I have not seen any Japanese Beetles on any of my Delphiiums, so I could not say, but then I see very few of them on any plants except the wild flowers.

Larkspur, certain geraniums, and mirabilis are all reported to cause problems for Japanese Beetles. The one I can personally testify to is the latter.

We had a REAL problem with them on the seed geraniums one year. They skeletonized leaves over night. But in the morning on the geraniums, there they'd be in some sort of coma. They were easy to catch in the mornings, but had already made a mess through the night. We just try to grow things they don't like - we've found that they don't care for oriental and day lillies (they like cannas), mums, california poppies and begonias. We still mix in a geranium here and there, but no mass plantings anymore.

The only thing that deterred them was a homemade garlic spray. We had some beautiful hibiscus one year, which is one of their favorites. If we sprayed the plants, they stayed away. But man, did that stuff stink, and had to be reapplied after rain.