Japanese beetles, or Popillia japonica, are serious pests. The adult beetle feeds on more than 300 species of plants, including roses, birch trees, mountain ash trees and vegetables. The grub feeds below the ground, chewing on plant roots and hurting the plant's ability to take in needed water and nutrients. Because the female lays its eggs in the lawn soil and the hatched larvae feeds on the turf roots, care is essential to protect the lawn and your plants.
According to Rutgers Department of Entomology, Harry B. Weiss and Edgar L. Dickerson discovered the Japanese beetle in August of 1916. They found the beetles at a nursery near Riverton, New Jersey. Specimens were sent to the United States National Museum, where H.S. Barben identified the beetles as a Japanese species. Weiss researched Japanese literature for more information and discovered the species was "a serious threat to agriculture." By 1921, infestation in New Jersey reached "approximately 213 square miles of territory."
The adult Japanese beetle is less than 1/2-inch long and iridescent green. The wing covers are copper colored. White patches of hair surround the sides and back of the body. The males and females have the same markings, but the female is larger than the male. The newborn larvae are 1/8-inch long and creamy white in color. As the larvae or grub grows, it curls into a "C" shape, similar in appearance to other grub species.
The life cycle of the Japanese beetle is one generation in a year. The female lays eggs in the soil, beginning mid-June. Within two weeks, the eggs hatch into grubs, which begin to feed on grass roots. By late August, the grubs are 1-inch long. As soil temperatures cool, the grubs burrow deeper into the soil and become inactive during the winter months. When the soil reaches 50 degrees F, they move up into the root area and begin feeding. After three to five weeks, they start the transformation into adulthood.
Because the grub destroys grass roots and the adult destroys foliage, it is important to control the population by eliminating the grubs in the lawn. Soil insecticides will not eliminate all grubs, but they will improve conditions when used on a regular basis. New products containing imidacloprid or thiamethoxam provide longer control than the previously recommended insecticides containing carbaryl or diazinon. You may apply the newer products to the lawn before the female lays her eggs; whereas, carbaryl or diazinon is applied in the fall.
Imidacloprid or thiamethoxam are granular formulations. Usually, you apply the insecticide with a lawn spreader, but always follow the manufacturer's directions for the amount to use per square foot. Once you have applied the insecticide, water the lawn immediately. It is important to move the residues into the root area or root system. Using a lawn sprinkler, wet the lawn and soil at least 1/2 inch in depth. Using an insecticide is not a one-time process. Because Japanese beetles migrate to new areas each year, you must continue to use the insecticide each spring.
Besides using insecticides on your lawn to eliminate grubs, you can remove the adult beetle so it doesn't attract more beetles. Pick the beetles from plants and place them in soapy water where they will drown. This approach is not overly effective if you have an infestation of beetles. You can also withhold watering your lawn since grubs do not do well in dry soil; however, dry conditions can damage the lawn. Avoid using Japanese beetle traps on or near your lawn. It brings more beetles into the area, which may not make it into the traps.