I have received numerous complaints about this product from multiple states. Until there are some valid, scientific experiments that show that this product is effective, we do not recommend it. We discussed this on a listserv for Extension entomologists. The title of the scientifically-reviewed journal article below unfortunately says it all:

Redmond, C. T. and D. A. Potter. 1995. Lack of efficacy of in-vivo and putatively in-vitro produced Bacillus popilliae against field populations of Japanese beetle (Coleoptera: Scarabaeidae) grubs in Kentucky. J. Econ. Entomol. 88: 846-854.

Unpublished studies from highly regarded researchers in other states have yielded similar results, unfortunately.
Many so-called "successes" are from people who dug a few times in their yard years after they applied the product and found few grubs. With the normal distribution of Japanese beetles in many home lawns, I am not surprised. A study by Cornell found that only 20% of the home lawns and golf course fairways required treatment (i.e. had more than 10 grubs/ square foot of turf).

Here is a link that might be useful: Redmond and Potter Article Summary

If you were to apply this the optimal time would be late July to mid August, time enough for the spores to get into the soil so the newly hatched larva would ingest the spores. That most people treat for grubs at the wrong time of the year, spring, is common knowledge since at that time the larva are too big for the controls to work and very quickly they pupate and are not ingesting anything anyway.
Bacillus popiilae is a natural occuring bacterium common in most soils, as is Bacillus thuringiensis, and a close relative (that I can't remeber the name of) that causes tetanus and few people have as much of a problem with grubs as they think. Marketing has produced enormous sales of products we usually don't need.