Grubs are the larval stage of a variety of soil beetles that breed in the ground. Small infestations of grubs will have little effect on your lawn, but a major colony of grubs may cause severe damage, making your lawn grass wilt and potentially die back. Knowing the grub life cycle and how to control it will help in managing grub damage.
According to Michigan State University, the most common symptoms of a grub infestation are browning or dying sod, causing bare spots in the grass. This is not always an indication of grubs, however, as poor maintenance practice will cause a similar effect. Sod coming easily out of the dirt with a light pull is a better indication of grub damage, as it indicates the roots are not strong.
Adult beetles, according to the University of Rhode Island, lay their eggs in July, with the larvae beginning their feeding process in August. Grubs begin feeding on the grass roots, then move down into the soil as the weather turns cold. During the winter grubs are inactive. Once the chance of frost is over in March, the grubs move up to the surface and feed lightly. Grubs finish feeding and go into their pupa stage. Adult beetles emerge from the soil in June or July and lay their eggs, starting the process over.
Beetles and chafers are the most common grub varieties. To identify between grubs, a magnifying lens or microscope is required to examine the raster pattern on the grubs end. A Y-shaped raster pattern indicates the presence of European Chafer larvae, while a V-shape indicates Japanese beetles. Home identification is difficult; most University Extension services provide a pest identification service for a small fee.
Moist soil encourages the growth of grubs, since beetles and chafers require a moist soil to hatch their eggs. In several years of good rainfall and irrigation, says the University of Missouri, grub population increases. When turf can withstand low moisture, controlled drought in the grass by ceasing irrigation will destroy grubs without the use of pesticides. Watering less in the hottest part of the summer in July and August will prevent adults from laying eggs as well.
Chemical control of grubs is best accomplished in the fall, when eggs are laid, than in the spring when grub damage is apparent says the University of Rhode Island. Grubs in the spring are more mature, with a harder exterior skin than the young grubs of the fall, meaning they are less susceptible to pesticides. Treatments between August 1 and September 15 are most effective in areas that have greater than eight to 10 grubs per foot.