There are a few sprays that you can use for Japanese beetles.

Insecticide recommendations for adult control

Removing beetles by hand may provide adequate protection for backyards, especially when beetle numbers are low. The presence of beetles on a plant attracts more beetles. Thus, by not allowing beetles to accumulate, plants will be less attractive to other beetles. One of the easiest ways to remove Japanese beetles from small plants is to shake them off into jars filled with soapy water.

With all insecticidal products, foliage and flowers should be thoroughly treated. The application may need to be repeated to prevent reinfestation. Follow label directions and avoid spraying under windy conditions. Never spray when bees are foraging. Be sure the insecticide is registered for use on the plant or crop you intend to spray. If it is a food crop, note the minimum number of days that must be observed between the date of the last application and the date of harvest.

Different chemicals are used on adults when feeding on foliage. Foliar sprays of contact insecticides kill adults and offer immediate knockdown, such as carbaryl, acephate, pyrethrins, and pyrethroids. Examples include pyrethroid products such as cyfluthrin (Tempo, Bayer Advanced Lawn & Garden Multi-Insect Killer), bifenthrin (TalstarOne, Onyx), deltamethrin (Deltagard), lambda cyhalothrin (Scimitar, Spectracide Triazicide), esfenvalerate (Ortho Bug-B-Gon Garden & Landscape Insect Killer) and permethrin (Spectracide Bug Stop Multi-Purpose Insect Control Concentrate and other brands). Both pyrethroids and carbaryl provide around 2 weeks of protection. For adults, repeated applications may be necessary because of the relatively short residual effect of the pesticides. Insecticides that are advertised as organic usually do not kill adults. Formulations with pyrethrins and PBO (piponeryl butoxide) are more effective. Neem products such as Azatrol may provide about 3-4 days deterrence of feeding. Insecticidal soap, extracts of garlic, hot pepper, or orange peels, and companion planting, are generally ineffective (Tables 1 and 2).

Soil-applied imidacloprid is systemic and translocated around the plant and kills adults when feeding on foliage. However, when adults feed on petals of shrub roses rather than the spiny leaves, imidacloprid is not effective. Soil-applied imidacloprid used on linden or basswood trees or any plant visited by bees or beneficial insects, can potentially kill any bees or beneficial insects feeding on the pollen and nectar in the flowers.

  It is best to understand the whole life cycle of them. It will give you the "advantage"! Please follow the link I have provided for not only treatment, but information about the Japanese beetle.

http://www.extension.umn.edu/garden/insects/find/japanese-beetles/