Browning grass and bare spots in your lawn are often an indication that the soil is too dry and the grass is dying. Sometimes, however, browning and bare spots in the lawn are an indication of a lawn grub infection. Lawn grubs, when left untreated, can cause massive damage.
Grubs are the larva stage of the European chafer and the Japanese beetle, says the Michigan State University Extension. Maturing grubs feed on the roots of lawn grass during both the spring and fall, causing grass to die due to lower water intake.
Thinning and weakening of the grass strand are the earliest signs of grub damage, according to the University of Missouri Extension. Wilting, even with regular watering, and browning of the grass occur later. Grubs attract birds, moles and shrews to the lawn, as they provide a healthy food source. Grass that is weakened will pull up easily from the lawn without any roots attached.
In July, beetles spend a two- to three-week period laying around 60 eggs in the soil. Eggs hatch about two weeks later, says the Cornell University Extension. During this stage, grubs feed on the roots for most of August. This is the damage usually apparent in the spring of the next year. Grubs molt from late August to early October, consuming more roots as they grow. When the temperature drops, the grubs dig down into the soil. When spring arrives, the grubs move back up toward the surface, feeding on what roots are left, before emerging from the soil as full-grown beetles. Spring root damage is apparent in the summer or fall.
Regular fertilization, thatch management and irrigation help prevent grub population, says the Michigan State University Extension, and help the lawn resist grub damage. Lawns have a grub threshold of five to 15 grubs per square foot for irrigated lawns, and five grubs per square foot for low-maintenance lawns. Tall fescue grass is resistant to grub damage, and can be over-seeded for cover over the winter months.
Chemical pesticides will work for any grub species, while organic pesticides are sometimes specific to whether grubs are European chafer or Japanese beetle. Organic products containing the active ingredient Bacillus popilliae are for Japanese beetles only, while those containing parasitic nematodes are acceptable for both. Beetles and chafers at the grub stage are only identified under a magnifying glass--the European chafer has a Y-shaped raster pattern and the Japanese beetle has a V-shaped raster pattern. Pesticides containing imidacloprid or halofenozide applied in July prevent the development of grubs in the lawn, no matter the species. Use pesticides containing carbaryl and trichlorfon in the spring and fall to kill grubs that are already developed.