My plant leaves are being eaten
Without a photo or a bit more information, it is difficult to determine which pest is attacking your plant. The castor bean (Ricinus communis) is not particularly prone to disease or pest infestations, so there isn’t an obvious answer. Spider mites are noted as an occasional problem (see Missouri Plant Finder https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b459, also found this Wisconsin master gardener site: https://wimastergardener.org/article/castor-bean-ricinus-communis/ )
Treatment really does depend on the particular insect you are dealing with. If you are not able to pick off the insects, a strong blast of water from your garden hose can sometimes dislodge them. Applications of insecticidal soap (available from your local garden centre- make sure to follow directions) can also help to rid your plants of some types of insect. When spraying with insecticidal soap, make sure to spray both the tops and underside of the leaves. As well, move the affected plant away from others for several days.