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Geraniums & Japanese Beetles

geranium image by Vasiliy Koval from

Japanese beetles, a bane of many gardeners, eat 300 types of flowers and plants. Geraniums have an intoxicating effect on these garden pests, providing a means of controlling them naturally. A study conducted in the 1920s discovered an unusual relationship between geranium plants and Japanese beetles--the beetles entered a comalike state when consuming the petals of this flower.

Japanese Beetles

Japanese Beetle image by Jim Mills from

Japanese beetles are a destructive garden pest, causing annual damages worth millions of dollars to home and commercial gardens across the country. Grubs or larvae feed on grass roots, weakening the structure and causing brown dead patches to appear while also attracting predators like moles that dig holes throughout the area their pursuit. Adults skeletonize leaves by eating the material between the veins of a wide range of plants including lucrative soybeans, maples and ornamental roses.

Geranium Plants

Geranium belongs to the family pelargonium. Although this perennial garden plant survives mild winters outdoors or is brought indoors until early spring, it also grows as an ornamental houseplant or annual in the northern areas. According to the University of Rhode Island Landscape Horticulture Program, geraniums spruce up an indoor or outdoor spot from spring through fall. The blooms usually feature solid bold colors, although some varieties are variegated.


Japanese beetles, unable to resist tempting geraniums, binge on the petals for 20 to 30 minutes before passing out as a result of the intoxicating effect for 12 to 18 hours. The pests lie on their backs with their feet up in the air and enter a stupor until the effect of the binge is over. Laboratory tests conducted to understand this relationship discovered that when given a choice between poisonous geranium and nutritious linden, the beetles always seemed to choose the former.


Once a beetle enters the comalike stupor, it lies on its back and becomes vulnerable to predators that feed on the bug. Beetles that regain consciousness if undiscovered eat more geranium petals, only to fall on their backs and enter the stupor again. Laboratory analysis of this behavioral cycle of addiction found a reduction in the generation of beetles the following year as a result of lack of time for reproductive activities.


Most of the addictionlike cycle tests were conducted in the controlled environment of a laboratory, in the absence of natural predators, where the beetles that woke up after the long stupor fed on more geraniums. Under normal conditions, a 12- to 18-hour stupor makes the immobile beetles an easy target for predators. The U.S. Department of Agriculture is conducting more tests.

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