This is such a great question, and I learned so much trying to answer it for you! Here’s what I have to share: in short, no, milky spore is not the best way to break the cycle of Japanese beetles, but can play a role in breaking the cycle. If you aren’t familiar, milky spore is a bacteria, that, in the 1930s, was discovered to kill Japanese beetle larvae – grubs – if they ingested it. It derives its name because it turns the grub from a clear yellow to a milky white. It is only effective on Japanese beetles, not any other types of grubs. The grub stage of their lifecycle lives in lawns, and the milky spore is applied to the lawn where it goes into the soil and the grubs hopefully consume it as they feed on roots, algae, and so forth. Sounds great in theory, but in reality, it’s not quite so simple. For milky spore to be effective, it has to be present in the soil when the grubs are actively feeding in spring and fall, the grubs have to physically ingest a minimum quantity of spores, and the temperatures must be 66-70°F. Very often, these conditions don’t line up, especially in colder areas where spring can be quite cool. That said, it’s worth applying milky spore in spring – exact directions and timing will depend on the formulation, so read your package directions – and working toward building up a population of milky spore in your soil to increase your chances of success. If you are digging in the garden and encounter any milky white grubs, leave them in the soil – their body will release more spores and help build the population of milky spore in the soil. So yes, work toward building up milky spore in your soil if you have a Japanese beetle problem, but pair it with other adult management strategies, like handpicking, spraying, and planting resistant plants.