The presence of Japanese beetles on your plants will draw in more Japanese beetles, from certain pheromones they release. If you can get rid of the beetles that are already present, then less new beetles will be attracted to your plants.
Here are a few simple methods for getting rid of Japanese beetles.
Pick them off and squish them
Yes, this is the simplest, lowest tech way to get rid of Japanese beetles if you're not squeamish, and if there aren't too many (you can decide what is too many for you). Do this every day while Japanese beetles are present in your area.
Japanese beetles are active for 4 - 6 weeks during the summer, but they may be present for only part of that time on your plants.
A variant of this method is to vacuum them off your plants. Go carefully so as not to further destroy your own plants! And empty the bag or cannister afterward, otherwise the dead bugs will start to smell.
Flick them into soapy water
This turned out to be a very simple and effective method for me to get rid of the Japanese beetles on my husband's canna lilies.
This is a variation of picking them off, but I didn't have to squish them and I didn't even really have to touch them except to flick.
I used about a tablespoon of dish soap in 2 cups of water in a plastic bowl. With bowl in one hand, I flicked the Japanese beetles into the bowl with the other hand. You can brush them into the water with a gloved hand if you don't want to touch them at all. It also works to tap the leaf with the bug over the soapy water.
The Japanese beetles die in the soapy water because it destroys their cell membranes, and their bodies dehydrate.