Ginger, why don't you try to freeze the buggers? And I don't mean just put the whole lot in a bag and in the freezer for a day or two. Some time ago I posted a thread of a lady who used warm ice (frozen nitrogen) to kill off all unwanted bugs in her viv. This will be so much easier for you should you bag the soil and then expose it to the nitrogen. If what ever is in the soil do not freeze rock hard solid and dies, then the lack of oxygen caused by the melting nitrogen will surely suffocate them!
And then you can add any bugs you desire afterwards as the nitrogen dissolves quickly and leaves nothing bad behind which will be bad for the frogs or live plants for that matter. As a matter of fact, this lady said that her plants exploded with live after the nitrogen treatment!