Controlling Japanese beetles (Popillia japonica) seems an endless labor. In the eight decades since the ravenous pests were discovered in this country, gardeners have tried a host of controls, ranging from the conventional--handpicking and spraying--to the wacky--spiking grubs to death with aerator sandals.
David Held and Daniel Potter of the University of Kentucky investigated a little known fact about these beetles: they love garden geraniums (Pelargonium). The entomologists found that Japanese beetles are also intoxicated by these plants. When given a choice of food, including their favorite linden leaves, adult beetles repeatedly chose geraniums even though this meal caused paralysis for up to 24 hours, and death in as many as a third of the population. The researchers suspect that geraniums contain a chemical that has narcotic effects on Japanese beetles.
Lab experiments using Orbit Series geraniums reveal that beetles prefer the flowers to leaves. The narcotic effects were similarly powerful whether the blooms were orange, red, or white; however, effects were significantly stronger in plants grown in full sun.
Although field research hasn't been done and the plants' effectiveness as a pest control is inconclusive, curious gardeners may want to try planting geraniums as a trap crop with other beetle favorites such as roses.
Shila Patel is a former managing editor at the National Gardening Association.
Photography by Suzanne DeJohn/National Gardening Association.