Cannas are a family of plants that are desirable for their large, graceful leaves and equally large, showy flowers. Home gardeners love them for their looks and for their resistance to many diseases, especially those caused by fungi. Unfortunately, these tuber plants are also a favorite of the Japanese beetle (Popillia japonica), a voracious garden pest that will quickly make skeletons out of the leaves of your cannas if left unchecked. Getting rid of these insect pests is not easy, but it can be done if you are persistent.
Look for signs of the bugs. Although these large beetles are easy to spot, you may first notice holes appearing in the leaves of your cannas. If you see the holes, look under the leaves for the bugs themselves.
Slip on some gardening gloves and handpick the bugs off the leaves. Although time-consuming, this is the best way to effectively rid the cannas of the Japanese beetles and will not take too much time if done each morning with your daily watering.
Drop the beetles into a bucket of soapy water to kill them so they don't simply return to the cannas after you pluck them off. Dump the bucket of beetle bodies far away from the cannas, because the scent of the dead Japanese beetles will attract more of the same.
Spray the plant with an insecticide made to kill Japanese beetles. This will kill any remaining, hidden bugs. Note that such insecticides will also kill beneficial insects and may prevent bees from pollinating the flowers, so avoid getting the insecticide on the canna flowers.
Continue to pluck off the insects each morning. Re-apply the insecticide after a rainstorm.