Q:

Are Geraniums truly effective against Japanese Beetles?

In 2010, the Agricultural Research Service recommended planting geraniums to help control Japanese Beetles. Geraniums Could Help Control Devastating Japanese Beetle (2010). However, I have not been able to find any follow-up research to prove their effectiveness.
The following two years, this report was repeated in several forums:
‘Orange Appeal’ Geraniums Lure Japanese Beetles to their Death (2011)
Japanese beetles meet their doom when dining on geraniums (2012)
After digging around a little, I found a publication from Michigan State University from April 1929:
Sun Helps Geraniums Poison Japanese Beetles (1929)
After all this time, is there any solid proof that geraniums are effective against this pest, or is this material just being re-hashed based on the ARS publication?

A:

No, there isn't. Most of the evidence you mention is anecdotal, or as you suggest, harks back to the 1920's, apart from the first and the final one you list. The penultimate one is again, anecdotal, but the writer suggests he is continuing to monitor the effects of geraniol, or at least the amount present in Ivy Geranium leaves, and its efficacy.
In truth, there is a commercial attractant which is more effective than geraniol alone, its combined with eugenol - the article below describes the attractant fully:
Youssef et al. 2009. Field evaluation of essential oils for reducing attraction by the Japanese beetle (Coleoptera: Scarabaeidae). J Econ Entomol. 102(4):1551-8. PMID 19736768, available online.
but the article itself is more about non attractants than attractants. It appears the commercial attractant is used to gather the beetles, then they're poisoned, rather than the attractant itself killing all of them. So if you were thinking a garden full of ivy leaved geraniums sited in full sun would wipe out the adult Japanese Beetle population, I'm sorry to say its not going to - but it might have some impact if other carnivorous insects get to the beetles while they're paralyzed.
You could download the information from the following link for interest's sake, but I think there's a charge for it:
Potter and Held 2002. Biology and management of the Japanese beetle. Annu Rev Entomol. 47:175-205. PMID 11729073.

