I live in London, Ontario, and the Japanese Beetles discovered the 7 large potted flowers on my balcony a week ago. Since then, they “flock” in around 8:45pm every night for around 30 minutes. So far, my only preventive solution is to bring all the flowers into my house for the night. What else can I do? If I covered each planter with a large, clear leaf bag for the night, would I harm my plants? Would that deter the beetles or would they just crawl into the leaf bags?
When Japanese beetles discover your garden they seem to do so in great numbers, as you have experienced. Experts say that the best method for control is to hand pick them in the early morning or late evening when they are less likely to be flying, and simply knock them into a pail of soapy water, where they will drown. Another option, similar to what you are suggesting, is to use what is called row cover cloth to drape plants, making sure that the edges are secured at the base of the plant, to prevent the beetles from landing. Gardeners and nurseries use this cloth to cover fruit trees or prized roses. It is an air permeable fabric that is closely woven so that insects cannot penetrate it. Row cover cloth is available in rolls from gardening specialty stores . Other fabrics that you could consider are tulle or mosquito netting. A covering that is breathable would be preferable to plastic, but covering in clear plastic overnight and removing in the morning would likely not harm your containers in the short term. We should see the end of these beetles in mid-August, when they mate, lay their eggs and gradually die off.
I know it is little consolation, but there are many gardeners in Southern Ontario who share your frustration with this pest. Here is a fact sheet from the Ontario Government that lists plants that are more and less susceptible to the Japanese beetle: https://www.omafra.gov.on.ca/english/crops/facts/92-105.htm