I'll start with the good news: it is possible to reduce your garden's Japanese beetle population by using organic methods and you don't have to completely eliminate this pest in order to save your plants. The bad news is that Japanese beetles are good flyers and they are glad to move into your garden from up to 5 miles away if they sense that it's a good place for them to be.
Now that we know what makes them want to move into your organic garden, we can create a strategy for discouraging them. According to the University of Kentucky Cooperative Extension Service, "When you remove beetles daily by hand from a plant, only about half as many are attracted to that plant compared to those on which beetles are allowed to accumulate." Japanese beetles tend to congregate in clusters that can easily be knocked off of your beloved plants into a bucket of soapy water where they will drown. Squishing them is an option for those of us who feel more aggressive towards them.
Removing the beetles will also decrease the amount of damage they do to their host plants. This further decreases the amount of beetles that will migrate to your organic garden. Harvesting your fruits (strawberries, apples, etc.) and vegetables (peppers, eggplants, tomatoes, etc.) before they become overripe or begin to rot will also decrease the number of beetles that you attract to your organic garden.
Strawberries and eggplants are two of the many garden crops that Japanese beetles are attracted to. Covering these and other affected crops with floating row cover will help but will also interfere with the pollination of these crops.
In your battle against the Japanese beetle, please refrain from using the popular pheromone traps--these attract many more beetles that they catch, leaving you with more beetles than you started with. If you absolutely can't resist using them, keep the traps well away from your favorite garden plants so as not to lure the beetles right to them.
The shiny adult Japanese beetles are most active for 6-8 weeks in early to mid-summer. By the end of their annual reign, they have laid their eggs and the grubs have hatched and have begun feeding. During the winter, these grubs will burrow into the soil and await next summer's bounty.
The biological controls available to organic gardeners can help keep these grubs in check. These controls include parasitic nematodes, Bt (Bacillus Thuringiensis), parasitic wasps and milky spore. These controls can often be purchased in garden centers or through mail-order bio-control catalogs.
Luckily, the beetles are only in full force for several weeks at a predictable time each year. We can use our knowledge about Japanese beetles and their habits to beat them at their own game. Now, let's get to work and take back the summer!