I asked our expert, Extension entomologist, Jeff Hahn:
"This is a real phenomenon; when JB feed on geraniums (Pelargonium spp.) they consume a toxin that well intoxicates them. This does not kill them but they are paralyzed for hours but eventually recover. Interestingly, they don't apparently learn from this as they will commonly go back and feed on geranium again. There is research that shows that interplanting geraniums does not protect other plants. In fact the research showed that there were more JB on roses when geraniums were present."
For more info on Japanese beetles:
Brace for Impact: Japanese Beetles are Coming
Japanese Beetle Management in Minnesota
Some Questions about Japanese Beetles
Companion Planting for Protecting Roses from JB.pdf