Hi Susan,
Japanese beetles feed on more than 300 different plants and lindens are one of their favorites. Defoliation of well established, healthy lindens (and other trees including apple) is usually not fatal. Defoliation is most harmful to recently planted trees (those planted in the last two to three years) and trees in poor health. Because hand picking beetles off a tree may not be a viable option, you might consider calling an arborist to treat.
There are also several option to treat the soil for grubs to disrupt the JB life cycle, including use of parasitic nematodes. Japanese beetles begin to lay eggs in soil mid to late June and is largely concluded by late July. While disrupting the lifecycle may help to reduce the JB population, this will not create a force field of protection to keep Japanese beetle adults from flying in from other areas. 

The following site offers facts on JB life cycle and management options.

https://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/

You can read more about the use of insect parasitic nematodes to control soil insects such as grubs here:
https://extension.colostate.edu/topic-areas/insects/insect-parasitic-nematodes-5-573/