Japanese Beetle Life Cycle

Life Cycle Stages

The life cycle of the Japanese Beetles consists of four stages of development called complete metamorphosis. These stages are:

  1. Egg
  2. Larva
  3. Pupa
  4. Adult
picture of Japanese beetle

Eggs
The eggs are white and oval and laid in the soil about 2 to 4 inches down where they can absorb moisture. Female Japanese beetles will leave the plant that they are feeding on to find ideal conditions and soil to lay eggs in. The females will lay anywhere between 1 to 5 eggs throughout the area that they choose.

When done with this egg laying process, the females return to a food source to continue feeding until another mating cycle occurs where this process is repeated all over again. This process will repeat itself until the female lays over 40 eggs throughout her life cycle.

Larvae
The eggs will develop into grubs and can spend the fall and winter months in the ground developing toward the pupation stage and ultimately into the adult. The larvae are typical white grubs and go through 5 instars, or molts, before pupating into the adult form.

The grubs survive on a combination of things including proper soil, moisture, and food sources. These food sources are often the grass or crop roots that they are laid by. As the grub develops it may become mobile and move toward either one of these conditions needed to survive, therefore causing an increased area of damage to the lawn or grass that they are developing and feeding in.

Pupae
The pupae are cream to reddish brown in color and are 1/2 inch wide. Once the larva or grubs are fully developed they enter the pupation stage and become adults. Once the adult stage is reached in the early part of the summer months, they leave the ground and immediately begin to look for plants to feed on.

Adults
These newly formed adult Japanese beetles begin to release pheromones. Pheromones are chemical substances that are released by insects or other organisms into the environment to communicate with others of the same species. These pheromones will attract other adult Japanese beetles that develop and leave the ground later.

Once the other adult beetles arrive to the plants, they begin feeding as well until the mating process begins. Mating is common on the food plants and several matings by both males and females are common. The adults are a metallic green color with copper – brown wings with a row of white hairs on each side of its abdomen.

Encounters & Concerns

Encounters
Homeowners encounter Japanese beetles during the early summer months as the beetle adults fly and gather in clusters to feed upon plant leaves, consuming the soft leaf tissue between the leaf veins, but not eating the leaf’s veins. The egg, larva, and pupa life cycle stages develop underground and unless soil is removed or dug into, these life stages will not be seen.

Observing Japanese beetles feeding on plants is quite common since the adult beetle feeds on about 300 species of trees, shrubs, ornamental, and fruit trees, in addition to vegetable crops. Damage to rose plants is one of the most reviled, yet common encounters homeowners experience with Japanese beetles.

Concerns
Homeowner concerns center around the damage this beetle causes on the property. Not only do these insects damage and destroy many kinds of plants, but also their swarming flights and feeding clusters are an annoyance. Also, damage to plant roots caused by the below ground beetle’s larval stage often results in root damage that can lead to bare spots in the home’s lawn, destruction of garden vegetable plants, and ruining the aesthetics from rose plants on the property.