The Japanese beetle (Popillia japonica Newman), a polyphagous scarab, feeds on certain palatable plants that are toxic, or reputedly toxic. Paralysis of the beetle after consumption of flowers of zonal geranium (Pelargonium × hortorum L. H. Bailey) has been documented, but factors affecting expression and range of this phenomenon are poorly known. Published anecdotes regarding toxicity of two other hosts, larkspur (Delphinium sp.) and bottlebrush buckeye (Aesculus parviflora Walt.), have not been empirically tested. For zonal geraniums, we compared toxicity with P. japonica of flowers versus leaves, sun-grown and shaded plants, and different-colored flowers. The progression of paralysis and recovery, as well as survival of paralyzed beetles under laboratory and field conditions, also were evaluated. Beetles became paralyzed after feeding on flowers of zonal geranium, but not by consuming leaves, suggesting the active compound is unique to flowers. Shaded or sun-grown flowers, and red-, white-, or salmon-colored flowers, were equally active. Beetles generally became paralyzed within 3 h of the onset of feeding. Most of them recovered when held in the laboratory, but paralyzed beetles placed in the field for 3 h did not recover. Flowers of Pelargonium inquinins (L.) L’Héritier, and Pelargonium zonale (L.) L’Héritier, which are the parental species of P. × hortorum, as well as ivy geranium (Pelargonium peltatum [L.] L’Héritier), another member of the type section Ciconium, all were active. Pelargonium × scarborovia, which belongs to the type section Pelargonium, was the only geranium species that did not cause paralysis. Although the active compounds have not been identified, our results suggest that light-activated flavonoids or anacardic acids probably are not responsible for geranium-induced paralysis. Contrary to published anecdotes, neither flowers nor foliage or larkspur and bottlebrush buckeye were toxic to Japanese beetles.