for sure off, I'm running with just 10/12 ancients right now and seeing over 100M Locust Swarm ticks.
Get that Bane of the Trapped leveled and swap out the boon for Zei's until your damage is better.
I normally roll Locust Swarm damage on shoulders/chest/offhand
You're missing crit damage on your amulet/gloves/1 ring. Locust Swarm does not crit but averages out it's damage from your crit chance/crit damage. This is probably alot of the issue.
Get a Harrington Waistguard or a Witching Hour.
Like Prov already said, you really need a Mask of Jeram in the cube.
Locust Swarm is really only there to kill trash, your gargs/dogs kill the elites/champions, and on T10 they currently pretty much 1 shot them with my gear.