Late July to early August, just before the eggs hatch and become the grubs. Those you may find now are simply too big for Bacillus popilliae to work on them now although and they will be hatching in a few weeks as the beetles we see every summer. Parasitic nematodes now may help.

When the soil temp at 8AM is over 55F

Looks like early june this year..

Or maybe when peonies are in fill bloom..

The best time was 3 years ago. Since you just asked the question, you probably missed that date.

There is no reason not to apply it yesterday. If you wait and apply them the day the bugs arrive, the disease will go to work in relatively small numbers. The milky spore disease creates billions more spores after spore number 1 is ingested by grub number 1. If the spores are not in the soil that day, you missed your opportunity.

Because the microbe is in the spore form, it is almost impervious to heat or cold. Go ahead and put it down now.

Thanks everyone.