sow bugs

I am a grower of organic edible flowers and we have an ongoing and terrible
time with sow bugs, especially with pansy and viola crops.  These bugs eat
the tender roots right at the base of the plant killing and damaging large
portions of the crop.  We have used oil traps, potato halves, diatom. earth,
and physically removing the insects from around the plants.  We haven't had
that much luck with any of these methods. Any ideas on how we could improve
this situation? Thanks!