Egg laying begins soon after the adults emerge from the ground and mate. Females leave plants in the afternoon, burrow two to three inches into the soil in a suitable area, and lay their eggs. They lay a total of 40 to 60 during their life. The developing beetles spend the next 10 months in the soil as white grubs. The grubs grow quickly and by late August are almost full-sized (about one inch long). Grubs feed on the roots of turf grasses and vegetable seedlings, doing best in good quality turf in home lawns, golf courses, parks and cemeteries. However, they can survive in almost any soil in which plants can live.
Mid-summer rainfall and adequate soil moisture are needed to keep eggs and newly-hatched grubs from drying out. Females are attracted to moist, grassy areas to lay their eggs; thus, irrigated lawns and golf courses often have high grub populations, especially during otherwise dry summers. Older grubs are relatively drought-resistant and will move deeper into the soil if conditions become very dry. Japanese beetle grubs can withstand high soil moisture, so excessive rainfall or heavy watering of lawns does not bother them.
As Japanese beetle grubs chew off grass roots, they reduce the ability of grass to take up enough water to withstand the stresses of hot, dry weather. As a result, large dead patches develop in the grub-infested areas. The damaged sod is not well-anchored and can be rolled back like a carpet to expose the grubs. If the damage is allowed to develop to this stage, it may be too late to save the turf. Early recognition of the problem can prevent this destruction.
Japanese beetles overwinter in the grub stage. When the soil cools to about 60°F in the fall, the grubs begin to move deeper. Most pass the winter two to six inches below the surface, although some may go as deep as eight to 10 inches. They become inactive when soil temperature falls to about 50°F.
When soil temperature climbs above 50°F in the spring, the grubs begin to move up into the root zone. Following a feeding period of four to six weeks, the grubs pupate in an earthen cell and remain there until emerging as adults.