Beneficial nematodes only work for insects still in the soil. When the temps warm up, ticks climb up the grass and get out of the soil, so the BN won't work against them. You don't say where you are, but if it is still pretty cool at night, you still have a chance to catch the ticks before they move out of range.
So what kills them AFTER they've moved to the trees? We live in Wylie and go to my in-laws that is just chock-a-block full of trees (and chiggers). They would be worried about hurting their lizards, fish, & chickens as well though (with whatever kills the chiggers).
Thanks for the help.