Finally put two brain cells together
and looked up "Japanese Beetles" in my (checked out from the library copy of) The Organic Gardener's Handbook of Natural Insect and Disease Control
(ed. by Barbara W. Ellis and Fern Marshall Bradley, 1992/1997):
(Some people have already recommended a couple of the methods suggested in this book, BTW.)
"Japanese Beetle (Popillia japonica).
In early morning, shake beetles from plants onto dropcloths, then drown them in soapy water; cover plants with floating row cover; apply milky disease spores or parasitic nematodes to sod to kill larvae; attract native species of parasitic wasps and flies; organize a community-wide trapping program to reduce adult beetle population; spray plants attacked by beetles with rotenone." (pp. 298, 299)
N.B. Rotenone, under conditions of extreme dosage and enhanced absorption, induced Parkinson-like symptoms in rats in a 2000 experiment. See https://en.wikipedia.org/wiki/Rotenone for info. Rotenone is still approved by the USDA for use on organic certified crops.
Also of interest for purposes of preventing Japanese Beetles from attacking one's garden:
Overwintering larvae deep in the soil move toward the surface in spring to feed on roots, pupating in early summer. Adults emerge, feed on plants, and lay eggs in late summer; eggs hatch into larvae that overwinter in soil. One generation occurs every 1-2 years." (Ibid., p. 299.)
So far, this book has answered every question I've asked it over the past three weeks. I may have to purchase a copy of it myself....it's due back at the library on Thursday, and I can only renew it if there are no Holds (= Reservations) placed on it.
USDA Zone 9, Sunset Zone 17