Adult beetles commonly skeletonize leaves and also feed on flowers and developing fruit of roses, grapes, Japanese and Norway maples, gray birch, elms, mountain ash and even tulip poplar and sycamore. A few Japanese beetles seems to bring out the Japanese beetle traps. They are not an effective control although they may trap and kill an impressive number of beetles.
Studies demonstrate that the traps draw the beetles to an area.
To use the traps, locate them away from plants you wish to protect. There are several more effective control measures for the adult Japanese beetle. Adults are sluggish early in the morning. They can be hand-picked from plants or shaken onto a cloth, then drowned in soapy water. Fine netting may be needed to protect valuable plants like roses or ripening fruit when numbers are high as apparently this year.