The Japanese beetle is the most damaging of all the beetles. The feeding grubs will eat away at the grass roots, which causes brown patches where the grass has died. Grubs can live underground during extremely cold winters in places like Wisconsin and Minnesota. To understand how the grubs survive a winter, you must know the cycle of the Japanese beetle.
In July, the Japanese beetles fly around looking to mate. After mating, beetles lay eggs in the grass. After the eggs are laid, the beetles stop flying and eventually die. It takes about two weeks for the grub larvae to hatch.
August to October
In August, the eggs hatch and the young grub larvae appear. The grubs are white and usually remain curled in a C-shape position. The young grubs burrow underground and start feeding on the grassroots. During September and October, the grubs grow to about 1/4 to 1 1/2 inch long and continue to feed on the grass roots. The most damage to lawns is done between September and October.
November to February
As the temperatures start to drop in November, the grub larvae burrow deeper underground. Grubs burrow about 11 inches underground to wait out the winter. From January to February, the grubs remain underground.
March to April
As the ground temperatures start to warm to about 50 degrees Fahrenheit, the grubs slowly make their way back to the surface to feed on the grassroots until June. During this time, the white grub feeding is slower than in late summer and early fall when the grub does the most damage.
In June, the grub larvae changes to a pupa and emerges from the soil as an adult Japanese beetle. The life cycle begins all over again. During June, before the grub turns to pupa, the white grub doesn't feed on the grassroots.