Q.Japanese beetles and African marigolds
Are you sure about these marigolds deterring Japanese beetles? I planted them all around my garden. The beetles snuggle in the blossoms and appear to mate there. Hardly the repelling I expected to see. I cannot show a picture since I pulled them all up. Had I seen this site before I did that I would have taken a picture for you.
Using plants to deter any pest is a questionable practice. Some swear by it and others have your experience. The few science studies I've seen do not show consistent evidence that specific plants can be used to deter pests or wildlife. Consider treating your lawn from Japanese beetle grubs. Contact your state extension service for the correct time to treat for grubs. Timing is crucial in grub control.