I have heard that soap & water is a good spray & was wondering what proportions to mix. We had a swarm of Japanese beetles on our nectarene trees. We mixed the soap spray pretty strong & got rid of the bugs, but the fruit was shriveling up & all the leaves came off.
You might check out applying neem. I'm not sure but don't think that soap/water will work for these nasty beetles unless you can shake them all out of your trees into the soap mixture.