The simple answer is, no, milky spore powder does not harm your bumblebee larvae or pollinators in general. Another pollinator friendly method used is beneficial nematodes. Both methods are integrated into the lawns and targets only the larvae stage of the Japanese beetle. However, research from the University of Minnesota Extension found these approaches to be of limited use as the only method of managing Japanese beetles (https://extension.umn.edu/yard-and-garden-insects/japanese-beetles). This makes sense because Japanese beetles can travel long distances when they are “called” by enticing pheromones from females, other feeding Japanese beetles, and the scent of their favorite plants, so getting rid of them in your lawn will only slow them down.

The most effective environmentally friendly method found for reducing the numbers of Japanese beetles is physical removal. Use a bucket with a little bit of soapy water and knock them off the plant into the bucket. The best time of the day to catch them is early morning or late afternoon/early evening. This can be tedious, but it is satisfying. This should be done as regularly as possible through the months when they are active (mid-June through August). Here is another website with more information https://mdc.mo.gov/trees-plants/diseases-pests/japanese-beetles

Good luck!