There has been some interesting research done into the behaviour of this species:
SIZE DOESN’T MATTER …for Japanese beetles, anyway.
In the animal world, big usually means best, but a certain Japanese beetle has evolved tactics to catch a mate, no matter what its size. The main difference between male Librodor japonicus beetles is the size of their jaws, which they use to fight other males. Japanese researchers studying the ‘pulling techniques’ of these beetles identified three main behavioural techniques. The largest male beetles simply wait at the mating site and fight off any smaller contenders. Medium-sized beetles have developed slightly larger wings, allowing them to fly around until they find unoccupied mating sites. But it is the smaller beetles that have adopted the most devious approach. They sneak around the mating grounds of the largest beetles, hoping to mate a female ‘behind the back’ of their larger opponent.
From BBC Focus magazine (here)
Coleoptera: Nitidulidae (sap beetles)
Four-spotted Sap Beetle = yotsuboshi-keshikisui
Location: Fureiai Forest Park (Google map)