Late June, early July when temperatures reach about 21 degrees C is when the Japanese Beetle begin flying around looking for plants to decimate. They continue through the summer reaching their peak in late July and August, then they gradually disappear. The list of plants they like is extensive. The best defense you have at this time is to make your garden less desirable so that they will go somewhere else.
Northumberland Master Gardeners would like to share some of the things they have done to combat Japanese Beetles in their gardens.
Joy Cullen writes: My first encounter with Japanese Beetles was three summers ago with just a few present in my garden. The population has since exploded. They nibble a few plants and trees: larch, gingko, purple sandcherry, but they devour climbing hydrangeas and roses. It is interesting that the climbing hydrangea in the sun is devoured but the one in shade is ignored. My control method so far has been hand picking in the morning. I found that spraying with a water/ammonia mix was ineffective. This year I am going to try a pot of geraniums beside the climbing hydrangea. Apparently Japanese Beetles are attracted to geraniums but when they chew on the leaves they become paralyzed and die of dehydration.
Lori Groves shares her experience: In my garden, Japanese beetles like to munch on the leaves of the bush beans, the paper birches and the wild plum tree. I placed 2 pheromone traps in the yard for only one summer. My conclusion was that they were somewhat effective, but there seemed to be many more beetles around than in the previous years without traps. For me, the remove-squish-and-drop method works best, which adds organic matter to the soil. One great discovery was that the Japanese beetles love to rest, gather and mate on the giant leaves of my rhubarb plants without eating them. The best time to catch the beetles hanging out on the rhubarb is in the early morning sunshine and at dusk. The beetles tend to be sluggish at these times. This has become my best strategy to reduce their numbers in my garden.
For additional information you can go to the link below to an article by a Halton Master Gardener, Cathy Kavassalis.