Grub's Life Cycle

A grub is actually the larval stage in the life cycle of various types of flying beetles or scarab beetles. One of the grubs you are most likely to encounter in your garden is the white grub, which is the larval form of the Japanese beetle.

While various species of scarab beetles have slightly differing life cycles, most follow a similar pattern. With the Japanese beetle and many other types of scarab beetles a life cycle is completed each year. However, there are some species in which a life cycle may take three years to complete.

Emergence

In most species, and depending upon the geographic location, the adult beetle emerges from its underground wintering in June or July. It comes above ground to search for food and to mate.

Eggs

In July, the female beetle begins laying up to 60 eggs in the soil, burrowing about three inches into the ground each time she deposits her eggs. The egg-laying process may take her two to three weeks to complete. The eggs will mature in the soil, taking about two weeks to hatch, depending upon the moisture, temperature and other environmental conditions.

Grubs

When the eggs hatch, small grubs in their first-stage of larval existence will emerge. They will appear to be creamy white in color and will be in a C-shape, having three pairs of legs. They will feed on the roots of grass and other plants throughout the month of August. The grub will molt two to three times from August until October. As they molt, they continue to grow larger and are eating more and more roots.

Overwinter

As the temperatures grow colder, the grub will go down into the soil about four to eight inches to survive until spring. They live just below the frost line and will return in the spring to feed on new roots before going into the pupa stage.

Pupa

In late spring, the grub stops feeding and enters the pupa stage during which metamorphoses will occur and from which it will emerge an adult in early summer. The grub is usually in the pupal stage for about three weeks. During the pupal stage, the grub doesn't eat. Pupae of the beetle can often be found in small cells about three to six inches below the soil.

Adult

The adult Japanese beetle can fly as far as a mile and feed upon all kinds of plants. Its favorite meals include roses, grapes and certain types of trees. While other scarab beetle adults may also be active at the same time as the Japanese beetle, they are less likely to be noticed because they aren't feeding on cultivated vegetation.

The adult beetles will congregate in clumps on bushes and trees to mate. Shortly after mating, the female beetle will begin depositing her eggs--beginning the life cycle once again.

References

• A Grub's Life: Egg to Beetle
• White Grub Life Cycle
• White Grub Management