Japanese Beetle Damage

Japanese Beetles (Popillia japonica) are a common annoyance for Iowa gardeners in June, July, and August. As a pest of nearly 300 species of trees, shrubs, and flowers (as well as fruits, vegetables, and turf), it is difficult to garden without being impacted. The Japanese Beetle population starts to thin out in late-August as the adults lay their eggs in the ground. The eggs hatch two weeks later, and small c-shaped grubs begin eating fine roots from nearby plants. These grubs grow larger through fall, eventually eating coarser roots. The grubs hibernate through winter and then emerge from the soil in mid-June. Individual beetles live for a month, with the largest population boom being in mid-July. The beetles appeared later this year than in previous years due to the cool weather, but it doesn’t appear that their populations were heavily impacted by the cold winter or wet spring.

While the larval phase causes damage to roots, the adult beetles will skeletonize the leaves and flowers of several deciduous trees and shrubs, as well as a few herbaceous plants. This damage is unsightly, but will not negatively impact the overall health of the plants, unless the plant is exceptionally small or already severely stressed. The beetles become active in mid-June and are drawn to the smell of injured plants. Once beetles find a plant, they use pheromones to attract more beetles. The key to controlling beetles is to begin control as soon as beetles appear.

Treatment of Japanese Beetles can take many forms.

  • Fill a bucket with dish soap and water. Shake beetles into the soapy water. This is easiest in the morning hours, when the beetles are more lethargic.
  • Physical barriers can be unsightly, but are incredibly effective at preventing damage. These are often found around grapes and other susceptible food crops.
  • There are several insecticides that effectively repeal or kill the beetles. Remember to use chemical that are non-toxic to bees when treating flowering plants (like roses). Neem oil is one option that is completely safe for bees, butterflies, and mammals.
  • Remove plants that Japanese Beetles favor, and replace them with less susceptible plants. Japanese beetles do not eat evergreen plants, and avoid lilacs, dogwoods, and forsythia.

Prepared by Lindsey Smith, Collections Curator