The adult beetles feed on a wide range of plant material. They skeletonize leaves and will also feed on flowers and developing fruit. 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 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.