Do Geraniums Control Japanese Beetles?

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Robert Pavlis

A popular claim is that geraniums will paralyze Japanese beetles and keep them away from your other plants. The pest eats the leaves and flowers and a chemical in the plant intoxicates them and they fall to the ground. They then die or birds pick them off, thereby controlling them in your garden.

Does this work? Can geraniums be used to protect other plants in your garden?

pick flower with three beetles sitting on it.
Japanese beetles eating geranium flower, source: Depositphotos
  • Japanese beetles are paralyzed when they eat too many flowers but not when they eat leaves.
  • Other plants are more attractive to these beetles than geraniums.
  • It is unlikely that geraniums will control Japanese beetles in the garden.

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What Does Science Know?

When Japanese beetles (Popillia japonica) eat the right kind of geranium they become incapacitated and fall to the ground. This phenomenon was first reported in 1920 and further examined by Charles H. Ballou in 1929 and has been studied more deeply in recent years.

As the beetles eat the plant they slowly become intoxicated and paralyzed. The effects are seen within 3 hours of the onset of feeding. “Paralyzed beetles held under laboratory conditions typically recover within 24 h, but symptomatic beetles exposed to field conditions usually succumb to predation or
desiccation”. The active ingredient causing the paralysis is an amino acid called quisqualic acid, which was isolated in 2010.

Contrary to anecdotal reports the beetles are only affected by consuming the flowers of the plant and not the leaves. The plants can be grown in sun or shade with equal effect and all flower colors intoxicate the beetles. Reports that white flowers are more effective are not true.

The plant involved is commonly called the geranium by gardeners but it is a Pelargonium. Flowers of Pelargonium inquinins, and Pelargonium zonale, which are the parental species of P. × hortorum, as well as ivy geranium (Pelargonium peltatum) were active. Pelargonium × scarborovia did not cause paralysis.

Anecdotes from gardeners also report similar effects for larkspur (Delphinium sp.) and bottlebrush buckeye (Aesculus parviflora) but testing has confirmed that they do not have similar effects on the Japanese beetle.

Should You Plant Geraniums to Control Japanese Beetles?

The above description sounds very promising but there are some key points to note.

  • There have been no field studies that indicate planting geraniums changes the population of beetles in a garden.
  • Geraniums are not a favored food source for Japanese beetles. They much prefer going after roses, Virginia creeper, beans, and fleece flower (Persacaria amplexicaulis).
  • The insecticidal potential of quisqualic acid is less than many presently registered insecticides.
  • Not all geraniums are effective.

Some common claims and the truth behind them.

  • White flowers work better than other colors – not true.
  • Geraniums will attract more beetles to the garden than they will control. This is possible, but unlikely since Japanese beetles prefer many other plants over geraniums.
  • This method does not kill the pest. That is not entirely true. When beetles were fed geranium flowers in a lab they did become paralyzed. When placed outdoors some of them died. However, there is no data to show that beetles feeding outdoors also die.

Should you try this method? It’s unlikely to change the number of Japanese beetles in the garden. If you grow plants that they prefer more, then they will probably leave the geraniums alone.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

4 thoughts on “Do Geraniums Control Japanese Beetles?”

  1. I have seen geraniums planted among roses in a rose garden and you are right, the beetles were fiercely feeding on the roses, never touched the geraniums. Hand picking them off the roses was more effective than anything else. the problem is that when they first showed up in the rose gardens the caretakers installed traps that eventually caused an inundation of beetles. It has been a huge problem ever since.

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