What is the best way to rid hanging Boston ferns of Japanese beetles?
Japanese beetles control:
Physical Removal and Exclusion
Removing beetles by hand may provide adequate protection for your plant, 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 early in the morning when the insects are sluggish. The beetles may be killed by shaking them into a bucket of soapy water.
More ways to control the critter: http://www.gardeners.com/how-to/control-japanese-beetles/5163.html
Hope this helps.