Lawn grubs are destructive pests that eat at the roots of grass, turning it first brown, then killing it off. Grubs are the larvae stage of beetles who call soil their home. Grubs are crescent-shaped, often white, and are found directly under the grass surface, at the roots.
Finding Food and Mates
In late June and early July, beetles exit the soil to find mates and food. Overwinter beetles exit the soil earlier, according to the University of Missouri. Brown beetles such as the masked chafer are difficult to see exiting the soil since they only emerge at night, according to the University of Illinois. Adult beetles travel up to a mile to find food and mates, spreading grubs around the area as they lay eggs.
In July female beetles will lay around 60 eggs over a two- to three-week period, according to the Cornell University Extension. The eggs are laid under the soil at the root surface, and will hatch around two weeks later according to the soil quality and temperature. The first-stage grubs feed on the roots of the grass for the majority of August. During this stage, grubs are unprotected against pesticides, making it the best time to attack.
Second and Third Stage
Beetles will molt their outer shells to form a new exoskeleton, starting their second and third stage of life. These beetles are harder to kill with pesticides. The grubs will begin to consume more roots, causing visible grass damage.
As the weather cools in autumn, grubs begin to move further down into the soil, making them difficult to reach with pesticides. The beetle begins to overwinter in its third stage of existence by burrowing below the frost line, ensuring survival during the winter.
Grubs move up in the soil once the last hard frost is gone and feed on the grass roots again for a short period, causing minimal damage. Grubs stop feeding late in the spring and turn into their pupae stage, where they are resistant to pesticides. In June or July, the life cycle starts over again as the pupae look for mates and food.