There are a few things to consider about applying milky spore. One consideration is that studies show it has limited effectiveness. A publication by CSU that was just released puts the effectiveness at less than 5% reduction in grub populations. That same publication had information on other types of controls that are more effective. Here is a link to this information.

http://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/

If you want to try the milky spore, apply it when larvae are active. You would apply it when they are actively feeding on turf grass roots which is May-Jun and Aug-Sept.

Another thing to consider is whether you have evidence of grubs in your lawn. If you had adult beetles in your garden last summer, it doesn't necessarily mean they originated from your lawn. The adults can travel up to five miles. If you had grub damage in your lawn you would notice your lawn looking drought stressed, even when you were adequately watering and you would be able to pull up clumps of grass easily because the roots would be damaged. The damage would have been most noticeable in Aug-Sept.

The link above will give you some good information about how to manage Japanese Beetles at all stages of their life cycle.

Hope this is helpful and we empathize.