Hi Steven,

Milky spore is advantageous because it survives winter, but it only kills 4% of Japanese beetle larvae. I support using entomopathogenic nematodes, but I'd like to point your attention to another biological control. grubGONE! is a product consisting of an entomopathogenic bacteria that primarily infects Japanese beetles. Like milky spore, it may infect and kill other, closely related grubs (e.g. masked chafer, oriental beetle, etc.). These closely related grubs rarely cause significant lawn problems. The bacteria is known as Bacillus thuringiensis var. galleriae, or B.t. var. galleriae for short. It can achieve 40% mortality, but it does not survive the winter, requiring reapplication. Milky spore and grubGONE! are applied as granules via a spreader. The product labels will have more specific instructions on which setting to select and how fast to walk as you spread. Entomopathogenic nematodes have more varied application methods, sometimes being mixed with water. Again, I'd defer to the instructions on the label. 

All of these organisms thrive in cool, moist, shaded conditions. Whichever organism(s) you select, ensure the lawn is well watered post application. 

However, it may not even be necessary to purchase these organisms. It's more important to confirm that Japanese beetle are actually responsible for the turf dieback, as many factors are responsible for dieback. The threshold for management is 6-8 Japanese beetle larvae per square foot of lawn. Even if they are present, if there are fewer than this threshold, then they are likely not responsible for the dieback. While these organisms are better for the environment than synthetic insecticides, they're a bit expensive to apply purely as a prophylactic measure. If you'd to schedule a site visit later in the season to evaluate your lawn, I can help with that. 

Even if they are responsible, Japanese beetle can usually be controlled through proper irrigation practices. These larvae are prone to desiccation, and are only such a problem because of the overabundance of our over-watered lawns. Simply allowing turf to dry out between waterings causes the larvae to shrivel up and die. If the turf itself is irrigation intensive (e.g. Kentucky Bluegrass), consider seeding with a more drought tolerant turfgrass (e.g. tall fescue, buffalograss, etc.). This fact sheet lists the pros/cons of each type of turf:

Turfgrass Species Selection Guidelines | CSU Extension

Let me know if you have further questions!

That is useful information. Yes, I always recommend grubGONE! over milky spore. Knowing you're also having problems with the adult beetles on roses, you may also be interested in beetleGONE! It's essentially the same product as grubGONE!, being the same bacteria. The difference is that grubGONE! comes as granules meant to be broadcast into lawns, while beetleGONE! is a spray meant to be applied to the adult beetles. 

And yes, watering in the grubGONE in early spring is the best time to apply it. This bacteria impacts Japanese beetle larvae most significantly when they are younger. And yes, the effects will last into summer as that's when the adults emerge. However, Japanese beetle can fly long distances and prevalent throughout Front Range counties. Because of this, the beetles would likely recolonize your property if you don't apply grubGONE! that year and/or abate other control measures. 

We're happy to help! I hope you don't have further pest problems, but if you do, I hope you'll reach out to us :)

Great questions. BeetleGONE! does not need to be applied to each beetle. It just needs to be applied to the foliage the beetles are eating. Besides infection through ingestion, it spreads between beetles like a disease. And it persists on leaves for 2-3 days after application. But you can re-apply it to maintain that persistence.