Japanese Beetles

Japanese beetles, an invasive beetle, emit a pheromone to attract other beetles to a plant or tree. They can defoliate a shrub or tree. A rather large population of Japanese beetles (Popillia japonica) emerged in mid‐June, and should remain active for a few more weeks. Japanese beetles were first observed in New Jersey in 1916 and have become a major defoliator of plants, especially those in the rose family.

The beetle inflicts its damage by skeletonizing a leaf, taking little bites until only the main veins remain. Waterford Park in Frederick has suffered heavy defoliation of little leaf lindens, dawn redwoods, and elm. The beetle is also fond of crabapple, crape myrtle and many flowers, especially roses.

The beetle emits a pheromone on its host plant when feeding to attract other beetles to the site. That is why it is not unusual to see multiple beetles feeding on a single leaf. Continued feeding results in the skeletonization of the leaf.

Sometimes entire trees can be skeletonized by this beetle, but this is not a common occurrence. In most cases, a large plant like a tree or shrub will not be killed by the beetle, but the vigor and appearance of the plant will certainly be compromised..

The adult beetle emerges from the ground in mid‐June and spends its time feeding and mating. The female lays her eggs on a grassy area by mid July, then perishes. The eggs hatch and a white grub tunnels into the ground. This grub begins feeding on grass roots, often causing brown spots to develop on the lawn. The grub overwinters under the lawn until it emerges as an adult the following June.

The grub form of this insect is susceptible to a disease called milky spore disease.

There are a number of methods for controlling Japanese beetles on the landscape. Milky spore bacterium, a biologic control for the grub stage of the beetle, is commercially available. This control can be effective, but it may take one to five years for the bacterium to build up to lethal levels in the lawn, and one must treat a fairly large area to ensure effectiveness.

Commercial traps lure the beetle to an area and trap them in a plastic bag. The drawback to this method is that more beetles are attracted to a site than are actually trapped, which is counterproductive.

Insecticides can be effective, but might need to be applied numerous times during rainy weather. Other methods of control include placing a barrier around the plant, picking the beetle off the plant, and the use of repellents.

An interesting control strategy for the beetle involves planting geraniums around the landscape. Japanese beetles quickly consume these petals of this flower and become paralyzed. Some individuals recover but many do not, the immobile beetles falling prey to other predators.

Article by FCFCDB member

Nature Notes for 7/19/2015